Ion Mobility as Explosive Detector

INTRODUCTION

Explosives are highly reactive substances which possess large amount of energy (potential energy) that leads to the production of explosions, they are usually accomplish with lighten, heat, sound and pressure when they are suddenly released. Explosive can be classified based on their sensitivity, velocity, composition and physical forms [23].

Ion Mobility Spectrometry is an analytical technique which is widely acknowledge in the area of detection of explosives as a result of it powerful sensitivity using electrophoresis techniques. The use of IMS as an explosive detector is based on the capability of IMS to detect almost any substance which can undergo ionisation and ability of detecting lightest element such as helium, to the most complex mixture and in complete organism [26].IMS is used in the detection of explosive compounds such as trinitrotoluene (TNT),Dinitroluene(DNT),Cyclo-1,3,5-trimethylene-2,4,6,8-tetranitramine(RDX),Pentaerythriol tetranitrate(PETN, Hexafluorobenezene (HFB),TETRYL(2,4,6-Trinitrophenylmethylnitramine), etc. [27].

IMS is widely used has a powerful analytical technique for detection of various explosive warfare chemical agent, trace level of nitro-organic explosives which may be found on hand bags and carry on-luggage’s [22] in various airport all around the world, all this has been made possible by the ionisation sources used and development in the drift tube materials. Most recently produced IMS are cheap and classy making it possible to carry out analysis on samples without pre-treatment [6].

IMS is carried out by allowing analyte which have undergo ionisation to be separated according to their movement in gas phase [1] .IMS also enable the possibilities of this Ionised analytes to be distinguished using parameters such as their masses, size, shapes and charges and it aid separation and identification of the ionised molecule which are in gas phase using their movement in carrier gas [2-4] .In IMS the analytes are converted to ionisable molecule at Atmospheric pressure in a weak electric field leading to the ions separated according to their movement in the gas phase [5]

This analytical measurement techniques of an ion mobility spectrometry to detect explosive involve the use of ion mobility spectrometer which is made up of important component such as ion source, ion gate drift tube and the detector plate [21].

IMS is a techniques mostly used in the aspect of detection of explosive as a result of the technique being able to easily detect object by the possibility of screening individuals who might have concealed explosive; a major employment in monitoring of site which might be contaminated with explosive substances or materials also including buried land mile which might have explosive [7].

IMS can detect explosives materials even if it is in small amount or large quantities and used in post offices for the detections of dangerous substances which might be present in mails [8].IMS have been used in the detection of min-explosive devices and expose charges in order to show individuals which might have come in contact with explosive based on the great analytical abilities to detect explosive in limited time, high sensitivity, and resolution power [10].The use IMS has a major detector of explosives, factors such as it been easy to use, low cost of production and maintenance , portability, and low level of detection limits are considered and the ability to hyphenate IMS with other analytical method such as GC,LC and MS so as to improve it sensitivity have been involved as ways of improving the technique and it detection, and most of this analysis are done in seconds [26].

The aim of this research on IMS as an explosive Detector includes; the principles, instrumentations, application, innovation of IMS based on limitation from the first invention to the recent years and possible ways of improving IMS for future application in detection of explosives.

HISTORY

In 1985,JUNE 23,the Canadian government initiated a trace detector which was as a result of the airplane crash that killed 329 people blown up at 31000 feet at the Atlantic ocean, although in 1987 the government developed a high speed detector that use ion mobility spectrometry to detect explosives[14].Ims has been a major technique of interest especially in the Detection of Explosive based on the high selectivity and Sensitivity with good resolution of the techniques in the detection of explosive without the sample to be detect undergoing any form of Pre-treatment or pre separation (textbook 23).It major use of interest have increased as a result of terrorism in various aviation sectors .In Ims ionisation occurs in gas phase chemistry of explosives in the air which permit part-per-billion(ppb) detection limit for explosive such as nitro-organic explosive[15-18 i.e 77-80 of textbook].Based on the increase in terrorism and crime rate two ion mobility spectrometers were discovered to make possible quick, non-invasive screening and this two models are 400 IONSCAN which was from Barringer Research Limited but now called Smith Detection[81 of textbook] and Itemizer which was produced from Ion track ,Inc. also now regarded as GE Interlogix. As a result of this two Models a new interest has been given to IMS and it has been accepted as a technique in analytical measurement [19].

The discovery of IMS became a major point of interest from the formation of ion and the behaviours of this ion when they are in the gas phase at ambient pressure. Researches on IMS were can be divided into two major periods which are the period IMS was discovered and Innovated(1850-1938) also the period IMS was foundational studied (1948-1970)[28].

The development of Ion mobility for the detection of explosive have been improved after the initial design which was described by Eiceman et al [20] in which the experiment were performed with pulse laser ionization (266nm.8nsec pulse ) as the source the use of naphthalene and anthracene as high vapour pressure organic compounds[21].furtherly, same compounds were also studied another source of ionisation called ⁶³Ni which are currently used in explosive detection[1].optimisation of ion mobility in achieving detection of ng for explosive so as to improve sensitivity by improving temperature of the spectrometer cell, high voltage pulse, applied electric field with appropriate resolution in order to obtain different features for various explosive species were carried out on the geometry spectrometer[21].Ion mobility spectrometry as also involved the use of various ionisation source such as electro-spray ionisation[46], thermal ionisation, corona spray and corona discharge ionisation[47,[48]],MALDI laser ionisation [48]. Based on the aspect of the high voltage (dc and pulsed) provided only very small current (< 1 mA) another major improvement is the flowrate (approximately 1 L/min) of both the carrier and drift gas which can be achieved but using a compact, palm-sized air pumps, and the recently IMS are easy to carry, used for field works based on the fact that the devices now operate on battery [21].

Researches also have shown the discovered of how IMS has been used in the detection of various explosives [22] after the incident in 1985, JUNE 23, when the Canadian government initiated a trace detector which was as a result of the airplane crash that killed 329 people blown up at 31000 feet at the Atlantic ocean, although in 1987 the government developed a high speed detector that use ion mobility spectrometry to detect explosives [14].There is also an increase in the publication of IMS over the past few years and till current date which shows interest in the technique and in its development [29 ]

Table 1: history in the Development on IMS

2.3 Application Area of ion mobility

Based on the development in ims is the ability to couple ims with other analytical techniques inorder to improve it detection ability The use of IMS as a detection can be for explosive such as 2,4,6-trinitrotoluene(TNT) ,2,4-Dihydro-5-nitro-3H-1,2,4-triazol-3-one (NTO), Pentaerythritol Tetrenitrate (PETN) which are detected at in positive polarity in air at temperature of 150 to 250 degrees[12].Cases where the it required to resolve explosive compounds that are available in the presence of Interferents ,IMS is regarded as a valuable technique when coupled with Gas chromatography[13]. IMS aid in the classification of explosive compounds which is usually by the use of Laser ION Mobility Spectrometry and the process is usually done with the transfer of the ion with Photo-ionised Naphthalene in ambient air and the wavelength of the laser is usually fixed(266nm).Common explosive are GE ,vapour TRACERS ,Smith Sabre 4000 and Russian built M0-2M and M0-8 although companies are coming up with non-radioactive ionisation procedures for IMS in the detection of explosive trace for instances Bruker “DE-detector including implant services”QS-B220 which can be referred to as the next Innovation of IMS[9].

PRINCIPLE OF IMS IN DETECTION OF EXPLOSIVES

THE DIAGRAM OF IMS [45]