Ion mobility spectrometry (IMS) is an analytical technique used to detect explosives and other chemicals. It works by ionizing vapor samples, separating the ions based on their mobility in an electric field, and detecting the ions. IMS provides fast, sensitive detection and is used in security applications like airports and public events. The document discusses the principles and components of IMS, including ionization methods, ion separation techniques, and factors that determine the resolution, sensitivity and detection limits of IMS systems. Commercial applications of IMS are highlighted, such as explosive detectors, air quality monitors, and portals used to screen people for explosive residues.

1. DETECTION OF EXPLOSIVES
BY ION MOBILITY
SPECTROMETRY
BY:
ASHISH KUMAR SHARMA (MS10043),

2. Outline:
 Introduction: What is IMS and principles of explosive
detection?
 IMS technique in detail: Ionization, separation,
detection and analysis of ions.
 Commercial applications of IMS.

3. INTRODUCTION
 Ion mobility spectrometry (IMS) is a sensitive analytical technique that is used for
detection, identification and monitoring of chemicals, mainly explosives, highly toxic
gases and drug interdiction.
 Basically, Vapors of these compounds are ionized according to atmospheric pressure
chemical ionization processes(APCI) and then the ions are separated on the basis of
their mobility in an electric field.
 Since terrorism and drug trade is ongoing global headache, therefore powerful
techniques all around the world are being developed to detect such narcotics and
explosives. IMS here has emerged as a powerful detection tool.

4. How can explosives be detected?
 The chemical components and taggants in an explosive are the ones
that make the detection feasible.
 Explosive devices filled by substances (explosive related
compounds, ERCs) with low vapor pressures can be detected by
their vapor phase, because they contain additives with high vapor
pressures.
 However, the concentration of these vapors decreases sharply with
distance due to convective flows.

5.  For example, in the analysis of an air sample, collected over a C-4
charge, no RDX vapor was detected; although, it was the main
component of the mixture.
 A high concentration of 2-ethyl-1-hexanol and a low
concentration of cyclohexanone were recorded. These are the
additives in the C-4 device.

6. Why IMS as detection tool for
explosives?
The following features of IMS makes it an effective method for detection of such
substances:
1. Low detection limits (concentration as low as ppb can be detected)
2. Fast Response of the technique (a few seconds)
3. Small size (wearable, handheld)
4. Low power (~4 X AA batteries)
5. Low cost
That is why small scale IMS are operational at airports and railway stations to detect
any explosive devices.

7. PRINCIPLE OF IMS WORKING
Standard IMS instrumentation is comprised of four major sub-components:
1. ion source region
2. ion gate
3. drift region
4. detector.
 In IMS, sample vapors are converted to ions at atmospheric pressure and those ions are then
characterized by their gas phase mobility in weak electric fields (drift region).
 In drift region, Ions move according to diffusion processes.
 Since, Different ions will have different mobility in a given drift region, they therefore get separated
and henceforth characterized.

8. What is ION MOBILITY?
 The ions moving through a gas (usually air at atmospheric pressure) under the
influence of a low strength electric field obtain a certain drift velocity
(mobility).
vd=KE
 The mobility coefficient, K, depends on:
1. the strength of the electric field,
2. the drift gas pressure and temperature
3. characteristics of the ion (mass & charge)
4. its interaction with the drift gas molecules.

9. K = [3∙e∙(2∙π)½ (1+α)]/[16∙N∙(μ.k.Teff)½ ∙ΩD∙ (Teff)]
Here, e: charge of an electron,
α : correction factor (usually below 0.02 under low electric field conditions),
N : number density of drift gas molecules.
μ : reduced mass of the ion mass(m) and drift gas (M) molecules [μ=m*M/(m+M)],
k : the Boltzmann constant,
Teff : effective temperature of the ion
ΩD : effective cross section for collision of the ion with the drift gas molecules.
UNIT OF MOBILITY COEFFICENT = cm2 V-1 sec-1

10.  In practice, a simplified formula is used for calculation the mobility at
standard temperature (273 K) and pressure (760 torr) conditions, called the
reduced mobility, K0,
K0 = K∙(273/T)∙(P/760)
 T and P represent the temperature and pressure of the drift gas.
 The time required for an ion to traverse a given distance is inversely
proportional to its reduced mobility.
 Thus, it is common practice to use a reference compound with known
reduced mobility, Kref, to calibrate the mobility scale.

