International Journal for Ion Mobility Spectrometry - B & S Analytik ...

INTERNATIONA L 

International Journal 

for Ion Mobility Spectrometry 

SOCIETY 

for 

ION 

MOBILITY 

SP ECTROMETRY 

5(2002)1 

Official publication of the 

International Society for Ion Mobility Spectrometry

Table of Contents 

Ion Mobility Spectroemtry - Basis Elements and Applications 

J. Stach & J.I. Baumbach 

Nitric Oxide as a Reagent Gas in Ion Mobility Spectrometry 

G. A. Eiceman, K. Kelly, E.G. Nazarov 

Measuring the temperature of the drift gas in an ion mobility spectrometer: 

A technical note 

C. L. Paul Thomas, N. D. Rezgui, A. B. Kanu, W. A. Munro 

1 

22 

31 

Papers presented at 

10 th International Conference on Ion Mobility Spectrometry, ISIMS 2001, 

August 12 - 17, 2001, Wernigerode, Germany 

The RIP positions in dependence on the moisture 

H. Bensch 

Mobilities of halogenated compounds 

M.J. Leonhardt, J.W. Leonhardt, H. Bensch 

Reporting Ion Mobility Spectrometry Data 

and the IUPAC/JCAMP-DX International Data Standard 

A.N. Davies, P. Lampen, H. Schmidt, J.I. Baumbach 

Negative Corona Discharge Ionization Source for Ion Mobility Spectrometry 

M. Tabrizchi & A. Abedi 

Ion Mobility Spectrometry of Alkali Salts 

M. Tabrizchi 

Temperature Corrections for Ion Mobility Spectrometry 

M. Tabrizchi 

39 

43 

47 

51 

55 

59 

Copyright © 2002 by International Society for Ion Mobility Spectrometry

Ion Mobility Spectrometry - Basic Elements and Applications 

J. Stach 1 , J.I. Baumbach 2 

1 

Bruker Saxonia Analytik GmbH, Permoserstr. 15, D-04318 Leipzig, Germany 

2 

Institut für Spektrochemie und Angewandte Spektroskopie (ISAS), 

Bunsen-Kirchhoff-Str. 11, D-44139 Dortmund, Germany 

Introduction 

Although first basic research on ion formation in 

air has been known since the eighties of the 

past century [1-3], with fundamental papers on 

ion mobility in electrical fields published already 

around 1905 by Langevin [4], ion mobility 

spectrometry (IMS) is considered to be one of 

the relatively new analytical methods. Almost 

100 years have passed from the origins to their 

successful analytical applications which, along 

with the first commercialized instruments [5-7], 

were mainly attributed to the extensive work of 

F.W. Karasek [8-34]. Major steps of 

development, of course, have not been taken 

into consideration by this judgement. This 

includes the follow-up (continuing) work of 

Hassé on ion mobility [35,36] as well as basic 

research on ion- molecule reactions [37-40] or 

experiments with drift tubes which were 

published in the sixties [41-43]. 

The principle ion mobility spectrometry is based 

on is very simple. Ions formed at normal 

pressure migrate in an electrical field against 

the direction of a carrier gas, i. e. air in the 

easiest case. Ion acceleration, along with 

permanent collisions between ions and gas 

molecules, leads to an average ion velocity 

over a certain path length. Ions of different 

mass and/or structure reach different velocities 

and, thus, get separated (Equ. 1). The quotient 

of ion velocity and electrical field strength is 

referred to as ion mobility. Under identical 

conditions the ion mobility constant K 0, which is 

corrected regarding pressure (P 0 = 101,325 

kPa) and temperature (T 0 = 273 K), is a 

substance-specific value. P 1 and T 1 

are the 

corresponding values for pressure and 

temperature measured in the drift tube. 

v = K E (1) 

K 0 = K (P 1/P 0) (T 0 /T 1) (2) 

A comparison of the basic principle of ion 

mobility spectrometry with other analytical 

methods seems very likely. Terms such as gas 

phase electrophoresis or plasma 

chromatography became popular in the 

relevant literature. However, the high 

expectations regarding the analytical 

performance of IMS resulting from these 

comparisons could not be confirmed [44]. It 

was its excellent ability of detecting chemical 

warfare agents [30,45,46] and, at the same 

time, of “real time monitoring” that, along with a 

clear design and a potential miniaturization of 

spectrometers, soon led to extensive 

military-oriented development programmes 

[47,48]. At the same time further major fields 

of application for the IMS were opened up. This 

includes the detection of explosives, drugs, 

pesticides or the detection of working place 

pollutants. Coupling techniques such as 

GC/IMS or IMS/MS extend the range of 

applications [49-56] considerably. The 

development of high resolution drift tubes 

provides new applications in life sciences 

espcially if Electro-Spray Ionization (ESI) 

-IMS/MS is considered [57]. However, the 

following review covers basics and applications 

of IMS from the point of view of mobile 

analysis. 


J.Stach and J.I. Baumbach.: "Ion mobility spectrometry..”, IJIMS 5(2002)1, 1-21, p. 2 

1 Basic Elements of Ion Mobility 

Spectrometry 

1.1 Function and Components 

The basic item of the spectrometer is the 

measuring tube consisting of a reaction 

chamber in which ions are generated by a 

suitable source, and a drift region at the end of 

which the detector is positioned. Reaction 

chamber and drift region are separated by a 

shutter grid. Normally the measuring tube is 

made of metal rings separated by isolators. 

This set-up is needed to produce an electric 

field with field gradients from150 und 300 V 

/cm. A drift gas, i. e. air in the easiest case, is 

pumped through the measuring tube from the 

Figure 1: 

Measuring tube (a) and components 

(b) of an ion mobility spectrometer 

detector’s side. Fig. 1 illustrates the set-up of a 

measuring tube and a block diagram showing 

the other components of an ion mobility 

spectrometer. 

Opening the shutter grid for a short moment 

enables a part of ions to get into the drift 

region. Due to the applied electrical field and 

the drift gas flow, ions having different masses 

and/or structures reach the faraday plate of the 

detector at different times. Thus, the recorded 

ion mobility spectrum shows signals registered 

at different drift times (ms) with the 

corresponding intensities (pA). 

1.1.1 Inlet Systems 

Depending on the problem of analysis various 

inlet systems have been decribed in literature. 

This includes systems with septa for a syringe 

injection [58], permeation or diffusion vessels 

[59,60] as well as inlet systems making use of 

thermal [61,62] or laser desorption [63]. The 

thermal desorption plays an important role in 

particular for the detection of drugs and 

explosives. In combination with suitable 

sampling and enrichment techniques drugs and 

explosives can be detected in the low ppt v 

concentration range [50]. Handheld 

spectrometers used to analyze the 

ambient air are fitted with a 

membrane inlet system [64,65]. 

Direct admission of ambient air is 

quite a problem since the formation 

of ions would be impaired by the 

simultaneous penetration of 

moisture. In instruments equipped 

with membrane inlet systems, the 

ambient air is pumped towards the 

membrane. Organic compounds 

contained in the ambient air 

permeate through the membrane 

and get into the measuring tube. 

This process can be described using 

the following equation: 

A (3) 

P i = 

m P r P P x 

F c H + P r A m P 

with Pi describing the partial vapor 

pressure of an organic compound in 

front of the membrane, Pr the 

permeability of the substance 

through the membrane and P the air 

pressure. Am is the permeability 

coefficient of the membrane and H 

its thickness. Fc describes the gass 

flow behind the membrane [52]. Permeability 

coefficients were determined for different 

materials [66,67]. In most cases 

dimethylsilicone membranes [64] are used due 

to the high retention capacity regarding water, 

which is of special importance for the formation 

of negative ions. However, the membrane inlet 

systems have an essential disadvantage: the 

detection limits deteriorate due to the small 

percentage of the sampling molecules 

permeating through the membrane. 



1.1.2 Measuring Tubes 

Measuring tubes of various designs have been 

tested and used in commercial spectrometers. 

First of all they vary in the dimensions of the 

overall measuring tube, the materials used, the 

shutter grid, the selected gas flow and the 

sample inlet systems. The design of the 

reaction chamber depends mainly on the 

selected ionization technique, with ß-emitters 

(e.g. 63 Ni) mainly used for this purpose [68]. 

Further ionization techniques will be described 

a) 

b) 

Reaction 

region 

Reaction 

region 

Drift gas 

Drift gas 

Drift region 

Drift region 

Figure 2: 

Uni- (a) and bidirectional (b) flow of drift 

gas through the measuring tube 

in Section 1.3. 

Usually the measuring tubes are composed of 

a stack of metal and isolator rings [8,10,69,70]. 

In most cases glass or ceramics is used as 

insulating material. The homogeneity of the 

electrical field depends on the radius of the 

metal rings and their distance [71]. Apart from 

the commonly used stack ring technique, 

ceramic tubes are used homogenously coated 

with conductive materials to generate an 

electric field [72,73]. Usually the measuring 

tubes have a length of about 10 cm. However, 

experimental setups with larger measuring 

tubes are also known. Brokenshire [74], for 

example, describes a high-resultion drift tube 

that has a length of about 50 cm and a 

diameter of 10 cm. 

Recently a drift tube has been described [75] 

which, based on a common reaction chamber, 

has two drift tubes and, therefore, can 

simultaneously separate positive and negative 

ions. In case of conventional tubes the polarity 

must be changed for detection of positive or 

negative ions. However, the polarity of the 

high-voltage can be changed by using suitable 

switching techniques in the seconds range, 

which results in a quasi simultaneous detection 

of positive and negative ions. 

Two different variants of drift gas flow are 

known. In the easiest case gas flow through the 

tube is in just one direction [76]. However, drift 

gas flow from the direction of the detector and 

the inlet system to the shutter grid is possible 

[69,70] as well and is mainly used in 

spectrometers with membrane inlet systems. 

The two variants are illustrated in Fig. 2. 

1.1.3 Shutter Grid 

The reaction chamber is separated from the 

drift tube by the shutter grid. There are two 

different setups in use: one setup according to 

Bradbury and Nielsen [77], and a second one 

according to Tyndall [78]. The Tyndall shutter 

grid consists of two grids made of wires that 

are paralleled at a distance of about 1 mm. In 

the Bradbury-Nielsen grid the rods are 

arranged at one level [79]. The grid potential 

primarily depends on the applied field strength 

and from the field gradient and, thus, on the 

position of the grid within the measuring tube. If 

an additional field is applied between the 

paralleled wires of the grid (approx. 600 V/cm) 

which is directed perpenticularly to that of the 

measuring tube, the shutter grid is closed. Ions 

will not pass the grid. If the additional electrical 

field is switched off, the ions may pass the grid. 

1.1.4 Detector and Aperture Grid 

The apperture grid is positioned approximately 1 

mm in front of the detector which is usually a 

circular disk. The apperture grid provides a 

capacitive decoupling between arriving ions and 

the detector. The main result of this decoupling 

is a small line width of the peaks in the ion 

mobility spectra. A decrease in signal intensity 

and grid vibrations causing an increased noise 

level might be disadvantageous [52]. 


⋅ 

2 

0 


1.1.5 Spectra Acquisition 

The acquisition of ion mobility spectra can be 

carried out in different ways. A single spectrum 

can be recorded in 25 to 50 ms. The short time 

needed to record one ion mobility spectrum 

provides even if real time monitoring is required 

the possibillity of spectra accumulation. 

However, the improvement of the signal to noise 

ratio is limited since the ion currents depend on 

the quotient of the grid opening time t 0 and the 

scan time T s [52]. If the grid opening time is 0.25 

ms and the scan time 25 ms only 1% of the 

formed ions reach the detector. t 0 on the other 

hand is an important parameter determining the 

resolution of the method. Short grid opening 

times needed for a sufficient resolution will 

reduce the ion current remarkably. 

A considerabel improvement can be achieved if 

the Fourier transform technique is used. Due to 

the applied opening times of the entrance and 

exit grid of the drift tube 25% of the ions can be 

detected by theory [80]. Experimantely an 

intensity gain by a factor of 1.4 to 2.4 can be 

REDUCED MOBILITY - K - cm /V sec 

3.3 

3.2 

3.1 

3.0 

2.9 

2.8 

2.7 

2.6 

2.5 

2.4 

2.3 

2.2 

2.1 

2.0 

+ 

(H0)NH 

2 n 4 

+ 

(H0)NO 

2 n 

+ 

(H0)H 2 n 

achieved. The reason for this is the increased 

noise level of the interferograms [52]. 

The dependance of the ion mobility on the field 

strength is used by the transverse field 

compensation technique for ion separation [81]. 

This method does not need a shutter grid. 

However, the experimental setup is different 

from the usual drift tubes. The analyser consists 

of two electrodes on which a strong high 

frequency field and the so called compensation 

voltage are applied. If the volatages are in 

correspondence with an particular ion mass, 

ions can pass the analyzer and reach the 

detector. 

1.2. Ion Formation 

Usually ion mobility spectrometers are equipped 

with a ß radiation source [10,68,73]. As a result 

of Atmospheric Pressure Chemical Ionization 

(APCI) processes in the reaction region of the 

measuring cell, reactant ions with positive and 

negative charges are formed. Sample molecules 

undergo ion molecule reactions with the reactant 

ions resulting in the formation of product ions. 

Radiation sources have a very good long term 

stability due to the long life time of the isotopes 

used. They are of special importance for hand 

held devices since no energy is required. 

