nitric oxide as a reagent gas in ion mobility spectrometry

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
Copyright © 2002 by International Society for Ion Mobility Spectrometry

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)
sample neutral
reactant ion
product ion
M
+
NO +
(M*NO) +
(5)
sample neutral
reactant ion
adduct ion
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 24
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
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 25
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.
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 26
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 -
Copyright © 2002 by International Society for Ion Mobility Spectrometry
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;

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 27
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
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 28
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
Ion
MH +
MH +
m/z (amu)
106
107
136
137
136
164
177
192
223
NO + (H 2O) n
Ion
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 +
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 29
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
Drift Time (ms)
2 4 6 8 10 12 14 16 18
Drift Time (ms)
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
Drift Time (ms)
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.
Copyright © 2002 by International Society for Ion Mobility Spectrometry

Eiceman, G.A. et al.: "Nitric Oxide as Reagent Gas ...”, IJIMS 5(2002)1, 22-30, p. 30
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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ACKNOWLEDGEMENTS
We are grateful to Don Shoff and Steve Harden
for cooperation in early studies by mass
spectrometry.
Copyright © 2002 by International Society for Ion Mobility Spectrometry