11. BASIC DESIGN OF IMS INSTRUMENT
Ions with high mobility, generally small ions, travel faster
than large ions and cover the distance between the
shutter and detector in a shorter time.

12. Working of IMS
 The working of Ion Mobility spectrometer can be divided into following
stages:
1. Collection of sample from test surfaces
2. injection of sample in spectrometer
3. ionization of the sample
4. separation of ions
5. analyzing the separated ions

13. Sample Collection
 IMS is a gas analyzer that analyzes only vapors. Therefore, sample needs to
be in vaporized state. Hence sometimes the samples are heated so as to
vaporize them.
 Different ways to collect samples are:
1. Aspiration method: The gaseous sample is aspirated inside the inlet of IMS,
using a flow booster, by creating exhaustion close to inlet.
2. Vortex flow method: Vortex airflow is created around the sample surface
and the gaseous sample and micro particles spiral upward toward the
inlet of the instrument.
Solid phase extraction, thermal desorption and laser desorption are other
alternative methods.

14. Photograph demonstrates the transfer of a substance
from the surface to the inlet of a sample collector by an
airflow swirling upward along the axis of the vortex.

15. Ionization of ERC
1. Atmospheric pressure chemical Ionization (APCI) – Here, the electron
beam passes through a reagent gas and the interaction of the reagent
gas with the electrons gives molecular ions of the gas. Due to ion-
molecular interaction, ions of ERC are formed in the second stage.
Common radiation sources employed are 63Ni, 3H or 241Am.
2. Photoionization
3. Surface Ionization
4. Electrospray Ionization (ESI)

16. Separation of Ions
 Two main methods are known for separating ions:
1. Time-of –flight ion mobility spectrometry (TOF-IMS)
2. Ion mobility increment spectrometry (IMIS)
Separation of ions generally depends upon the ion mobility, drift gas and
electric field.

17. TOF-IMS
 The separation chamber is a cylindrical hollow tube
through which the drift gas is blown at a flow rate of Vt.
 The ions travel through the length l under the effect of
Electric Field E and are separated by their velocities,
proportional to their respective mobility coefficients. The
average drift time tdi for an ion is:
tdi = l/(Ki(0)E) = l2/(Ki(0)Ut)
 A spectrum of Ion current (Id) vs. the drift time (td) is
recorded.

18. IMIS
 IMIS is based on the drift of different ions under the
application of alternate electric field provided by
an external alternate voltage source.
 Ions oscillate as a result of alternating field moving
perpendicular to the direction of drift gas. Velocity
of ions, Vi depends on the difference in their
mobility in the low strength and high strength
electric fields, that is, on the function α(E/N).
 The spectrum thus obtained is of current I vs. Uc.
 Spectrum of I vs. T is also obtained by converting Uc
to (|Uc-Uc0|)/Vu; where Vu (V/sec) is the scanning
rate at which the spectrum is recorded.

19. Recording of Separated Ions
 Recording of ions is carried out using an electrometric system of current
recording. This is essentially a setup of a collector and a current amplifier.
 Errors are sometimes induced in the recording process mainly because of:
1. The Lag Effect of the system : induced because of the use of RC chain
(alternating voltage).
2. thermal noises and fluctuations associated with the electrometric system.
3. Other possible sources that induce such errors are the various instrument
parameters, such as temperature fluctuation, drift gas velocity change,
etc. These result in the background interfering ions.

20. PERFORMANCE CHARACTERISTICS OF IMS
The performance of an IMS can be judged by the following criteria:
I. Ion Separation and identification.
II. Resolution.
III. Rapidness (Time taken to get the results)
IV. Sensitivity.
V. Detection Limit

21. Capability of Identification
 The capability of identification of an instrument means its ability to separate out
and identify the unknown compounds with the known ones.
 parameters that determine the identification capability of IMS are:
I. Average Drift time of ions through drift space (td) for IMS.
II. Compensation voltage of the drift of ions (Uci) for IMIS.
Different compounds will have different values of td and Uci. This is because of their
different mobility coefficients (K0(0)) and the functions of mobility increment, α(E/N).
 Generally, α(E/N) ∞ Uci & K0 ∞ 1/td.
 Both K0(0) and α(E/N) decrease with greater m/z ratio.

