Miniature DMS-IMS detector for enhanced revolving power Abstract ...

IJIMS 10(2007)1 - 5
Miniature DMS-IMS detector for enhanced revolving power
E.G. Nazarov * , A.G. Anderson * , E.V. Krylov * , S. L. Coy * , R.A. Miller * , D. Burchfield @ ,
and G. A. Eiceman#
* Sionex Corporation, Bedford, MA
@ Hamilton Sundstrand, Pomona, CA.
# New Mexico State University, Las Cruces, NM.
Abstract
Ion mobility based gas analyzers provide high sensitivity detection and identification of
trace levels of ions in gases. Because these analyzers operate at ambient pressure, they
are of reduced complexity and suitable for field applications. However, dealing with the
real world complexity of mixtures, field applications continue to be a challenge.
Currently there are two types of ion mobility based gas analyzers. These are based on
Ion Mobility Spectrometer (IMS) and Differential Mobility Spectrometer (DMS) technologies.
The two methods operate on different principles of ion separation. In IMS, ion
species separation is a function of the coefficient of mobility (K 0 ) of the ions, resulting in
different drift times. In DMS, ion separation is a function of the electric-field-dependence
of mobility ((E) parameter). For enhanced ion identification it is desirable to use the value
of the comprehensive mobility coefficient, K(E) = K 0 (1+(E)). Combining DMS and IMS
will take advantage of complementary data on a given ion species and enhance chemical
identification.
The new Sionex portable tandem DMS-IMS instrument includes a DMS for rapid separation
of positive and negative ions, and two short (1.5cm) IMS drift tubes for further resolution
of DMS pre-filtered ions. The sensor system is compact; measuring only
2"×2.5"×3". The design has been optimized through computer modeling using the
Sionex-proprietary microDMx TM modeling software package. In the tandem system,
DMS provides compensation voltage (Vc) separation while the IMS provides drift time
separation. Figure 4 shows a DMS-IMS images for positive toluene ions in nitrogen (left
frame, 45ppb) and in air (right frame, 17.5ppb). The right frame shows that in air, toluene
forms four ion species that can be resolved using the tandem system. The current prototypes
have high sensitivity (ppb level) for a wide range of chemicals including CWA simulants,
explosives, TICs, TIMs, and environmental pollutants.
Introduction
An ion mobility spectrometer includes stages for ionization, ion separation by electric
field and ion current measurement. Ion separation is based on the ion mobility coefficient.
Two main types of ion mobility spectrometers are most popular at present: the low-field
Ion Mobility Spectrometer (IMS, Figure 1 top right) and the Differential Mobility Spectrometer
(DMS Figure 1 top left). The two devices operate on different principles of ions
separation. In IMS, ion species are separated by their coefficient of mobility 1 (K 0 ). In
DMS ion species are separated by their field mobility dependence 2 ((E)). Enhance resolving
power results if the ion mobility based gas analyzers utilize both K 0 and (E) properties
for ion separation. Combining DMS and IMS 3 takes advantage of the comprehensive
coefficient mobility, K(E) = K 0 (1+(E)). Prototype of Sionex DMS-IMS 2 tandem system
(Figure 1, bottom) provides this complementary ion information.

IJIMS 10(2007)1 - 6
Experimental
The new Sionex portable tandem DMS-IMS 2 instrument (Figure 2, left) has two steps of
ions separation: on the first, DMS provide pre-separation of positive and negative ions
according differential mobility , and then two short (1.5cm) IMS drift tubes (one for each
ion polarity) provide further separation according absolute value of ion species mobility.
The sensor system is compact, measuring only 2"×2.5"×3". The design has been optimized
through computer modeling using the Sionex-proprietary microDMx TM modeling
software package (Figure 2, right). The DMS section provides compensation voltage
(Vc) separation all ion species formed in ionization region while the two orthogonal located
IMS provide drift time (DT) separation for both positive and negative ions simultaneously.
Results and Discussion
Since the trend lines of the mass resolution (Figure 3, left) for the IMS 1 and DMS 4 are
quasi-orthogonal, a DMS-IMS tandem system has enhanced power for ions specie identification.
Figure 3 ( right frame) shows that the combination information obtained in the
first and second steps and presented in two-dimensional space (drift time vs. compensation
voltage) provides the enhanced resolving power of the DMS-IMS system. Combined
with possibility to scan of the separation voltage (RF voltage) in the DMS, the
analytical space can be extended to three-dimensional, and provide further information
related to ions fragmentation too.
Figure 4 shows DMS-IMS 2-D spectra for positive toluene ions in nitrogen (left, 4.5ppb)
and in air (right, 17.5ppb). The left frame shows that in air, toluene forms four ion species
which do not be resolved in each of segments of system, but using the tandem system
all these species can be detected and resolved independently.
Conclusions
DMS-IMS tandem provides the complementary chemical information provided by a
DMS-IMS tandem systems improves the confidence level for detection, separation, and
identification of ion species in real-world situations. The current prototypes have high
sensitivity (ppb level) for a wide range of chemicals of current practical interest, including
CWA simulants, explosives, TICs, TIMs, and environmental pollutants.
References
[1] Mason E.A.; McDaniel E.W.; Transport Properties of Ions in Gases. 1988, Wiley, New York.
[2] Buryakov I.A.; Krylov E.V.; Nazarov E.G.; Rasulev U.Kh., Int. J. Mass Spectrom. Ion Proc., 1993,
128, 143-148.
[3] Anderson A.G.; Krylov E.K.; Nazarov E.G.; Miller R.A.; Coy S.L., PITTCON, February 2007,
Chicago, USA, p.59.
[4] Krylov E.; Nazarov E.G.; Miller1 R.A.; Tadjikov B.; Eiceman G.A., Journal of Physical Chemistry A.
2002, 106(22), 5437-5444.

IJIMS 10(2007)1 - 7
Figures
DMS = Adjustable Ion Filter
IMS = Time-of-Flight Spectrometer
Peak position (CV) depends on {(E); SV} Peak position (DT) depends on {K 0 ; E; L}
DMS-IMS tandem
Peak position depends on {(E); K 0 ; Vc; E}
Figure 1. Differential Mobility Spectrometer (DMS) (top left); drift-tube Ion Mobility
Spectrometer (IMS) (top right) and DMS-IMS tandem (bottom).
Figure 2. Sionex portable tandem DMS-IMS instrument (left). Optimization tandem
system with microDMx TM modeling software (right).

IJIMS 10(2007)1 - 8
Figure 3. Trend lines of DMS and IMS mass resolution (left). Simulation of the
combined analytical space of DMS and IMS based on those trend lines (right).
Figure 4. DMS-IMS 2-D spectra for positive toluene ions in nitrogen (left, 4.5ppb) and
in air (right, 17.5ppb), showing resolution of multiple ions enhanced by the extended
analytical space.