Beside ionization with radiation sources 

ionization techniques like photo [82], laser 

[83,84], corona discharge [85] or electrospray 

ionization [85] are well known in ion mobility 

spectrometry. The use of thermal ionization on 

hot surfaces [52,86,87] and ionization due to 

adduct formation with alkaline ions [52,56] was 

described as well. 

B 

% TOTAL IONIZATION 

100 

80 

60 

40 

20 

+ 

(H2O) n-1H+H2O 

n=4 

+ 

(H2O) nH 

n=3 

0 

10 20 40 60 80 100 120 140 160 180 200 

TEMPERATURE °C 

Figure 3: 

Dependence of ion mobility K o (a) and the water content (b) 

of the reactant ion clusters (H 2O) nH + from temperature, 

authorized by: S.H. Kim et al., Anal. Chem. 50 (1978) 

Copyright © 2002 by International Society for Ion Mobility Spectrometry 

n=2 

1.2.1 Radiation Sources 

63 

Ni radiation sources used in ion mobility 

spectrometers are identical with those of 

electron capture detectors known from gas 

chromatography. 63 Ni emitts ß radiation with an 

average energy of of 0.067 MeV and a half life 

time of 85 years. For some applications Tritium 

which emitts ß-particles isused as radiation 

source. The average energy is much lower and 

amounts to 18 keV. 

1.2.1.1 Formation of Reactant Ions 

Electrons emitted from the 63 Ni foil collide with 

atoms or molecules of the drift gas. If nitrogen is 

used as drift gas the released energy amounts 

to 35 eV per collision. Nitrogen is ionized as long 

as the energy of the radiation is higher than its 

ionization potential of 15.58 eV (Equ. 4). This


starting reaction which can be observed also in 

air initiates further competing reactions: 

N 2 + ß - → N 2 

+ 

+ e - (4) 

N 2 

+ 

+2N 2 

→ N 4 

+ 

+ N 2 

(5) 

N 4 

+ 

+ H 2 

O → H 2 

O + + 2N 2 

(6) 

H 2 

O + +H 2 

O → H 3 

O + + OH (7) 

H 3 

O + + H 2 

O + N 2 

→ (H 2 

O) 2 

H + + N 2 

(8) 

(H 2 

O) 2 

H + + H 2 

O + N 2 

→ (H2O) 3 

H + + N 2 

(9) 

The formation of positively charged water 

clusters of the type (H 2 

O) n 

H + as described by 

reactions 4-9 was proved by “high-pressure” 

MS-investigations [88,89]. The number n of 

water molecules contained in the cluster 

depends on the temperature and water content 

of the drift gas. Fig. 3 illustrates a typical 

example of the temperature dependance of n 

and the resulting change of the ion mobility 

constant. At a temperature of 25 °C , a 

pressure of 700 Torr and a rel. Humidity of 20% 

the clusters contain 5 to 8 water molecules [90]. 

However, IMS/MS investigations indicates that 

the peak formed by the reactant ions is not only 

formed by the water clusters described above. 

a) 

POSITIVE ION CURRENT 

2 -1 -1 

K=2.07 0 cm v s 

0 16.7 17.9 

41.0 

DRIFT TIME (ms) 

2 -1 -1 

K=1.93 0 cm v s 

As recorded IMS/MS spectra clearly show, the 

clusters contain beside water nitrogen or other 

drift gas components [59, 65, 91, 92], (see Fig. 

4). 

Considering the formation of negatively 

charged ions, sample molecules are ionized by 

the attachment of electrons in case that 

nitrogen is used as drift gas. If air is used as 

drift gas O 2 

- 

ions as reactive species are 

dominant. In humid air attachment of water can 

be observed. The formed ions have the 

composition (H 2 

O) nO 2- . Additionally, ions with 

the composition (H 2 

O)OH - and O 4- , CO 4- , CNO - , 

Cl - , CN - , partly also in hydrated form, were 

detected [93-95,69,91]. Fig. 5 shows a typical 

example. 

1.2.1.2 Formation of Product Ions 

The formation of positively charged product 

ions depends on the protone affinity of the 

substance to be ionized. Since the protone 

affinity of water is very low, the water clusters 

described above ionize a great number of 

organic compound classes. This includes 

alkenes, alcoholes, thiophenes, ethers, 

aldehydes, ketones, esters, amines, nitriles, 

a) 

NEGATIV ION CURRENT 

DRIFT TIME (ms) 

2 -1 -1 

K=2.16 

0 

cm v s 

0 16.0 

41.0 

b) 

RELATIVE INTESITY (%) 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

36 

+ 

(H 2 O) 2 NH 4 

+ 

(H 2 O) 3 H 

54 

55 

+ 

(HO)H 

2 4 

+ 

(HO)H 

2 5 

73 

82 

83 

91 

Figure 4: 

IMS/MS Coupling: Ion mobility spectrum of positively 

charged reactant ions (a) and corresponding MS 

spectrum (b), providing information about all formed 

reactant ions, authorized by: G.E. Spangler et al., Int. 

J. Mass Spectrom. Ion Proc. 52(1983) 267. 


101 

M/z (u) 

+ 

(HO)NH 

2 4 2 

+ 

(HO)NH 

2 5 2 

119 

129 

+ 

(HO)NH 

2 4 4 

+ 

(HO)NH 

2 4 6 

157 

b) 

RELATIVE INTESITY (%) 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

(HO)O 

2 2 

50 

CO 3 

- 

O 4 

- 

60 

64 

68 

- 

(HO)O 2 2 2 

- 

(H 

2O)CO 

3 

78 

82 

86 

M/z (u) 

- 

(H 

2O)O 

4 

- 

(H 

2O) 3 

O 

2 

- 

(H 

2O)CO 

4 

- 

(H 

2O) 2CO 

3 

88 

94 

96 

100 

- 

(HO)CO 

2 2 4 

Figure 5: 

IMS/MS-Coupling: Ion mobility spectrum of negatively 

charged ions (a) and corresponding MS spectrum (b), 

authorized by: G.E. Spangler et al., Int. J. Mass 

Spectrom. Ion Proc. 52(1983) 267.


(H 2 

O) n 

H + + AB → (AB)H + + n H 2 

O (10) 

(H 2 

O) n 

H + + AB → (AB)(H 2 

O) m 

H + + n-m H 2 

O (11) 

(AB)(H 2 

O) m 

H + + AB → (AB) 2 

H + + m H 2 

O (12) 

phosphororganic compounds etc. The reactions 

that are dominant for the formation of positive 

product ions from a substance AB to be 

detected are summarized in Equ. 10-15 

[96-99]. Whereas for low concentrations only 

Equ. 10 and 11 apply, in case of higher 

concentrations the formation of dimerice 

product ions according to Equ. 13 is observed. 

Fig. 6 shows a typical ion mobility spectrum. 

Clusters containing more than two sample 

molecules may result in dependance of the 

analyte, its concentration and the temperature. 

As IMS/MS studies have shown clustering of 

product ions with drift gas molecules has 

generally to be taken into account [46,100]. 

Also in case of product ions the dependence of 

cluster formation on temperature has to be 

considered. At higher temperatures proton 

exchange processes dominate. 

The reactions taking place in case of negative 

product ion formation are closely related to the 

ionization processes observed in an ECD 

[34,101]. The major reactions relevant to IMS 

are summarized in Equ. 13-15: 

[(H 2 

O) 3 

O 2 

] - + AB → [AB] - + 3H 2 

O + O 2 

(13) 

[(H 2 

O) 3 

O 2 

] - + AB → [(AB)O 2 

] - + 3H 2 

O (14) 

[(H 2 

O) 3 

O 2 

] - + AB → A - + B + O 2 

+ 3H 2 

O (15) 

Figure 6: 

Ion mobility spectrum of positive diethyl-methylphosphonate 

(DEMP) ions illustrating the formation of monomer and 

dimer ions 


Halogenated 

hydrocarbons, but also 

cyano- or nitro 

compounds are subject 

to a dissociative 

charge transfer reaction analogous to Equ. 15. 

With high analyte concentrations this reaction 

leads to the formation of adducts. Analogously 

the formation of O 2 

- 

adducts is observed. 

Reactions strongly depend on the analyte 

concentration, temperature and on the humidity 

of the measuring tube. Equ. 16-18 summarize 

reactions of halogenated compounds. The 

composition of the poduct ions was confirmed 

by IMS/MS investigations. Tabl. 1 illustrates on 

the example of halothane the distribution of 

product ions [102] depending on the 

M - X + O 2 

- → X 

- + M + O2 (16) 

M - X + O 2 

- → (M - X) O2 

- 

(17) 

M - X + X - → (M - X) X - (18) 

concentration of the analyte. 

Proton abstraction is another typical reaction 

for the formation of negative ions as observed 

for aromatic or acid compounds such as 

phenoles [31]. 

1.2.1.3 Quantitative Aspects of Ionization 

For quantitative description of ionization the 

following prozesses have to be taken into 

consideration: 

• primary ionization, i. e. formation of 

reactant ions, 

• ion-molecule reactions for the formation 

of product ions, 

• recombination of positive and negative 

ions, 

• diffusion of ions towards the sides of 

the measuring tube and 

• transport of ions from the reaction 

chamber due to drift gas and electric 

field [103]. 

The formation of reactant ions depends 

on the activity of the ß-ionization source 

(C A 

) and the radiation energy (W). 

Assuming that one ion pair is produced 

per 35 eV of primary energy each, 1 x 10 6 

C A W of ion pairs are produced per sec 

and unit volume. If no sample molecules 

are available in the source, the quantity of 

reactant ions in the ion source results 

from recombination reactions and the


Table 1: 

Dependence of the distribution of halothane product 

ions on the concentration, determined by IMS/MS 

investigations at a temperature of 40 °C [102] 

relative occurrence 

product ions 10 ppb 100 ppb 500 ppb 

Cl - 

31 

(H 2O) Cl - 41 

(H 2O) 2 Cl - 55 

Br - 100 46 15 

- 

M O 2 24 

M Cl - 100 50 

M Br - 50 100 

discharge of particles due to diffusion and the 

drift gas. Thus, in case of usual drift gas flows 

and 63 Ni activities between 10 -5 to 10 3 mCi, the 

quantity of ions is between 10 4 and 10 11 

ions/cm 3 . 

The formation of product ions may be 

described by simple rate laws assuming that 

the total number of charged particles remains 

constant. For a simple charge transfer reaction 

(analogous to equation 13) Equ. 19-21 can be 

derived, with n R standing for the number of 

reactant ions, n p for the number of product ions 

and n 0 the number of sample molecules 

available for the formation of product ions, t for 

the time and K the rate constant of the 

bimolecular reaction. 

The detection limits reached using ion mobility 

spectrometers with ß-ionization sources are 

mostly in the ppb-range. They stronly depend 

on the substances and vary with the 

Figure 7: 

Intensity of reactant and product ions in 

dependence on sample concentration for a charge 

transfer reaction 


composition of the drift gas, with its water 

content having a predominant influence. 

1.2.1.4 Ionization of Mixtures of Analytes - 

Use of Reactant Gases 

On the assumption that a stationary equilibrium 

has established in the reaction chamber the 

ionization of substance mixtures may be 

regarded as a result of competing reactions 

strongly influenced by the proton and electron 

affinity of the sample molecules. Compounds 

with a high proton or electron affinity are 

preferably ionized. The signal intensity of 

product ions depends on the product of proton 

or electron affinity and concentration. Thus, for 

a substance mixture A, B, C the correlation 

shown in equation 22 using the proton affinity 

PA follows. 

dn P 

/dt = -(dn R 

/dt) = K n 0 

n R 

(19) 

n R 

= n R 

° (e -Knt ) (20) 

n P 

= n R 

° (1 - e Knt ) (21) 

ΣReactant-Ionen≈(PA A 

[C A 

] + PA B 

[C B 

] +PA C 

[C C 

]) 

(22) 

In addition, in the case of substance mixtures 

also reactions among the sample molecules 

themselves (e.g. the formation of "mixed" 

dimeres according to Equ. 23) may be 

observed. The consequence for the detection 

of single substances is a strong matrix 

dependance. 

[(AB)(H 2 

O) 2 

H] + + CD → [(AB)(CD)H] + + 2 H 2 

O 

(23) 

Due to the influence of the proton or electron 

affinity on the APCI processes the selectivity 

[105] may be increased by selecting suitable 

reactant gases, as illustrated in Fig. 8 by 

means of a hypothetical substance mixture. 

Acetone [106-109] or chlorinated hydrocarbons 

[105,110,111] are reactant gases that are 

frequently used e. g. for the detection of both 

chemical warfare agents and explosives. 

Whereas in the case of acetone proton bridged 

dimeres react as reactant ions, chlorid ions are 

formed by dissociative charge transfer 

reactions from chlorinated hydrocarbons (Equ. 

16). Dopant gases can also be used to prevent 

a potential peak interference of reactant ions 

and product ions as observed e. g. for the


Figure 8: 

Ionisation of substance mixtures in dependence on the 

proton affinity of the reactant gas (acord. to G.A Eiceman 

and Z. Karpas, Ion Mobility Spectrometry, CRC Press, 

Boca Raton, 1994, p. 48) 

detection of hydrazine and 

monomethylhydrazine, if dibutyl ketone is used 

as dopant gas [112]. 