22. RESOLUTION
 Resolution of ion mobility spectrometer can be defined as its ability to
characterize the ions with very similar values of K0(0) and α(E/N).
 For TOF-IMS, Resolution is defined as the ratio of drift time for the ions to
their peak width at half height:
Rt = tdi/wti
 For an IMIS, Resolution is defined as the ratio of the compensation voltage
to peak width at half height:
Ru = Uci/wui
 Thus, by reducing the peak half width, resolution can be improved.
 Resolution of the peaks can also be changed by changing the drift gas.

23. SENSITIVITY:
 Sensitivity in IMS is defined as the ratio of the amplitude of the ion current of an
analyte to the analyte flow which caused this current:
S = I0/F0 (C/mol)
 Sensitivity(S) can also be expressed as follows:
S=Fkikeks (C/mol)
Where; Ki : coefficient of ionization efficiency
Ks : coefficient of ion transmission through chamber
Ke : coefficient of decrease in ion current amplitude peak.
 Thus, the sensitivity of the instrument is dependent upon how effectively it ionizes
the analyte(ki), dispersion of ions in the chamber(ks) and loss of some of the ions
during current recording(ke).

24. RAPIDNESS:
 Rapidness of IMS is judged by the time it takes to provide the readings:
ts = ti + tnt,pi,p + te
ti = Vex/Vs is the transportation time of sample from the inlet to separation chamber.
tnt,pi is the average time of separation of ion mixture;
tpi is the time necessary for resolution of analyte peak;
te is the delay time in recording signal.
 For the detection of ERC by TOF-IMS, general experimental values are: tt= (1.5-5)*10-2 sec &
tnt = 1-10 sec.
 The separation chamber volume also determines the amount of time the instrument takes
to provide readings. Greater the chamber volume(Q), greater is the time for recording the
spectrum.

25. DETECTION LIMIT
 Detection limit is the smallest value that can be reported from an
instrument at a certain level of confidence :
DL = kS/m
where, ‘S’ is the standard deviation for the blank sample,
‘m’ is the calibration sensitivity
‘k’ is the coefficient corresponding to a certain confidence level.

26. Schematic diagram of a differential
mobility spectrometer.
Ions are carried by a stream of air from
the ion source into the space
between two plates. A waveform with
alternating high (20 kV/cm)
and low (1 kV/cm) electric fields is
applied to one plate and the
other plate is grounded. Ion a is pushed
toward the upper plate, ion
c toward the bottom plate, while ion b
passes through and reaches
the detector. By changing the
compensation voltage, different ions
(a or c) will reach the detector.

27. IMS IN COMMERCIAL USE
 Explosive and drug vapor detector.
 Air quality monitoring on International Space Station.
Photograph of the GID-3 (Smiths Detection, U.K.) mobility spectrometer for
continuous monitoring of airborne vapors for specific detection of chemical
warfare agents.

28. The Vapor Tracer, a handheld explosive analyzer (made
by GE Interlogix).

29. Sentinel II portal for screening humans for explosives
residues: streams of air are used to sample the subject’s
body and the air is drawn through ports at the bottom of
the portal into a pre-concentrator and IMS detector
(Smiths Detection).

30. Two views of miniaturized mobility
spectrometers. A model of a
lightweight chemical agent detector
or LCAD is shown and was
developed for the U.S. Army. A
variant, the LCD, was developed for
use by the U.K. armed
forces. Both are from Smiths
Detection.

31. Conclusion:
 IMS is a highly sensitive and powerful detection technique that can be
employed for explosive detection, drug detection and air quality
monitoring. It rapidly developing and hence has a great future.

32. Refrences:
1. Buryakov, I.A.; Detection of explosives by Ion Mobility Spectrometry, J.
Anal. Chem.,2011,vol.66,pp 674-694.
2. Karpas, Z.; Bulletin of the Israel Chemical Society; Issue No. 24, December
2009.
3. ION MOBILITY SPECTROMETRY (Second Edition), Eiceman, G.A.;Karpas,Z.;
ISBN 0-203-50475-5 Master e-book ISBN.
4. Asbury, G.R.; Hill, H.H., Jr., Using different drift gases to changes separation
factors (α) in ion mobility spectrometry, Anal Chem. 2000, 72, 580–584.
5. Ion mobility spectrometry: recent developments and novel applications,
by Dr. Abu B. Kanu & Prof. Herbert H. Hill, LPI,2004; Spectrometry
Techniques.

33. THANK YOU!!!!!!!!!!