Thermal mobile trace analysis

With a focus on trace forensic detection of explosives, especially for use in counterterrorism and to counter narcotics investigations, Fetterolf et al. [75] evaluated the use of ion mobility-mass spectrometry for explosives determinations. In this, explosives residues were collected on a membrane filter by a special attachment on a household vacuum cleaner. Although subsequent thermal desorption and analysis required only 5 s, fimits of detection for most common explosives were as low as 200 pg. The persistence of explosives on hands and transfer to other surfaces were also examined as were post-blast residues of NG on fragments of improvised explosive devices constructed with double-based smokeless powder. Finally, postblast residue from C-4, Semtex, and other explosives was found by IMS analyses on items of forensic and evidentiary value. These few out of many examples demonstrate that mobihty spectrometers are well suited tools for laboratory and on-site investigations, before and after the use of explosives. 

Until the 1980s the only ionization techniques used for trace analysis on-line with chromatography were electron ionization (El, formerly known as electron impact ionization) and chemical ionization (Cl). These closely related ion sources are discussed below but for now it is sufficient to emphasize that both sources require introduction of analytes in the gas phase, and that the sources are located within the high vacuum chamber of the mass spectrometer thus it is essential to severely limit the amount of mobile phase that can enter the mass spectrometer in order to maintain the necessary vacuum conditions. As a result, for quantitative analyses it is necessary to either restrict the flow rate of the mobile phase or to develop external on-Une devices that remove the bulk of the mobile phase prior to introduction into an El or Cl source. This is fairly easy to achieve for GC, but remains difficult for HPLC (see below). The restriction to vapor phase analytes implies that El and Cl are applicable to only thermally stable and volatile compounds, and sometimes chemical derivatization is necessary to achieve this (Section 5.2.1b). 

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