1.2.2 UV-Ionization 

Photoionization is a method that has been 

specially introduced for the detection of 

unsaturated or aromatic hydrocarbons 

[113-116]. However, using ultraviolet lamps 

with different radiation energies a broad 

spectrum of organic compounds can be 

ionized. Usually cylindrical gas-filled cathode 

lamps containing xenon (8.5 - 9.6 eV), krypton 

(10.0 eV), hydrogen (10.2 eV), argon 

(7.2 - 11.6 eV) and helium (11.3 - 20.3 

eV) are used. The ultraviolet lamps 

may be fitted vertically [82] or axially 

to the drift tube [117-120]. In particular 

in case of axial arrangement a 

possible ionization of the sample 

molecules behind the shutter grid is 

disadvantageous. The formation of 

photoelectrons can be utilized for the 

detection of negative product ions. 

Equ. 24 describs the ionization 

process. The desactivation reactions 

summarized in Equ. 24-27 have to be 

considered as well due to their 

influence on the ion yield. 

In case of substance mixtures charge 

transfer reactions have to be taken 

into account, i. e. compounds with a 

lower ionization potential (IP) are 

preferred during ionization. Fig. 9 

illustrates this fact by means of a 

series of IMS spectra, in the course of which 

benzene, toluene and p-xylene were sampled 

AB + hν → AB* → AB + + e - (24) 

AB* → A + B (25) 

AB*+ C → AB + C (26) 

AB + + e - + C → AB + C (27) 

additionally[121]. 

The range of detectable substances may be 

enlarged by using reactant gases. Thus, using 

reactant gases such as acetone the detection 

of phosphonates gets possible by 

means of UV-IMS [120,121]. 

In comparison with ß ionization, 

UV-ionization has the advantage 

that quantitative analyses can be 

carried out in a large concentration 

range, since linearity is not limited 

by the restricted number of reactant 

ions. Moreover, the total drift time 

range may be used due to absence 

of reactant ions. 

Figure 9: 

Sequence of ion mobility spectra of positive ions 20 ppmv benzene 

(IP=9,2eV), 20 ppmv benzene + 20 ppmv toluene (IP=8,8eV) and 

20 ppmv benzene + 20 ppmv toluene + 17 ppmv p-xylen 

(IP=8,5eV). With the concentrations used just the signal of the 

compounds with the lowest ionization potential appears. 

1.2.3 Laser Ionization 

The use of laser ionization for IMS 

is mainly described in the papers of 

Lubman et al. [83,84]. Both 

Nd:YAG as well as ArF-Excimer 

lasers were used. The dependence 

of IMS spectra on the radiation 

energy and the cross section was 

investigated [122]. Possibilities for 



selective ionization using lasers 

with different wave lengths were 

demonstrated [123]. 

Applications of the laser 

ionization on surfaces [124,125] 

and the Matrix-Assisted Laser 

Desorption Ionisation (MALDI) 

are also known [126]. 

1.2.4 Corona discharge 

Although well-known from mass 

spectrometry [127], corona 

discharge sources have only 

been described in a few papers 

[85,120,128,129] in connection 

with ion mobility spectrometers. 

A major advantage of corona 

discharge sources is that a 

higher yield of reactant ions can 

be gained. The processes 

running in the sources are nearly identical with 

the known APCI processes [130]. However, in 

comparison to usually used 

63 

Ni-radiation 

sources fragmentation reactions may be 

observed depending on the design of the 

corona discharge source (see Fig. 10). 

1.3 Ion Separation and Ion Mobility 

Ions formed in the reaction chamber move in 

the electric field against the drift gas towards 

the detector. According to Equ. 1 the velocity of 

the ions on their way from the shutter grid 

towards the detector is proportional to the 

electrical field strength and the ion mobility 

[131-134]. As shown by Equ. 28, K or the 

reduced mobility constant K 0 contain properties 

of the formed ions and of the used drift gas: 

In equation 28 q describes the number of 

charges ze with e = 1,602 . 10 -19 C, N stands 

for the density of the drift gas (molecules /cm 3 ), 

k is the Boltzmann constant (1,381 . 10 23 J/K) 

and µ is the reduced mass of an ion - drift gas 

molecule (atom) pair as described in Equ. 29 

with M the mass of a drift gas molecule or atom 

K = 3/16 q/N (2π/µkT) 1/2 (1 + α)/Ω D . (28) 

and m the ion mass. 

µ = m M / (m + M) (29) 

α in Equ. 28 is a correction value, which is 

under 0,02 if m > M [52]. T indicates the 

temperature of the drift tube in °K. Ω D is the 

collision cross section of ion and drift gas 

molecules. According to equation 28, ion 


Figure 10: 

Corona discharge ion mobility spectrum of positive ions of 

pinacolyl-methyl-phosponate containing numerous fragment peaks. 

The ion mobility spectrum obtained by means of ß-ionization ( 63 Ni) 

contains monomere and dimere product ions only. 

mobility depends on the temperature, the 

charge, the reduced mass and the collision 

cross-section. Ω D is influenced by the ion or 

molecule size, their structure and polarizibility. 

If the same drift gas is used, mobility is largely 

controlled by the reduced mass. In case of very 

large ions µ is in the range of M. In this case 

mobility largely depends on Ω D and, thus, on 

ion structure. In the middle range, that means 

for the most IMS relevant compounds, a 

dependence of ion mobility on mass and 

structure is observed. 

The influence of the structure on ion mobility 

has been investigated in numerous studies on 

isomeric compounds [24, 28,135-137]. Karpas 

found the following classification of the 

influence of structure on mobility for substituted 

hydrocarbons [138]: 

• linear < branched 

• primary < secundary < tertiary 

• aliphatic compounds < aromatic compounds 

• amines < amides. 

Due to the major influence of the structure 

correlations between ion mobility and ion mass 

reveal errors up to 20%, if compounds 

belonging to different substance classes are 

considered. Ion mobility-ion-mass functions for 

homologous series of compounds allow the 

determination of the ion mass with errors < 5%. 

Fig. 11 illustrates the mass-mobility-functions 

for selected classes of substances [48,138]. 

The temperature dependence of K and Ω D is 

given with T -1/2 and T -2 . Thus, K 0 is to a great 

extent independent on the temperature of the 

measuring tube [133,139,140]. Due to the


Ion 

mass 

[amu] 

400 

Phosphor organic 

compounds 

The resolution of ion mobility spectrometers is 

usually determined according to Equ. 30 

[143]: 

R = t d / 2 t 1/2 (30) 

300 

200 

Oxygen containing 

compounds 

lg m = -0,452K+2,951 

0 

lg m = -0,524K+3,123 

0 

Aromatic 

hydrocarbons 

lg m = -0,382K+2,870 

0 

100 

1,0 1,5 2,0 

2 

K 0 [cm/Vs] 

Figure 11: 

Ion-mass-ion mobility functions for selected 

compound classes 

standardization of K regarding pressure and 

temperature (Equ. 2) the temperature 

dependence of the density N of the drift gas on 

ion mobility is compensated. 

As systematic investigations with nitrogen, air, 

argon, argon/methane mixtures and carbon 

dioxide have proved [84], the influence of the 

drift gas on ion mobility is essentially 

determined by the polarity of the drift gas 

molecules. The considerably lower ion 

mobilities in carbon dioxide may be explained 

by an enlarged cluster formation. 

With the same drift gas is used the measuring 

results of different spectrometers should be 

comparable since ion mobility is standardized 

as regards temperature and pressure. 

However, small deviations in the experimental 

conditions, e. g. changes of the humidity of the 

drift or sampling gas, may also cause 

differences in measured K 0 values. Thus, 

K 0-values (e. g. [141]) published in literature are 

not applicable in each case. Using different 

calculating methods [56, 142] the calculated ion 

mobility constants tally to a high degree with 

the values determined by experiments. 

In Equ. 30 t d descibes the drift time of the 

peak and t 1/2 the peak width. Thus, the 

resolution strongly depends on the peak form, 

which is influenced by numerous factors. This 

includes e. g. the grid opening time, peak 

broadening by diffusion or Coulomb 

interactions and ion molecule reactions in the 

drift tube [144]. The resolution may be also 

affected [52] by field and temperature 

gradients or pressure fluctuations caused by 

the pumps. 

1.4. Coupling techniques 

1.4.1 Coupling using chromatographic 

methods 

Due to its excellent detection performance 

IMS is predestined for coupling with 

chromatographic methods [145]. Beside the 

registration of the total ion current |I reactant ions - 

I product ions| selective detection is possible by 

acquisition of complete ion mobility spectra or 

by recording selected K 0 windows. 

Already in 1972 an interface for GC/IMS 

coupling was introduced by Karasek and Keller 

[14]. At first packed columns were used as e. g. 

reported for the separation of mono-, di- and 

trichlorotoluene [29]. Problems arised from 

column leakage [146] and from the long dwell 

times of molecules in the reaction chamber 

resulting in additional ion molecule reactions. 

Improvements were gained by using capillaries, 

by reducing the reaction chamber, and by using 

an unidirectional gas flow [76]. The influence of 

the drift gas composition, e. g. the oxygen 

content, was studied on the detection of 

halogenated organics [147]. Selectivity of 

GC/IMS has 

been proved by detection of 

2,4-dichlorophenylacetic acid in soil samles 

[148]. Due to the use of ceramics as insulating 

material it became possible to operate the IMS 

measuring tube even at temperatures of 300°C 

[149]. A further progress was made by using an 

axial direct coupling (s. Fig. 12) [150]. 

In particular in the past few years miniaturized 

GC/IMS couplings [152-154] with optimized 

reaction chambers have been developed [155]. 

The use of multi-capillary columns e. g. for the 



electrical lenses [103]. The design is similar to 

that known from mass spectrometers using 

APCI ion sources (APCI = Atmospheric 

Pressure Chemical Ionization) [166]. 

IMS/MS experiments have been mainly used 

for investigating ionization processes running 

in ion mobility-spectrometers. However, from 

this point of view it always has to be 

considered that modifications in the 

composition and structure of ions may occur 

due to the transfer into the vacuum system 

[91]. 

Different scan-techniques can be used in 

IMS/MS experiments [56]: 

Figure 12: 

GC/IMS coupling techniques; lateral (a) and 

axial coupling (b), providing at the same 

time the possibility to feed a make-up gas 

for better eluate swirling [151] 

detection of explosives has also been 

described [156-157]. An extension of 

applications to nonvolatile materials can be 

achieved using the Pyrolysis/GC/IMS coupling. 

The influence of the mobile phase on the 

resulting drift gas composition and, thus, on the 

formation of product ions has to be considered 

when coupling IMS with SFC (Super Fluid 

Chromatography) [158]. This effect can be 

suppressed by using measuring tubes with an 

unidirectional gas flow [159-162]. 

Couplings between ion mobility spectrometers 

using ß-radiation sources and fluid 

chromatographs were implemented at first in 

analogy to HPLC/MS coupling by means of a 

moving-belt Interface [21]. A “direct” coupling is 

possible by using Corona Discharge or Elektro 

Spray Ion sources [163,164,57]. 

1.4.2 IMS-MS Coupling 

Coupling of IMS with mass spectrometry has 

already been described in the first papers on 

“Plasmachromatography” [10]. For coupling 

usually measuring tubes are used that are 

provided with a “pinhole” in the centre of the 

detector. The interface between drift tube and 

mass spectrometers consists of a system of 

1. Acquisition of IMS spectra, recording is 

also possible by means of the MS detector. 

2. In case of a permanently opened shutter 

grid the IMS just ac as ion source for the 

MS. APCI mass spectra are recorded if 

radiation sources are used. 

3. Mass analysis of IMS peaks (see e. g. Fig. 

6 and 8). 

4. If the quadrupole is set to a certain mass 

and the IMS operated in the usual manner, 

ions of the selected mass are displayed in 

the ion mobility spectrum. 

Further results regarding the composition and 

structure of ions formed in the IMS can be 

obtained by IMS/MS/MS experiments [166]. 

2 Applications 

In the past few years a large number of IMS 

applications have been reported. This includes 

the detection of chemical warfare agents, 

drugs, explosives, environmental pollutants as 

well as applications in the field of working place 

monitoring. Nevertheless, IMS is no universal 

analysis method. The complex and partly 

incalculable ionization mechanisms mainly with 

substance mixtures always require a thorough 

evaluation of the analysis problem. 

Laboratory instruments, stationary devices for 

on-site measurements, which are programmed 

for selected compounds, or handheld 

instruments are generally available for 

corresponding applications. Typical on-site 

applications of IMS are described in the 

following chapters. 

2.1 Chemical warefare agents 

At present the detection of chemical warefare 

agents is the most important and most common 



Figure 13: 

Ion mobility spectrum of sulphur mustard (spectrum of negative ions, 

C = 400 µg/m 3 ) 

application of IMS. However, only a few papers 

have been published on this subject in literature 

[30,45,46,120,167]. 

In addition to phosphororganic compounds, 

namely nerve agents (e. g. tabun, sarin, 

soman, VX), blister agents (e. g. sulphur 

mustard, nitrogen mustard, lewisite), choking 

and blood agents (e. g. phosgene, prussic acid) 

can be detected by means of IMS. Whereas 

nerve agents have a very high proton affinity 

and, thus, form positive ions, blister, blood, and 

choking agents are strongly electrophilic and 

form negative ions. Thus, modern handheld ion 

mobility spectrometers make use of bipolar 

measuring tubes in order to assure a more or 

less continuous detection of all groups of 

warefare agents. The automatically operating 

instruments identify and quantify the warefare 

agents that are stored in appropriate spectra 

libraries. The achieved detection limits are in 

the lower ppbv range. The response times of 

the instruments depend on the concentration, in 

case of concentrations near the detection limits 

they are in the minute’s range. High 

concentrations are displayed after a few 

seconds. 

