The increasing terrorist threat internationally has made it crucial to detect all kinds of explosives and other threat substances in order to provide security for important locations such as airports, border crossings, embassies, seaports, governmental buildings, power stations or transportation systems. A number of techniques for detecting threat substances are known, such as X-ray screening, fluorescence quenching, neutron and gamma-ray spectroscopy, LC-MS, UV gated Raman spectroscopy, laser induced breakdown spectroscopy, electrochemical and immunosensors, chemiluminescence, SPME-HPLC, GC-ECD, GC-SAW devices, GC-differential mobility spectrometer. More recently, metal oxide semiconductor (MOS) nanoparticle sensors have been used for the detection and discrimination of low concentrations of explosives.
Ion mobility spectrometry (IMS) has been shown over the past 20 years to be a reliable method for trace detection of explosives, drugs, chemical warfare agents, toxic industrial chemicals and various organic environmental pollutants, due to its low detection limit, relatively fast response, hardware simplicity, and portability. IMS-based equipment is presently used in vulnerable places, such as airports, for screening of both people and carry-on luggage.
Although IMS technology has been successful in many areas, it is undesirably limited in cases where the sample material is presented in complex matrices. Under these conditions, when other materials are liberated with the analytes of interest, those other materials can selectively compete in the ionization process. Their ionization levels may be less than those for the analytes of interest, so they competitively react in the ionization process and greatly reduce the sensitivity and selectivity of the IMS.
IMS is a gas-phase ion separation technique that operates under atmospheric pressure. A drift tube consisting of a reaction region and drift region is the main element of the IMS instrument. In conventional IMS instruments, the electric field is created by a series of conducting guard rings, and in more simplified drift tube designs the ion drift tube is formed of single-piece, conductive glass tube. This more recent drift tube design is disclosed in U.S. Pat. No. 7,081,618, which describes a reaction-ionization/drift tube chamber constructed with one or more single-piece conductive ceramic or glass tubes having specified conductivity. The glass tube or ceramic is used in place of the stack assemblies of metal and ceramic annular components that were typically used in previous drift tubes. This approach provides a simpler design, fewer parts and improved performance for fast switching of ion polarity during a scanning mode.
Ion mobility spectrometers for detection of explosives, narcotics and other contraband are disclosed in U.S. Pat. Nos. 3,699,333, 5,027,643, and 5,200,614. U.S. Pat. No. 5,491,337 shows still further improvements to ion trap mobility spectrometers and U.S. Pat. No. 6,690,005 describes a pulsing mechanism for ions entering the rings-stack drift tube with front trapping capability and switching of ion polarity entering the drift chamber. U.S. Patent application 2002/0134933 provides a method for detecting both positive and negative mobility spectra wherein the first and second selected switching times are less than 20 msec and 15 msec, respectively, with a transition time of less than 5 msec.
U.S. Pat. No. 7,528,367 describes an ion mobility spectrometer with an inlet that communicates to an ionization chamber and a drift chamber. Stacked grid electrodes with applied potential hold ions between them until they are pulsed into the drift chamber. This patent claims sharper peak shape and improved resolution. U.S. Pat. Nos. 6,124,592 and 6,407,382 describe methods to separate and store ions by exploiting mobility characteristics of the ions by applying an electric field to trap the volume of ions prior to pulsing them into the ring stacked drift chamber. U.S. application 2009/0113982 discloses a multi-dimensional detection system based on the ultraviolet detection of molecules produced in the thermal decomposition of explosive compounds separated by gas chromatography.
Meanwhile, the combination of GC and IMS has been established for the use of the IMS as a detector to the effluent from a GC, wherein the IMS has been interfaced to the GC effluent column and operates continuously and is used no differently than other conventional GC detectors such as flame photometric, flame ionization and electron capture detectors.
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