Ion mobility spectrometery

DETECTION OF EXPLOSIVES

BY ION MOBILITY

SPECTROMETRY

BY:ASHISH KUMAR SHARMA (MS10043),

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.

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)

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.

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.

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.

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.

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

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.

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.

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).

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.

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.

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.

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.

The Vapor Tracer, a handheld explosive analyzer (made

by GE Interlogix).

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).

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.

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.

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.

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