In addition to military applications, the detection 

of chemical warfare agents is of importance in 

the field of CWA disposal even decades after 

the two world wars. As the events in Japan 

have shown, the use of warefare agents in 

terroristic attacks cannot be excluded. In both 

cases ion mobility spectrometers may be used 

both as alarm units for personal protection and 

for on-site analysis [168]. 

In the field of CWA disposal IMS is also used 

for detection of “old” warfare agents from the 


World War I and II. Till today 

large quantities of 

ammunition have been found 

on training grounds, old 

production sites and battle 

fields. Fig. 13. shows a typical 

ion mobility spectrum of 

sulphur mustard (HD) found 

in leaking bottles on a training 

ground. For ion mobility 

spectra of sulphur mustard 

the formation of product ions 

of the composition HD*O 2 

- 

is 

typical. In addition Cl - ions are 

formed due to a dissociative 

charge-transfer eaction. As a 

result HD*Cl - adducts can be 

formed in case of higher HD 

concentrations as well [169]. Fig. 15 illustrates 

the selectivity of IMS using a ß-ionization 

source to warefare agents on the example of 

IMS and GC/MS investigations made with a 

sample of sarin. Sarin was found in a shell 

originating from the Second World War. The 

composition of the gas phase above the 

sample was determined by means of 

enrichment on XAD followed by thermal 

desorption and GC/MS. The total ion current 

chromatogram is shown in Fig. 14a. More than 

30 compounds were identified, with the major 

compounds listed in Fig. 14a. Fig. 14b shows 

the ion mobility spectrum of the same sample. 

The spectrum indicates just dimeric product 

ions [170] due to the high sarin concentration. 

2.2 Explosives 

The very good detectability of explosives and 

chemical warfare agents by means of IMS has 

already been found in the seventies. Karasek 

and Denney were able to detect 10 ng TNT at a 

signal/noise ratio >1000 [171]. The detection 

limit for ethyleneglycoldinitrate (EGDN) is about 

500 pg. If reactant gases are used which form 

Cl - ions still 30 pg EGDN are detectable [172]. 

However, the extremely low vapour pressure of 

explosives normally requires an enrichment. 

The use of particle collectors in connection with 

a thermal desorption of the enriched explosives 

has proved for this purpose[173]. 

Explosives are identified by peaks which are 

related to [NO 3] - , [M-H] - , [M . Cl] - and [M . NO 3] - 

ions. The formation of dimeric product ions, 

[M 2] - , is also possible. These processes 

strongly depend on temperature [174]. Adducts 

with Cl - ions are caused by chlorine containing


Intens. 

x10 7 

4 

3 

2 

Figure 14: 

Total ion flow chromatogram of a 

sarin sample, obtained by 

enrichment on tenax with 

subsequent thermal desorption 

and GC/MS separation(a, left). 

Ion mobility spectrum of sarin 

(spectrum of positive ions, 

C = 600 µg/m 3 - 

b, below). 

1 

0 

0,0 2,5 5,0 7,5 10,0 12,5 15,0 17,5 Time [min] 

b) 

1: Methylchloride 2: Acetone 3: Propyl chloride 

4: Benzene 5: Toluene 6: Sarin 

7: Chlorobenzene 8: Ethylbenzene 9: o,m,p Xylene 

10: Diisopropylphosphonate 

reactant gases. 

Due to the low detection limits for explosives 

and the short analysis times IMS is a 

interesting method for safety areas. The 

detection of explosives in or on pieces of 

luggage, on clothing or on the skin using IMS 

has been described in literature [176]. 

2.3 Drugs 

During the last years IMS has become an 

important on-site and laboratory method for 

drug detection due to the low detection limits, 

its high selectivity, and short analysis times. 

Also in case of drugs it is necessary to enrich 

the sample on a carrier material (wire, teflon 

and others), followed by a subsequent thermal 

desorption. The compounds are usually 

detected by means of the pseudo moleculer ion 

or specific adduct ions. Karasek et al. [181] 

demonstrated by IMS/MS investigations that in 

the case of heroin [M] + , [M . H 2] + , [M . CH 3CO 2] + 

ions and with cocain [M] + , [M . C 6H 5CO 2] + and 



[M . CH 3CO 2] + ions are formed. Nitrogen was 

used as drift gas. In addition to natural drugs 

such as heroin or cocain, numerous synthetical 

drugs can be detected [177-180]. Table 4 

contains a selection of natural and synthetical 

drugs that have been investigated by means of 

IMS. A survey of forensic IMS applications was 

published by Karpas [50]. 

Numerous tests have proved that the detection 

of drugs using IMS is very reliable and that 

there are only a few interferents. Thus, 

detection is possible without further sample 

preparation e. g. in pieces of luggage, 

containers, letters [183]. 

2.4 Environmental relevant compounds – 

Working place monitoring 

Due to the complexity of ionization 

mechanisms, applications of IMS in particular in 

the field of environmental analyses and working 

place monitoring require a careful evaluation of 

the analytical problem. Above all, the influence 

of matrix on peak selection and calibration has 

to be considered. In case of complex substance 

mixtures a coupling method should be generally 

preferred. Table 4 contains a survey of 

compounds or classes of compounds which 

can be detected by means of IMS using 

ß-ionization in the range between 5 ppb and 1 

ppm. In the following some applications will be 

given as examples. 

Monitoring of 2,4- and 2,6-toluendiisocyanate 

[184] is a typical application of IMS in industrial 

facilities. Both isomers are reported to have 

detection limits of about 5 ppb v. The linear 

measuring range is between 0 and 50 ppb v, the 

response time runs to a few seconds. The 

detection of toluendiisocyanate is interfered by 

amines (concentrations in the ppm range) and 

halogenated compounds in higher 

concentrations. Fig. 15 shows a typical ion 

mobility spectrum. 

Meanwhile the detection of hydrogen flouride 

by means of IMS has been widely used [185]. 

As reactant gas methylsalicylate is used to 

avoid an overlapping of peaks of HF ions and 

[(H 2O)xO 2 ] - reactant ions. The detection limit is 

approx. 0,5 ppm v, with the linear range running 

to 10 ppm v. Interferences regarding NO x, HCl, 

chlorine or H 2S are low. 

Karpas et al. described a method for the 

detection of bromine [186]. The detection limit 

reached is 10 ppb v. Considering the equilibrium 

between Br- and Br 3- ions, which is dependant 

on concentration, a linear range between 10 

and 500 ppb v is reached. 

For the detection of diamide and monomethylhydrazine 

[187] the detection limits are reported 

to be < 10 ppb. The linear measuring range is 

between 10 ppb and 1 ppm. Using 5-nonanone 

Table 2: 

Survey on selected explosives, their vapour pressure at room temperature and the K0-values of the 

product ions used for identification. The values given in brackets are detection limits reached by using the 

corresponding product ion [175,176] 

Compound 

Letter 

IMS-Peaks, K 0(Detection limit /pg) 

Vapour pressure a) 

Symbol 

[M-H] - 

[M.Cl] - 

[M.NO 3] - 

Nitroglycerine 

NG 

1,45 

1,34 (50) 

1,28 (200) 

2,8 [mg/m 3 ] 

Dinitrotoluene 

DNT 

1,57 (200) 

- 

1,0 [mg/m 3 ] 

Trinitrotoluene 

TNT 

1,45 (200) 

- 

- 

56 [µg/m 3 ] 

Cyclotrimethylen-trinitr 

amine 

RDX 

- 

1,39 (200) 

1,32 (800) 

0,056[µg/m 3 ] 

Pentaerythrittetranitrate 

PETN 

1,21 (80) 

1,15 (200) 

1,10 (1000) 

0,090[µg/m 3 ] 

Ammonium nitrate 

- 

1,93 (200) a 

33 [µg/m 3 ] 

a) 

Detection as [NO 3] - 



Figure 15: 

Ion mobility spectrum of negative ions of 2,4-toluendiisocyanate, C = 20 ppb v 

Table 3: 

Detection of selected drugs using IMS 

Compound 

Acetylcodeine 

Molecular 

weight 

341 

K 0 [cm 2 /Vs] of 

important ions 

1,09; 1,21 

Carrier 

gas 

Air 

T [°C] 

220 

Method 

therm. Des. 

Literature 

178 

Amphetamine 

135 

1,66 

Air 

220 

therm. Des. 

178 

Barbital 

184 

0,99; 1,50 

N 2 

230 

GC/IMS 

177 

Bromazepam 

316 

1,24 

220 

therm. Des. 

180 

Cannabinol 

310 

1,06 

Air 

220 

therm. Des. 

180 

Chlordiazepoxid 

300 

1,18 

Air 

220 

therm. Des. 

182 

Cocain 

Codein 

303 

299 

1,16; 1,50; 1,84 

1,18; 1,21 

N 2, air 

Air 

153, 

220 

220 

therm. Des. 

therm. Des. 

181,182, 

178,179 

Diazepam 

Heroin 

Morphine 

285 

369 

285 

1,21 

1,05; 1,15 

1,04; 1,14 

1,22; 1,26 

Air 

N 2 

N 2, air 

Air 

220 

153 

220 

220 

therm. Des. 

therm. Des. 

therm. Des. 

therm. Des. 

178,179,18 

0 

181 

178,182 

178 

Opium 

1,55 

Air 

250 

direct, wire 

50 

Oxazepam 

287 

1,23; 1,28 

Air 

220 

therm. Des. 

180 

Triazolam 

343 

1,13 

Air 

220 

therm. Des. 

178 



Table 4: 

Survey of compounds detected by means of IMS using ß-ionization in the field 

of environmental analyses [54,56,112] 

Compound 

Chlorine 

Bromine 

Iodine 

Hydrogen fluoride 

Hydrogen chloride 

Hydrogen iodide 

Prussic acid 

Phosgen 

Sulphour dioxide 

Nitrogen dioxide 

Nitric acid 

Ammonia 

Hydrazine 

Hydrogen sulphide 

Detection limit 

[ppb v] 

100 

100 

5 

100 

100 

100 

100 

100 

100 

100 

100 

100 

10 

1000 

Classes of compounds 

Alcoholes 

Aliphatic amines 

Aromatic amines 

Ethers 

Esters 

Ketones 

Phenoles 

Chlorinated aromatic 

compounds 

Polychlorinated biphenyles 

Carbon tetrachloride 

Dry etching gases 

Detection limit 

[ppb v] 

100 

5 

5 

100 

10 

10 

100 

100 

100 

500 

100 

Acetonitrile 

Acetaldehyde 

Aniline 

Cyclohexanone 

Nitrobenzene 

Toluendiisocyanate 

Vinylacetate 

10 

100 

5 

10 

5 5 

100 

Figure 16: 

(a) Total ion chromatogram (TIC) of a benzene fraction (Kp. 65-80 °C) with 20 ppm v of highly volatile 

chlorinated hydrocarbons (dichloromethane, chloroform and 1,2 dichloroethane), 

(b) IMS spectrum of the gaseous phase acquired above the sample. 



as reactant gas a separation of reactant and 

product ion signals is achieved [112]. 

Another advantage of IMS is illustrated in Fig. 

16, which shows an ion mobility spectrum of a 

benzene fraction (bp. 65-80 °C) containing 20 

ppm v of highly volatile chlorinated 

hydrocarbons. The chlorinated hydrocarbons 

form a Cl- peak with K 0 = 2,78 cm 2 /Vs due to a 

dissociative charge transfer reaction in the IMS. 

Thus, this signal may be used for detection of 

the total amount of volatile chlorinated aliphatic 

compounds [188]. Since the detection limits for 

chlorinated hydrocarbons strongly depend on 

the compound [189], the standard mixtures 

required for calibration have to be adapted to 

the analytical problem. The respective matrix 

has to be considered also. 

The determination of certain classes of 

substances is only possible if the respective 

compounds, e. g. by means of a dissociative 

charge transfer reaction, form the same type of 

ions. Typical examples are chlorinated, 

brominated and iodized hydrocarbons or some 

cyano compounds. 

Further applications are known from the 

semiconductor industry (detection of etch 

gases [190]) and from electronics (detection of 

de-gassing from electronic equipment [191]). 

Investigations of the use of IMS for 

determination of narcotisvon have also been 

made [102]. The headspace analysis of soil 

samples for benzine by means of IMS using UV 

ionization was described by Eiceman et al. 

[192]. 

3 Summary 

Approximately since 1970 ion mobility 

spectrometry has undergone a continuous 

development. The continuous growth in 

publications may be evidence for it. Efficient 

laboratory instruments and rugged, 

miniaturized and automated devices for on-site 

analysis are available. As far as the latter are 

concerned both stationary devices for building 

and plant monitoring and hand-held 

spectrometers can be used. Applications from 

many sectors of the analytical chemistry are 

known. 

Device components have been optimized 

according to the respective applications. IMS 

systems to be used for fast on-site analysis 

have a membrane inlet system. Portable or 

laboratory devices are fitted with a shutter grid 

system for sampling. Sample enrichment 

followed by a thermal desorption is typical, for 


example, for the detection of drugs or 

explosives. 

Ionization is mainly achieved by using 

ß-radiation sources. They do not need energy, 

which is especially important for hand-held 

spectrometers, and they are stable over many 

years. Ionization of compounds is steered by 

their proton and electron affinities. A possible 

influence of the matrix may be bypassed by 

using a reactant gas. The detection 

performance of the instruments allows most of 

the compounds to be determined. Chemical 

warfare agents, explosives or drugs can be 

detected in the lower ppb-range. The linear 

range for the detection of many compounds is 

very limited. There are alternative ionization 

techniques available such as UV or corona 

discharge ionization. UV ionization depends on 

the ionization potential of the substances to be 

analyzed and on the energy of the radiation 

released by the lamp. The linear range for the 

detection of the compounds is not limited. 

Using reactant gases APCI processes can be 

initialized resulting in the ionization of 

compounds which, normally, cannot be 

recorded by means of UV ionization. By using 

corona discharge ionization a higher ion output 

can be gained. However, fragmentation 

reactions have an unfavorable influence. 

The formed ions are separated in bipolar 

measuring tubes, i. e. positive and negative 

ions are detected by voltage reversal. Also 

known are measuring tubes which, proceeding 

from a common reaction chamber, have two 

drift tubes and, thus, are able to simultaneously 

separate positive and negative ions. 

Separation of formed ions in the drift tube of 

the measuring chambers depends on their 

mass and structure. In comparison to gas 

chromatography or mass spectrometry IMS has 

a low resolution capacity of IMS is low. This 

may be compensated for, as e. g. in the case of 

matrix influences, by selecting a suitable 

ionization technique. The detection of complex 

mixtures requires coupling techniques made 

available by GC/IMS and IMS/MS. 

IMS covers manifold fields of applications. 

Major detectable substances are chemical 

warfare agents, drugs, explosives or some 

ecologically relevant compounds that can be 

efficiently ionized using ß-radiation sources. In 

most cases mobile IMS are used for these 

applications.


Literature 

[1] W.C. Röntgen, Science 3 (1886) 726. 

[2] J.S. Townsend, Philos. Trans. R. Soc. London 

A193 (1899) 129. 

[3] J.J. Thomson, G.P. Rutherford, "Conduction of 

Electricity Through Gases",Dover, New York, 

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NITRIC OXIDE AS A REAGENT GAS IN 

ION MOBILITY SPECTROMETRY 

G. A. Eiceman, K. Kelly, and E.G. Nazarov 

Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, USA 

ABSTRACT 

Nitric oxide was introduced into the ion source 

of an ion mobility spectrometer in air at ambient 

pressure so conventional hydrated proton 

(H 3O + (H 2O) n) ion chemistry could be replaced 

with ionization reactions based on charge 

exchange, hydride abstraction, and adduct 

formation. The addition of NO(g) from 1 to 60 

mg/m 3 at temperatures from 125 to 250°C 

resulted in progressive replacement of 

H 3O + (H 2O) n with NO + (H 2O) n reaching a plateau 

of ~0.9 for [NO + (H 2O) n]/[NO + (H 2O) n + 

H 3O + (H 2O) n]. Mobility spectra for three 

chemical substances were obtained through a 

range of temperatures from 100 to 250°C at 

50°C increments with 5 to 10 values for [NO] at 

each temperature. Ions were mass-identified 

using a mobility spectrometer/mass 

spectrometer. The chemicals were 2,4-lutidine, 

di-tert-butyl-pyridine (DTBP), and 

dimethylmethylphosphonate (DMMP). The 

core product ion for 2,4-lutidine with a reagent 

ion of H 3O + (H 2O) n was mass identified as MH + 

while the product ion with NO(H 2O) + was an 

adduct ion M*NO + . The adduct ion of 

2,4-lutidine was thermally unstable above 

150°C and underwent dissociation to M and 

NO + . The mobility spectrum for DMMP was 

MH + with hydrated proton reactant ions and 

chemistry was unaltered with the addition of 

NO(g) as reagent gas. The product ion for 

DTBP was MH + with H 3O + (H 2O) n and addition of 

NO(g) resulted in formation of fragment ions 

(M-CH 3) + , (M-(t-butyl)) + and (M-(t-butyl))H + (NO). 

These results demonstrate that NO(g) can 

serve as a reagent gas for the formation of 

product ions through reactions other than 

proton transfer using a conventional 

beta-source. At temperatures below 125 o C, the 

hydrated proton could not be replaced even 

partially with NO(H 2O) + though the level of 

NO(g) reached 140 mg/m 3 . 

INTRODUCTION 

In ion mobility spectrometry (IMS), hydrated 

protons H 3O + (H 2O) n have been used commonly 

as the reservoir of charge for atmospheric 

pressure chemical ionization (APCI) reactions 

(1). The reactions between analyte and the 

hydrated protons or reactant ions leads to 

product ions at ambient pressure through what 

might be described best as a displacement 

reaction as shown in Equation 1: 

In addition to the hydrated proton, other 

reactant ions can be observed at minor 

intensities in a clean IMS drift tube and these 

include NH 4+ (H 2O) n, and NO + (H 2O). The level 

of hydration, i.e. value for n, is dependent upon 

temperature and moisture. The ratios of 

intensities for these reactant ions in scrubbed 

air or nitrogen at temperatures of 150+ o C are 

roughly 1:1:10 for NH 4+ (H 2O) n, and NO + (H 2O), 

M 

+ 

H 3O + (H 2O) n 

MH + (H 2O) n-1 

+ 

H 2O 

(1) 

sample 

neutral 

reactant ion 

product ion 

water 

neutral 

M 

+ 

MH + (H 2O) n-1 

M 2H + (H 2O) n-1) 

+ 

H 2O 

(2) 

sample 

neutral 

protonated 

monomer 

proton bound dimer 

water 

neutral 

Received for review April 30, 2002, Accepted July 15, 2002 


Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 23 

and H 3O + (H 2O) n, respectively (2). The 

advantages of APCI reactions include both high 

selectivity and detection limits of µg/m 3 or 

lower. Also, ions are near-thermal energies in 

APCI reactions so mobility spectra are 

comparatively simple and are comprised largely 

of the intact protonated sample. In instances 

when the sample vapor levels are large, proton 

bound dimers (M 2H + (H 2O) n-1) can form as 

shown in Equation 2. 

However, the simplicity of APCI reactions and 

the principles that underlie selectivity can 

present disadvantages for chemical 

measurements. For example, protonated 

monomers or proton bound dimers provide little 

structural information about a sample. Also, 

substances that exhibit low proton affinities 

may not be ionized or detected in the presence 

of matrices or interferences that have elevated 

proton affinities. In short, reactions through 

proton transfers in APCI can be considered 

somewhat inflexible for user-tailored response 

in analytical measurements. One means to 

impart some control over the selectivity of APCI 

reactions is to use alternate reagent gases (3). 

This is generally accomplished by adding a 

reagent gas into the ion source so the 

conventional reactant ions can be replaced by 

alternate reactant ions. The most common 

approach has been to employ reagent gases 

that have relatively high proton affinities and 

these gases are added to the ionization region 

at concentration of ~500 µg/m 3 . Alternate 

reactant ions may exclude from ion formation 

those molecules with proton affinities below 

that of the reagent gas for comparable 

concentrations. Thus, substances that are not 

ionized should not appear as interferences (4). 

Apart from the hydrated protons, the next most 

intense reactant ion, NH 4+ (H 2O) n, can serve as 

an alternate reactant ion with elevated proton 

affinities but unfortunately introduces 

complications largely through the formation of 

cluster ions. 

Another reactant ion available for ion formation 

in IMS is NO + (H 2O) n and the basis for ionization 

with NO + (H 2O) n would be principally charge 

exchange. Thus, patterns of relative response 

to a sample using source chemistry with 

NO + (H 2O) n might be changed from that 

observed with hydrated proton reagent ions. 

Studies in chemical ionization mass 

spectrometry (CI-MS) have shown that small 

amounts of NO(g) in the ion source can result 

in the nitryl ion [NO + ] as the predominant 

reactant ion and that the chemistry of the nitryl 

ion was substantially different from the 

hydrated proton clusters. Though such 

reactions were studied at pressures of 1-10 

torr, the findings have relevance for IMS 

studies at ambient pressure. The three reaction 

paths identified through CI-MS studies included 

hydride abstraction, charge exchange, and 

adduct formation as shown in Equations 3 to 5, 

respectively: 

These reactions are dependent upon the 

properties of the sample (M) and some patterns 

in selection of pathway for ionization can be 

discerned. For example, substances with high 

ionization energies (>9.3 eV) will not undergo 

charge exchange (Equation 4) and instead may 

react via Equations 3 and 5. Hydride 

abstraction is seen with alkanes and with 

molecules containing heteroatoms or 

aromaticity. Molecules with high electron 

density can be expected to form adduct ions. 

Little if any fragmentation has been observed in 

CI-MS studies when NO was used as a reagent 

gas (5-10) except in isolated instances such as 

with highly branched molecules (11). Because 

of the variety of reactions possible, it was even 

suggested that nitric oxide will ionize most 

compounds regardless of the ionization 

potential (12). Thus, possibilities exist for 

M + 

NO + 

(M-H) + 

+ 

HNO 

(3) 

sample neutral 

reactant ion 

hydride abstracted 

product ion 

neutral 

M + 

NO + 

M + 

+ 

NO 

(4) 


reactant ion 

product ion 

M 

+ 

NO + 

(M*NO) + 

(5) 


reactant ion 

adduct ion 



addition control in selectivity of response and 

this motivated a study of NO + as a reactant ion 

for IMS. A potential complication to this 

concept (not seen at low pressure) is the 

formation of ion clusters with water or other 

small neutrals at ambient pressure in IMS and 

APCI-mass spectrometry. 

The chemistry of NO + (H 2O) n has been 

described previously in IMS measurements for 

only a single chemical (13) and the main 

product ion was an adduct per Equation 5. 

Nonetheless, several environmentally important 

chemicals might be detected preferentially 

through charge exchange rather than proton 

transfer reactions. Such chemicals include 

benzene and substituted benzenes. If NO(g) is 

a suitable reagent gas, drift tubes containing 

conventional ion sources (i.e., 10 mCi of 63-Ni) 

might be easily adapted to selective detection 

of benzene, toluene and xylenes. A second 

interest is to determine if NO-based ionization 

chemistry can be used to resolve or clarify 

response to mixtures when the components 

may under ionization through different and 

distinctive paths. For example, the product 

ions for two substances with the same drift time 

from a traditional proton-based ionization 

sources, may differentiated by formation of 

cluster ions, proton abstracted ions or M + ions 

simultaneously. 

In this present study, the mobility spectra for 

several well-known chemicals in IMS were 

studied using hydrated proton and NO + with 

mass-identification of ions using IMS/MS. 

Mobility spectra were obtained through a range 

of concentrations of NO(g) and concentrations 

of samples. Also, the temperatures needed to 

obtain reagent ions from NO(g) were 

determined. 

Table 1. The drift tube temperature was set 

using an model 6100 controller (Omega 

Engineering, Inc., Stamford, CT) and a model 

MIS-2 meter (Panametric, Inc., Waltham, MA) 

was used to monitor both temperature (via an 

Omega series CCT-23 0/400C transducer) and 

moisture (Panametric probe). Moisture was 

kept between 5-10 µg/m 3 throughout the 

experiments through the use of in-house 

supplied air treated using an Addco model 737 

Pure Air Generator (Clearwater, FL), several 

scrubbing towers containing 5Å molecular 

sieve, and a commercial drier (R&D 

Separations GC-2 Disposable Moisture Getter, 

Alltech Associates, IL) and monitored using a 

Panametric (Waltham, MA) “Moisture Image 

Series 2” hygrometer probe. 

The mass spectrometer was a model API-III 

tandem mass spectrometer (PE-SCIEX, 

Toronto, Canada) and the drift tube was a 

conventional discrete ring design with the 

following specific features: insulating rings of 

high-temperature stable Teflon (PTFE); 

press-fit seals on the Teflon rings to provide a 

first level of pneumatic isolation; an aluminum 

shell (drift tube casing) with Teflon seals to 

provide a second level of pneumatic isolation; 

access to electric utilities at an end cap; and 

preheated drift gas. The drift tube was 

equipped with a dual shutter design though 

generally the shutters were full open to gain 

maximum ion intensity in the MS. Ions 

reaching the end of the drift tube were passed 

through an additional distance of 3cm equipped 

with a radiator surface and allowed the body of 

the IMS to be cooled. The end of this length 

was fitted into a Teflon socket placed into the 

high voltage flange of the API-III. 

EXPERIMENTAL SECTION 

Instrumentation 

The GC-IMS was a 

Hewlett-Packard (Avondale, PA) 

model 5880A gas chromatograph 

equipped with a 30 m long, ID 0.25 

mm, df 0.25 µm RTX-50 capillary 

column (Restek Corporation, 

Bellefonte, PA), split/ splitless 

injector, flame ionization detector 

(FID) and ion mobility detector. 

The ion mobility spectrometer was 

designed and built at New Mexico 

State University in Las Cruces, NM 

and specifications are listed in 

2,4-Lutidine 

C 7H 9N 

MM 107.15 

CAS Registry 

Number: 108-47-4 

DMMP 

C 3H 9O 3P 

MM 124.08 

CAS Registry 

Number: 756-79-6 

DTBP 

C 13H 21N 

MM 191.31 

CAS Registry 

Number: 585-48-8 



Chemical and Reagents 

All chemicals were reaction or analytical grade 

and were used as received. The solvent for 

samples was methylene chloride (Fisher 

Scientific, Houston, TX). Standards were 

prepared from three chemicals including 

2,4-lutidine (Sigma Chemical Company, St. 

Louis, MO), 2,6-di-tert-butylpyridine or DTBP 

(Aldrich Chemical Co.), and 

dimethylmethylphosphonate or DMMP (Aldrich 

Chemical Co.). 

Two stock solutions were made for chemicals 

at concentrations of 130 mg/m 3 in methylene 

chloride. These were a mixture of 2,4-lutidine 

and DTBP and a second mixture of DMMP and 

DTBP. These were necessary owing to the 

poor chromatographic resolution between 

2,4-lutidine and DMMP. A 5% mixture of nitric 

oxide in nitrogen (Matheson Gas, 

Montgomeryville, PA) was scrubbed over 5A 

molecular sieves to remove water and was 

introduced directly into the source region of the 

IMS via a 10 cm length of uncoated fused silica 

capillary column. The flow rate of the nitric 

oxide was measured using a microliter scale 

bubblemeter as 5 to 400 µl/min providing a final 

concentration in the drift tube of 1-60 mg/m 3 

with a drift gas flow rate of 300 ml/min. 

Procedures 

Retention times and purity of solutions were 

first determined using the GC-FID. With the ion 

mobility spectrometer as the detector, flow 

rates of nitric oxide were selected on the basis 

of the relative intensity of the NO + peak in the 

mobility spectrum. The initial value for NO + 

was determined and NO(g) was gradually 

increased until the hydrated proton peak was 

barely visible. Under these conditions, the 

system was kept from excessive levels of 

NO(g). The instrument was allowed to stabilize 

between each increment of nitric oxide and all 

TABLE 1: 

Parameters for gas chromatograph-mobility spectrometer 

Gas Chromatograph 

Initial Temperature (/C) 

Initial Time (min) 

Program Rate (/C/min) 

Final Temperature (/C) 

Final Time (min) 

Splitless time (min) 

Split time (min) 

60 

0 

20 

200 

5 

0 

.75 

Mobility Spectrometer 

Length of Drift Region (cm) 

5.2 

Length of Ion Source Region (cm) 

1.0 

Insulator Ring Inner Diameter (cm) 

Insulator Ring Outer Diameter (cm) 

Insulator Ring Thickness (cm) 

Conducting Ring Inner Diameter (cm) 

Conducting Ring Outer Diameter (cm) 

Conducting Ring Thickness (cm) 

Shutter Diameter (mm) 

Collector Diameter (mm) 

Aperture Diameter (mm) 

Detector Aperture Distance (mm) 

1.9 

5.1 

1 

1.9 

4 

0.2 

13 

9.5 

10.3 

~1 

Amount of 63 Ni in Source (mCi) 

Electric Field in Drift Region (V/cm) 

Frequency of shutter pulse (Hz) 

Drift Gas (air) Flow Rate (ml/min) 

Temperature of Drift Gas (°C) 

(preheated) 

Delay in spectra (ms) 

Shutter Pulse width(µs) 

No. of points digitized per spectrum 

Collector-Aperture Voltage (V) 

No. of averages per spectrum 

10 

* 

216 

34 

200 

150-250 

2 

208 

640 

60 

64 

* = The electric field changes slightly with temperature due to thermal expansion of the Teflon. V/cm 

at 250/C = 216, V/cm at 200/C = 217.6, V/cm at 150/C = 219 

Insulating Rings: High Temperature Teflon - Conducting Rings: 303 Stainless Steel 

Shutters and detector were obtained from Graseby Dynamics and used without modification. 



Intensity 

0 2 4 6 8 10 

Figure 1: 

Mobility spectra for reactant ions with different flows of 

NO(g) into the ion source at 250°C. 

other flows and moisture remained constant 

throughout the study. One-microliter injections 

of solutions were made into the chromatograph 

injector by splitless methods. Spectra were 

taken from the time of injection until the last 

chemical eluted. Background spectra were 

taken from the time prior to the first chemical 

elution. The maximum spectra for each eluted 

chemical was found using WASP software and 

interface card (Graseby Dynamics, Watford, 

UK) and spectra was deconvoluted using Peak 

Fit ver 5.0 (SPSS Science, Chicago, IL) 

software to determine peak areas and 

positions. This procedure was used with 

temperatures of 250°C, 200°C, 150°C, 125°C, 

and 100°C for all chemicals in both solutions. 

Mass spectra for the ions were obtained using 

a diffusion vapor generator and a steady level 

of vapor flow into the source region of the drift 

tube. The last ring was floated at 400V DC and 

the interface plate for the MS was 200V DC 

enabling the facile transfer of ions to the mass 

spectrometer. The temperature of the drift tube 

and ionization region was 180°C. Air obtained 

from a zero air generator (Whatman) was used 

as drift gas and carrier gas. The mass spectra 

were all obtained with 500 accumulations due 

to the low ion concentrations and a mass range 

NO(g) 

NO(g) 

NO(g) 

NO + (H 2 O) n 

+ 

+ 

+ 

of 10 and 250 amu. 

H + (H 2 O) n 

NH + 4 (H 2 O) n 10 mg/m 3 

Drift Time (ms) 

H 2O + 

+ 

N 4 

e - 


8.3 mg/m 3 

5.8 mg/m 3 

4.0 mg/m 3 

3.0 mg/m 3 

0 mg/m 3 

RESULTS AND DISCUSSION 

Reactant Ion Peaks and NO Reagent 

Gas 

The mobility spectra for positive polarity 

with clean air, without any NO reagent 

gas, at 250°C showed the expected 

patterns of reactant ions as identified by 

Karasek et al as NH 4+ (H 2O) n, NO + (H 2O) n, 

and H 3O + (H 2O) n where n was calculated 

here to be 2-3 based upon comparisons 

of reduced mobilities as shown in Figure 

1 (bottom spectrum). The intensities of 

NH 4+ (H 2O) n, NO + (H 2O) n, were low here 

compared to those of others and had 

reached these low levels after months of 

flow under only cleaned gas and 

temperatures of 250°C. An addition of 

NO(g) from 1 to 60 mg/m 3 in the 

ionization region of the drift tube caused 

and increase in intensity for the peak for 

NO + (H 2O) n as shown (and labelled) in 

spectra in Figure 1. This response toward 

NO(g) was proportional (bottom to top spectra 

in Figure 1) and the increase in peak height for 

NO + (H 2O) n was accompanied by decreases in 

the peak area for H 3O + (H 2O) n. This can be 

understood as conservation of charge with 

charge transferred or relocated in the 

NO + (H 2O) n peak; however, these results do not 

disclose the pathway(s) for this change. 

Presumably, the initial steps to form NO + (H 2O) n, 

could arise by one of several reactions as 

shown in Equations 6-8: 

Though the exact pathway is not known for the 

formation of NO + (H 2O) n in these studies, the 

practical importance is that the NO(g) could be 

controlled and an alternate reactant ion, 

NO + (H 2O) n, could replace up to 90% of the 

standard reactant ion H 3O + (H 2O) n. 

The yield or efficiency of converting NO(g) into 

NO + was found to be temperature dependent 

as shown in Figure 2 in plots of peak intensity 

for NO + versus NO(g) in the ion source 

atmosphere. At 250°C, the response curve for 

NO + was sharp suggesting a high yield with the 

reaction(s) responsible for ionization of NO(g). 

As the temperature was decreased, the slope 

for plots of peak intensity versus NO(g) also 

NO + 

NO + 

NO + 

+ 

+ 

+ 

H 2O 

2 N 2 

2 e - 

(6) 

(7) 

(8) 

decreased. Initially, this change was gradual;


Intensity of NO + 

T=250°C 

T=225°C 

T=200°C 

T=150°C 

T=175°C 

T=100°C 

0 33 66 100 132 166 

NO(g) (mg/m 3 ) 

T=125°C 

Figure 2: 

Peak intensity for NO + versus NO(g) at temperatures 

between 100 to 250°C. 

however, a dramatic change occurred in the 

region between 125 and 100°C where the 

response curve became nearly flat. In all these 

experiments, total charge in the reactant ions 

was conserved and the peak intensity for 

H 3O + (H 2O) n declined as the peak intensity 

increased for NO + (H 2O) n. At temperatures 

below 125°C, attempts to form NO + (H 2O) n as a 

reagent ion, even with excessive levels of 

reagent gas (140 mg/m 3 ), were unsuccessful 

as shown in Figure 3. This lack of formation of 

NO + (H 2O) n at low temperature may be 

explained using these possibilities: 

1. The precursor ions needed to form NO + 

were not available at low temperature. That 

is, the precursor ions were not formed in 

the source region of the drift tube. 

2. The NO + (H 2O) n may have been formed in 

the source region but underwent 

decomposition in the drift region. At low 

temperatures, residence times in the drift 

region increased so such losses might be 

pronounced at low temperatures. 

The first possibility was explored by adding 

2,4-lutidine (which forms adduct ions) at 100°C 

to the drift tube even though nitric oxide 

reactant ions were not evident. The 

expectation was that even short lived reactant 

ions would have opportunity to react with 

sample vapors. No adduct ions were found 

and this suggested that the ions needed for 

reactions were not being formed. The second 

possibility was explored by extending residence 

times for ions at 125°C (where NO + (H 2O) n was 

observable) by lowering the electric field 

strength on the drift region. Ions of NO + (H 2O) n 

were observed even with comparatively long 

residence times in the drift region discounting 

the second possibility, namely, ion instability 

with increased residence times, was not too 

much of a consideration. Since NO + has been 

formed under a range of temperatures in mass 

spectrometry studies, the absence of NO+ in 

air at ambient pressure below 125°C likely 

occurred through the gas phase ion molecule 

reactions that precede or compete with 

formation of NO + (H 2O) n in the ion source 

region. For example, ion-cluster formation is 

promoted at low temperatures and necessary 

precursors to form NO + in a beta source, i.e., 

+ 

N 4+ , N 2 or H 2O + may have been removed 

through reactions with water at 100°C at a rate 

faster than production of NO + was possible. 

Regardless of the causes for this ion behavior, 

these results offer practical guidelines for 

operating IMS drift tubes with NO reagent gas: 

nitric oxide is not a viable reagent gas at 

temperatures below 125°C. Thus, further 

studies to ascertain the usefulness of 

NO + (H 2O) n as an alternate reactant ion were 

made with drift tube temperatures ≥125°C. 

Mobility Spectra for Prospective Chemical 

Standards in IMS Using NO Reagent Gas 

Reduced mobilities today are generally 

calculated from formulas for electric field 

strength, drift tube dimensions, ambient 

pressure, drift tube temperature and others. 

Since some of these terms may have 

significant uncertainty, chemical standards 

have been proposed as a means of calibrating 

drift scales worldwide (14). In the discussion 

below, findings are described for three 

prospective chemical standards (15) and the 

effects of NO + (H 2O) n on mobility spectra and 

gas phase ion chemistry are shown. 

2,4-Lutidine (2,4-Dimethyl pyridine) 

The reaction chemistry of 2,4-lutidine proceeds 

via proton transfer to form a protonated 

monomer MH + (H 2O) n (another product ion, a 

proton bound dimer, is thermally stable only at 

temperatures below 100°C and when vapor 

levels of 2,4-lutidine are increased). Thus, the 

product ion peak at 200°C as seen in Figure 4 

(bottom trace) was mass-identified as 

MH + (H 2O) n with a drift time of 6.58 ms (Table 

2). The calculated value for K o was 2.06 

cm 2 /Vsec and this comparable to accepted 

values of 1.95 cm 2 /Vsec; this demonstrates the 

magnitude of uncertainty in calculations of K o 

values and the value of chemical standards for 



calibration of mobility axes (15). Addition of 

NO(g) in the ion source resulted in the 

formation of increased intensity of NO + (H 2O) n 

as shown in the second trace of Figure 4 and in 

a new product ion peak that is partially resolved 

~6.93 ms. As the amount of NO(g) was 

increased, both the NO + (H 2O) n and this 

additional product ion increased in intensity. At 

the highest level of NO(g) used in this 

experiment, the new product ion was more 

intense than the protonated monomer as seen 

in Figure 4 (top plot). The mobility for this ion 

(1.85 cm 2 /Vsec) was smaller than that for the 

protonated monomer suggesting a ion size 

larger than that for MH + (H 2O) n. Since NO has a 

strong binding energy with pyridines, an adduct 

ion of M*NO + was considered plausible as 

shown in Equation 5 and was mass identified 

along with(M-H) + NO as shown in Table 2. This 

last ion may have formed in the supersonic 

expansion region where collision induced 

dissociation is known to occur and may have 

arisen via Equation 9: 

Attempts to isolate the MH + from MNO + using 

the second shutter of the drift tube was 

unsuccessful in injection specific ions into the 

MS owing to close drift times for these two ions. 

had a drift time of 9.19 ms or K o value of 1.40 

cm 2 /Vsec (referenced to 2,4-lutidine). This was 

mass-identified as MH + as shown in Table 2. 

When NO(g) was used as the reagent gas, 

additional peaks appeared at 8.08 and 8.87 ms 

or K o values of 1.59 and 1.45 cm 2 /Vsec, 

respectively (referenced to 2,4-lutidine). These 

peaks increased in intensity with increased 

levels of nitric oxide and correspondingly, the 

intensity of the protonated monomer declined. 

The identities of these peaks were 

mass-identified as fragment ions of the 

compound and included (M-CH 3) + , M-t(butyl) + , 

and M-t(butyl) + HNO (see Table 2). There was 

no peaks with drift times >10 ms, thus, there is 

no evidence for an adduct ion as seen with 

2,4-lutidine or rather no adduct ion had a 

lifetime sufficiently long to survive the drift or 

ion source regions. Tertiary butyl groups have 

been found to undergo fragmentation with 

NO(g) in chemical ionization MS studies and so 

agree with these findings. 

In summary, DTBP undergoes fragmentation 

through hydrocarbon chain losses and this 

apparently occurs in the reaction region (i.e., 

kinetically fast compared to ion drift). This is 

notably more complex than 2,4-lutidine and 

NO M NO + 

(M-H)*NO + 

+ 

HNO 

(9) 

cluster ion 

adduct ion 

Di-t-Butyl Pyridine 

The mobility spectrum for DTBP with hydrate 

proton reactant ions is shown in Figure 6 

(bottom trace) for 200°C where the product ion 

creates possibilities of a temperature 

TABLE 2: 

Mass-Identified product ions from IMS and IMS/MS of test chemicals with reagent ions of hydrated 

proton, H 3O + (H 2O) n, and of NO + (H 2O) n. 

Chemical 

2,4-Lutidine 

DTBP 

H 3O + (H 2O) n 

m/z (amu) 

108 

192 


MH + 

MH + 

m/z (amu) 

106 

107 

136 

137 

136 

164 

177 

192 

223 

NO + (H 2O) n 


M-H + 

M + 

M-H + (NO) 

MH + (NO) 

(M-t-butyl) + 

(M-t-butyl) + NO 

+ 

M-CH 3 

MH + 

MH + (NO) (trace) 

DMMP 

125 

249 

MH + 

M 2H + 

125 

249 

MH + 

M 2H + 



Intensity 

Intensity 

H + (H NO + 2 O) n (H 2 O) n 

35.7 mg/m 3 

2 4 6 8 10 12 14 16 18 

H + (H 2 O) n 

NO + (H 2 O) n 


2 4 6 8 10 12 14 16 18 


15.0 mg/m 3 

4.9 mg/m 3 

0 mg/m 3 

Figure 3: Mobility spectra for 2,4-Lutidine at 200°C with 

varying concentrations of nitric oxide. 

35.7 mg/m 3 

15.0 mg/m 3 

4.9 mg/m 3 

0 mg/m 3 

Figure 4: 

Mobility spectra for DTBP at 200°C with varying 

concentrations of nitric oxide. 

dependence. The mobility spectra were 

explored up to 250°C without substantial 

changes in profiles. 

the product ion was unchanged suggesting 

that neither cluster ions nor fragment ions 

were formed. Since binding energies are 

not available for NO + with DMMP or related 

compounds, there is no precedents or 

supporting observations for these results. 

However, the ionization potential of this 

molecule is well above the recombination 

potential of nitric oxide and this makes 

charge exchange impossible. Molecules with 

ether groups have been seen to react by 

hydride abstraction, but the proximity of the 

phosphorous makes these predictions 

unreliable. When analyzed, DMMP showed 

no change in either intensity or drift time of 

the product peak when nitric oxide was 

introduced. Studies with mass-identification 

of the ions by IMS/MS (Table 2) confirmed 

that no product ions with DMMP were 

formed or were sufficiently stable to survive 

passage through the ion source and drift 

regions. Indeed, peak shape suggests that 

the any ion formed between DMMP and NO + 

such as M*NO + would have decomposed 

before injection into the drift region. 

CONCLUSIONS 

The reagent ion chemistry of a mobility 

spectrometer was changed from ordinary 

proton based ion chemistry to reactions 

based upon charge exchange, adduct 

formation and hydride abstraction. 

Consequently, traditional drift tube might be 

configured for the ionization of chemicals 

best accomplished by these reactions rather 

than proton based reactions while still 

maintaining the common beta emitting 

NO + (H 2 O) n 

H + (H 2 O) n 35.7 mg/m 3 

Dimethylmethylphosphonate (DMMP) 

Mobility spectra for DMMP are shown in 

Figure 5 and the reaction chemistry with the 

reaction ion H + (H 2O) n is known to produce 

protonated monomers and proton bound 

dimers. A protonated monomer is apparent in 

Figure 5 (bottom trace) at a drift time of 6.61 

ms or K o values of 1.94 cm 2 /Vs and the 

proton bound dimer is seen at drift time of 

9.37 ms or K o values of 1.37 cm 2 /Vs 

(referenced to 2,4-lutidine). Increases in 

NO(g) at 250°C had the expected influence 

on the reactant ion peak but the drift time for 

Intensity 

2 4 6 8 10 12 14 16 18 


15.0 mg/m 3 

4.9 mg/m 3 

0 mg/m 3 

Figure 5: 

Mobility spectra for DMMP at 200°C with varying 

concentrations of nitric oxide. 



sources such as 63-Ni. The creation of the 

NO + reagent ion was unfavorable below 125°C 

and concentrations of 10-600 µg/m 3 for NO(g) 

were needed in the source for other 

temperatures to obtain the desired ionization 

chemistry. 

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Chem. 46(8), 710A (1974) 

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1794-1797 (1984). 

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[9] N. Einolf and B. Munson, Int. J. Mass Spectrom. 

Ion Phys., 9, 141, (1972) 

[10] B. Jelus, B. Munson, and C. Fenselau, Biomedical 

Mass Spectrometry, 1, 96, (1974) 

[11] D. Hunt, T. M. Harvey, Anal. Chem., 47, 1965, 

(1975) 

[12] S.C. Subba Rao and C. Fenselau, Anal. Chem. 50, 

511, (1978) 

[13] F.W. Karasek and D.W. Denney, Anal. Chem., 46, 

633, (1974). 

[14] G.A. Eiceman, International Workshop in Ion 

Mobility Spectometry Mesalaro NM 1992 

[15] J. Stone, E.G. Nazarov, and G.A. Eiceman, 

submitted. 

ACKNOWLEDGEMENTS 

We are grateful to Don Shoff and Steve Harden 

for cooperation in early studies by mass 

spectrometry. 


MEASURING THE TEMPERATURE OF THE DRIFT GAS IN AN ION 

MOBILITY SPECTROMETER: A TECHNICAL NOTE 

C. L. Paul Thomas 1 , N. D. Rezgui 1 , A. B. Kanu 1 , W. A. Munro 2 

1 DIAS, UMIST, PO BOX 88, Manchester, M60 1QD, UK. 

2 Graseby Dynamics Ltd, Park Avenue, Bushey, Watford, Hertfordshire, WD2 2BW, UK. 

ABSTRACT 

The importance of accurate temperature 

control in ion mobility spectrometry is 

emphasised along with the advantages of 

temperature programmed ion mobility 

spectrometry. The problems inherent in 

inferring a homogeneous temperature 

distribution in the drift gas from a temperature 

sensor mounted in the wall of the drift tube are 

considered. An alternative method for the 

measurement of drift gas temperature based 

upon reduced mobility standards is proposed. 

The drift time of the proton bound dimer of 

2,4-lutidine (K 0 = 1.43 cm 2 V -1 s -1 ) in the 

positive mode was used to measure the 

temperature of the drift gas as it was heated 

and cooled between 323 K and 473 K. 

Differences between the temperature of the 

drift gas and the temperature of the instrument 

varied between – 16 K and 26 K, corresponding 

to errors in the assignment of reduced mobility 

of up to 6.3 %. 

Other methods of measuring drift gas 

temperature by inserting thermocouples into 

the gas drift gas exhaust and pressure 

measurement ports were found to be 

unreliable. Finally, the effect of the use of 

heated transfer lines and gas preheaters ion 

the drift gas temperature was investigated and 

no significant effect on the temperature of the 

drift gas was observed through the use of 

either or both of these devices. 

Introduction 

Temperature is acknowledged as an essential 

factor in ion mobility spectrometry. Indeed 

useful additional information about the chemical 

processes effecting the product ions in the 

mobility spectrum may be obtained by studying 

their mobilities at different temperatures [1]. 

Temperature programming an ion mobility 

spectrometer enables experiments of rich 

complexity and subtlety to be undertaken to a 

highly efficient manner [2]. The accurate 

monitoring of temperature of the drift gas, 

therefore, is an important element in the 

operation of ion mobility spectrometers. Most 

importantly, because at a fundamental level 

accurate temperature measurement is needed 

to properly define reduced mobilities. However, 

generating a temperature programme, 

accurately and reproducibly, in the drift gas of 

an ion mobility spectrometer is a non-trivial 

exercise. Consideration of the construction of 

ion mobility spectrometers quickly reveals why 

this is so. Typically a heater surrounds the 

ionisation source, drift tube and detector of an 

ion mobility spectrometer. The sensor used to 

monitor and control the instrument’s 

temperature is normally placed close to or in 

direct proximity to the heating element. In using 

such an arrangement, the assumption is often 

made that the temperature of the drift gas is the 

same as that of the control thermocouple. This 

is not likely to be correct. The internal 

structures and mixtures of materials used to 

fabricate an ion mobility spectrometer, and the 

flows of gasses through the system coupled 

with the gas supply connections and insulation 

used, suggest that: 

• not only will the drift gas temperature be 

different to the temperature of the control 

thermocouple; but also that, 

• the drift gas temperature is likely to vary 

throughout the instrument. 

Under isothermal conditions it is possible to 

overcome this problem by calibrating the 

instrument against a standard material and 

deducing a cell constant, (Z (T,P)), that may be 

applied to enable reduced mobilities of other 

Received for review April 5, 2002, Accepted July 12, 2002 


C.L.P. Thomas et al.: Measuring the temperature of the drift gas ...”, IJIMS 5(2002)1,31-36, p. 32 

compounds to be estimated. In essence 

heterogeneity in the drift gas temperature 

distribution is accounted for at the same time 

as the heterogeneity in the electric field along 

the drift tube. 

The reduced mobility of an ion (K 0) in an ion 

mobility spectrometer may be related to: the 

observed drift time (t d); the drift length (d); the 

electric field strength (E); temperature (T); and, 

pressure (P) by the expression [3] 

K 0 = d 

Et d 

& 273 

T & p 

760 

(1) 

Under pressure regulated isothermal conditions 

this expression may be simplified to 

K 0 = z (t,p) 

t d 

(2) 

as d, E, and T and P are constant throughout 

the experiment. If the temperature of the 

system is changed then ideally another 

calibration is needed and a new value for the 

cell constant invoked. However, time and 

budget pressures, may tempt hard pressed 

laboratories to adopt a single value for a cell 

constant, Z, and apply a temperature and 

pressure correction, after allowing the system 

to stabilise to the new operating conditions, 

Equation 3. 

K 0 = z 

t d 

N 2 

& p T 

Labview TM data processing 

P, T 

Test atmosphere generator 

Figure 1: 

A schematic diagram of the experimental arrangement used. The ion 

mobility spectrometer was mounted on the top of a converted gas 

chromatography oven. The mixing flask was fitted inside the oven and 

the gas lines connecting the test atmosphere generator to the switching 

valve, and hence to the flask and ion mobility spectrometer were 

silco-steel heated to 423 K. 


(3) 

Ion mobility spectrometer 

In such a situation, the accurate measurement 

of the temperature of the drift gas becomes a 

determining factor in the reliability of the 

experiment. Indeed, if temperature 

programming is to be used then the issue 

becomes more important still as the validity of 

the assumption that drift gas and heater 

temperatures are the same becomes less and 

less credible. Consequently, this study was 

concerned with studying the drift gas 

temperature during a temperature programmed 

ion mobility spectrometer experiment. The 

research sought to develop a calibration 

methodology for drift gas temperature 

measurement, and control, during temperature 

programmed ion mobility spectrometry. Further, 

as part of this study the effects of preheating 

the drift gas and using heated sample transfer 

lines on the temperature of the drift gas were 

also investigated. 

Instrumentation 

The approached used has been described 

before, [1], and Figure 1 is a diagram of the 

experimental system. Nitrogen (obtained from 

boiling off liquid nitrogen stored in a stainless 

steel Dewar flask) was passed through gas 

purification media (molecular sieve and 

charcoal adsorbent tubes) into a distribution 

manifold. Gas flows were controlled by needle 

valves and monitored with 

calibrated flowmeters. The 

Exhaust 

Heated 

zone 

Mixing flask 

2,4-lutidine vapour was 

produced using a permeation 

source housed in a temperature 

controlled stainless steel vapour 

generator. If required the 

concentration of the probe 

could be programmed through 

the use of exponential mixing; a 

4-way valve was used to switch 

the vapour in and out of the 

mixing flask. Permeation 

sources were constructed from 

PTFE tubing (4 cm long, o.d. 

6.25 mm with a wall thickness 

of approximately 0.8 mm.) [4]. 

The tubing was sealed with 5 

mm diameter glass beads. 

Vapour sources were 

conditioned separately in a 

purpose built assembly,


Table 1: 

Experimental and instrument parameters 

Parameter 

Heated transfer line temperature 

Drift gas flow rate 

Inlet gas flow rate 

Gas identity 

Vapour generator temperature 

2,4-lutidine concentration 

2,4-lutidine purity 

2,4-lutidine monomer reduced mobility 

2,4-lutidine dimer reduced mobility 

IMS cell temperature 

Drift tube length 

Inlet potential 

Source type 

Source potential 

Shutter 1 potential, moving grid 

Shutter 2 potential, fixed grid 

Number of field defining electrodes 

Potential dropped across cell 

Screen grid potential 

Detector potential 

Number of field defining electrodes 

Potential dropped across cell 

Screen grid potential 

maintained at 323 K for several days before 

they were inserted into the vapour generator. 

Care was taken when calibrating and 

transferring the permeation sources to avoid 

contaminating them. The conditions under 

which the test atmospheres were generated are 

summarised in Table 1. 

A high temperature ion mobility spectrometer 

was used for this work (Graseby Dynamics Ltd, 

Bushy, Watford, UK). The ceramic drift tube of 

the cell was approximately 4.25 cm long and 

the cell was capable of operation at 

temperatures of up to 523 K. Field defining 

electrode were wound around the outside of the 

ceramic tube, and a Type k thermocouple (RS 

components Ltd, Corby, Northamptonshire, 

NN17 9RS, UK) was attached to exterior wall 

drift tube approximately at the mid point in 

between the field defining electrodes. 

Surronding the whole assembly was a 

cylindrical heater, with an air gap of ca 

1 cm between the drift tube and the 

heater. Circuler flanges mounted at 

each end of the drift tube held the 

Faraday plate detector and screening 

grid at one end and the shutter 

electrodes, reaction region and 

ionisation source at the other. The 

cylindrical heater clamped around 

these two flanges. The thermocouples 


Setting 

150 

200 to 220 

9 to 11.4 

Nitrogen 

50 

0.497 

99 

1.95 

1.43 

373 to 523 

4.25 

0 

63Ni, 370 

0 

60 

100 

8 

1000 

1063 

1100 

8 

1000 

1063 

used in this work were Type K 

thermocouples with a welded tip 

and insulated with glass fibre. The 

thermocouple wires were 0.3 mm 

°C in diameter, and with insulation the 

cm 3 min -1 

whole was 1.5 mm in diameter and 

cm 3 min-1 

approximately 2 m long. The 

°C 

temperature rating of the unit was 

g m -3 223 K to 533 K. 

% The operating parameters used 

cm 2 V -1 s -1 are given in Table 1. Mobility 

cm 2 V -1 s -1 spectra were captured along with 

K drift tube pressure and 

cm temperature with a data acquisition 

V 

card (National Instruments) and 

MBq 

processed using Labviewä. Ion 

V 

V mobility spectra were collected at 

V regular intervals and saved. The 

parameters used in the signal 

V processing can be found in Table 

V 2. As well as the temperature of 

V the exterior drift tube wall other 

temperature measurements of the 

V 

ion mobility spectrometer were 

V 

obtained from Type K 

thermocouples located : 

• in the pressure measurement port sited in 

the detector flange; and, 

• the drift gas exhaust port, sited in the inlet 

flange assembly. 

Given that both ends of the drift tube contained 

delicate structures (Shutter grids and detector 

assemblies for example) no attempt was made 

to directly insert a thermocouple in to the drift 

tube region. 

Experimental 

Experiments were undertaken based on a 

programmed temperature step. In each case 

the temperature of the drift tube was varied 

either from 473 K to 323 K or vice versa. 

Throughout the temperature programme 

temperatures and mobility spectra were 

recorded and analysed for the correlations 

between them. Figure 2 shows examples of the 

Table 2: 

Data acquisition parameters. 

Parameter 

Number of spectra averaged 

Number of data points sampled per scan 

Frequency of data acquisition 

Width of ion packet 

Delay from start of acquisition 

Setting 

100 

512 

50 KHz 

180 µs 

0 µs


Instrument temperature / K 

470 

440 

410 

380 

350 

Heat up 

programme 

0 500 1000 1500 2000 2500 3000 

Time / s 

Figure 2: 

An example of two temperature programmes used in the course 

of this work. Note that the heating and cooling rates were not 

high with experiments typically lasting 30 to 40 minutes. 

temperature programmes used in the cooling 

and heating stages of the experiment. 

Comparison of direct temperature 

measurement methodologies. 

The first experiment compared the temperature 

of the drift tube thermocouple to the 

temperatures recorded by thermocouples in the 

drift-gas exhaust and pressure measurement 

ports. The temperature of the ion mobility 

spectrometer was initially set at 473 K and then 

programmed to reduce to 323 K. 

Measurement of the drift gas temperature 

through the use of 2,4-lutidine reduced 

mobility standard. 

Drift gas temperatures were estimated from the 

drift times of 2,4-lutidine, through the use of the 

relationship below derived from Equation 1, 

T = 0.359 dP 

EK 0 t d 

. (3) 

The ion mobility spectrometer was stabilised at 

a temperature of 473 K and 2, 4-lutidine 

introduced into the ionisation region at a 

constant concentration of 0.497 g m -3 . The drift 

tube temperature was then reduced to 363 K, 

while the temperature from the heater 

thermocouple was monitored and continuously 

recorded along with the pressure of the drift 

tube. During this time ion mobility spectra were 

also continuously recorded. Once the drift tube 

temperature had stabilised at 363 K the 

process was reversed, and the temperature 

was increased to 473 K; once again with 

spectra, pressure and temperature 


Cool down 

programme 

continuously monitored. 2,4-lutidine 

was chosen as it a widely reported and 

generally accepted reduced mobility 

standard, with a stable reduced 

mobility value across the temperature 

range used in this work. 

Effects of heated gas supply and 

sample transfer lines 

In the third experiment 2,4-lutidine was 

used to: 

• investigate the effect on the 

temperature of the drift gas of 

incorporating a pre-heater in the 

drift gas supply line. This preheater 

consisted of 1m of 1/8’’ o.d. copper 

tubing wound around the outside of 

the heater that surrounded the drift 

tube and insulated with mineral 

wool; and, 

• connecting a heated sample transfer line to 

the inlet port of the instrument, 

approximately 20 cm long of 1/8” PTFE 

tubing traced with heating cord and 

maintained to 150 °C. 

In these experiments the temperature of the 

drift gas was monitored through the use of 

2,4-lutidine during temperature programmed 

ion mobility spectrometry in a manner similar to 

that described for the second experiment 

Results 

Comparison of direct temperature 

measurement methodologies. 

Figure 3 shows the relationship of the 

estimates of gas temperatures obtained from 

the thermocouples placed in the exhaust and 

pressure ports of the instrument during the 

temperature programmed cool down. It shows 

that the thermocouples placed in the gas 

streams were substantially cooler than the 

thermocouple mounted in the drift tube wall. 

Although the thermocouples were mounted in 

insulated gas lines the discrepancies between 

them and the drift tube wall are most likely to 

be attributable to cooling of the thermocouple 

junction through conduction of heat down the 

thermocouple wires. Note that as the 

temperature differential between the drift gas 

temperature and the ambient laboratory 

temperature, 296 K, reduces as the experiment 

progresses, this effect becomes less 

pronounced. Despite several modifications to 

this approach, involving extensive insulation of 

the thermocouple wires and heated sheath


Estimated temperature / K 

Figure 3: 

A comparison of the estimated temperatures of the drift gas obtained from 

thermocouples located in the exhaust line directly at the point at which the drift 

gas exits the instrument, and the pressure measurement port located at the 

rear of the instrument, against the instrument temperature as indicated by the 

cell wall thermocouple temperature. Note the solid line indicates an exact 

agreement between the thermocouple in the cell wall and those positioned in 

the gas lines. 

gases, a practicable, reliable and accurate 

direct measurement of the drift gas 

temperature was not achieved. Consequently, 

this approach was abandoned. 

Measurement of the drift gas temperature 

with 2,4-lutidine as a reduced mobility 

standard. 

Figure 4 shows the ion mobility spectra 

obtained during the cool-down and heat up 

Signal / V 

(with 1.5 V offset between spectra) 

273 

273 323 373 423 473 523 

20 

18 

16 

14 

12 

10 

523 

473 

423 

373 

323 

8 

6 

4 

2 

Pressure Port 

Exhaust Port 

0 

0 5 10 15 20 25 

Drift time / ms 

463.9 K 

456.4 K 

448.0 K 

437.6 K 

426.6 K 

416.9 K 

408.0 K 

397.2 K 

387.6 K 

376.7 K 

366.9 K 

Cell Temperature / K 

Signal / V 

(with 1.5 V offset between spectra) 

Figure 4: 

Example ion mobility spectra obtained during the cool-down (A) and 

heat-up (B) stages of the experiment 


20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

stages of this 

experiment. Two peaks 

are discernible and these 

are the monomer and 

dimer forms of a 

protonated 2,4-lutidine 

molecule. As the 

temperature reduces 

during cool-down, 

bridging between the two 

peaks, and peak 

broadening, become 

evident at about 380 K. 

These phenomena are 

also seen during the heat 

up stage, but the 

distortion of the peaks is 

evident to a higher 

temperature, about 415 

K. 

Perhaps, as the 

temperature of the 

instrument decreases 

2,4-lutidine starts to 

adsorb onto the inner surfaces of the 

instrument and the resultant build up leads to 

neutral species entering the drift region. During 

the heat-up stage of the study, the adsorbed 

species require significantly higher 

temperatures before they are removed from the 

system. (Following this experiment it was 

necessary to dismantle the whole of the test 

atmosphere generation system and clean it to 

remove the last traces of 

2,4-lutidine from the 

assembly. Further, the 

ion mobility spectrometer 

464.8 K 

455.0 K 

446.3 K 

434.3 K 

425.7 K 

415.8 K 

406.2 K 

395.1 K 

384.0 K 

377.2 K 

0 5 10 15 20 25 

Drift time / ms 

required several days at 

ca. 473 K with gas 

flowing through it to 

remove the 2, 4-lutidine 

residues) 

The 

pressure 

measurements taken 

during these experiments 

showed no sign of 

change as the 

temperature of the cell 

varied. The drift tube is 

made of a temperature 

resistant ceramic and as 

such the increase in the 

length of the drift tube 

due to thermal expansion 

was assumed to be


Temperature difference / K 

0 

-20 

-40 

-60 

-80 

negligible. At this stage it is helpful to note that 

the electrical field controls are isolated from the 

drift tube, and as such were not effected by the 

changes in temperature of the drift tube. Thus, 

the change in the drift time of the 

2,4-lutidine-monomer peak was attributed 

solely to the change in the drift gas 

temperature. 

At the beginning of the run the system had 

been run at a stable temperature for over 25 

hours and as such a cell constant was derived 

based on Equation 3, see Equation 4. 

T = A t d 

300 350 400 450 500 

Transfer line and preheat 

Preheat 

Transfer line 

No temperature conditioning 

Cell Temperature / K 

Figure 6: 

Comparison of the temperature differences observed between the 

cell wall temperature and the drift gas temperature under different 

gas-temperature conditioning regimes within the experiment as the 

instrument was heated from 323 K to 473 K. 

(4) 

Here the term, A, denotes the cell constant 

used to estimate the average temperature of 

the drift gas in the instrument. The variation of 

the drift time of the 2,4-lutidine monomer during 

the cool-down and heat-up stages was then 

used to estimate the drift gas temperature. 

Figure 5 is a comparison of temperatures 

estimated from drift time against the indicated 

instrument temperature obtained from the 

thermocouple temperature mounted in the drift 

tube wall. These data show a significant 

hysteresis between the cell wall temperature 

and the estimated drift gas temperatures. 

During cool-down the drift gas is hotter than the 

cell wall, and the opposite is the case during 

heat up. Further, the differences in 

temperature are not trivial during cool down the 

difference is between 6 and 26 K, compared to 

a – 2 to -16 K difference on 

heat-up. Such errors in temperature 

measurement correspond to errors 

in the estimate of reduced mobility 

of between 1 % and 6.3 %. 

Heated gas supply and heated 

sample transfer lines. 

Figure 6 shows the effects of 

incorporating a heated sample 

transfer line, residence time 

approximately 2 s, or preheating the 

drift gas with an approximately 1 s 

residence time, or adopting both of 

these procedures on the 

temperature of the drift gas as 

determined by 2,4-lutidine during a 

heat-up stage. No significant effect 

was observed across the 

temperature range 50 to 200 °C in 

either the heat-up, or cool-down, 

stages. In these tests the heat 

capacity of the drift tube and the surrounding 

structures was significantly greater than the gas 

inputs to the system and no discernible heating 

or cooling effect was observed either through 

the use of a gas pre-heater or heated sample 

transfer lines. 

Summary 

The behaviours reported in the tests described 

above have been observed in a range of 

different experiments over a period of 2 years, 

and with other reduced mobility standards, in 

particular dimethylmethylphosphonate. The 

central finding is that the temperature of the 

drift gas in ion mobility spectrometers may be 

obtained through the use of reduced mobility 

standards, under conditions of highly regulated 

drift gas purity. Further, in the experimental 

system used in this study a reliable calibration 

could be obtained that related the cell wall 

temperature to the estimated drift gas 

temperature. 

References 

[1] Munro AM, Thomas CLP, and Langford ML, Anal 

Chim Acta., 375 (1998) 49-63. 

[2] Ewing RG, Eiceman GA, and Stone JA. Int. J. Mass 

Spec. 193 (1999) 57-68. 

[3] Eiceman GA and Karpas Z, “Ion Mobility 

Spectrometry” CRC Press, Boca Raton, Fl, USA 

(1994). 

[4] Nelson GO, “Gas mixtures preparation and control, 

Lewis Publishers, Chelsea, MI, USA (1992).

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