Anal. Chem. 2009, 81, 7189–7196
A NIST Standard Reference Material (SRM) to Support the Detection of Trace Explosives William A. MacCrehan* Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8392 SRM 2905 Trace Particulate Explosives was developed to simulate the residues produced by handling plastic and military explosives. A series of nine candidate materials were prepared by coating chromatographic supports with either Composition C-4 ((containing RDX (hexahydro1,3,5-trinitro-1,3,5-Triazine) and HMX (octahydro-1,3,5,7tetranitro-1,3,5,7-Tetrazocine)) or TNT (2,4,6-trinitrotoluene). Criteria for selection of the best material for the SRM included: coating efficiency, extractability with organic solvents, thermal storage stability, consistency of the particle size with fingerprint residues, and suitability for calibration of trace explosives detectors. The final base material selected for the SRM was octadecylsilane-modified silica (C18) with a nominal 20-30 µm particle size. Four materials comprise the SRM, with two nominal concentrations of explosive, 0.01% and 0.1% (mass fraction) for both C-4 and TNT, respectively. The final certified concentrations were determined by liquid chromatography (LC) with ultraviolet absorbance detection (LC/UV) and a liquid chromatography with mass spectrometric (LC/MS) method using negative ion atmospheric pressure ionization (APCI-) with an acetate ionization additive that improves quantitation. The SRM was tested on a table-top field explosives detector based ion mobility spectrometry (IMS). Sensitive and reliable determination of trace explosives residues is a key element in preemptive detection of terrorist threats and post-blast forensic evaluation of evidence. In addition to the chromatographic techniques familiar to analytical chemists, there is a plethora of new “field” explosives detectors that include handheld, table-top, and even portal devices. Although there are numerous approaches addressing the need for rapid detection including sensors, test kits, and even mass spectrometers, at this time, the most frequently deployed technology for explosives detection is based on ion mobility spectrometry. IMS detection is very rapid, comparatively simple, requires no sample preparation, and may be miniaturized. However, the primary means of “calibration” of these detectors is with vendor-supplied materials, such as sampling swipes pre-coated with easily detected explosives or simulants. This calibration step is used to adjust the mobility drift time under the ambient conditions to qualitatively identify the explosives. Generally, no determination of the signal response versus amount of explosive is involved in this vendor-recommended protocol. 10.1021/ac900641v Not subject to U.S. Copyright. Publ. 2009 Am. Chem. Soc. Published on Web 07/28/2009
Recently, the ASTM International has published a simple, independent approach to evaluating explosives detector performance. ASTM E2520-07 Standard Practice for Verifying Minimum Acceptable Performance of Trace Explosives Detectors uses test solution calibrants in the microgram per milliliter concentration range to verify the ability of these instruments to detect trace explosives deposited on sampling swipes.1 However, this simple test, designed to be used by first responders, such as policemen, firemen, and transportation security agents, only tests the final step in the determination - the response of the detector to pure analyte deposited from solution. A more thorough understanding of the collection and measurement of trace explosives is required to ensure the most reliable detection. The residues that remain following the handling of explosives differ markedly in particle size from components comprising the bulk materials. For example, although the military specification (MIL-C-45010A) for the plastic explosive Composition C-4 requires crystalline particles of RDX of 100-800 µm, the distribution of residues that remain on the fingers after handling explosives is much smaller2,3 because of crushing of the material. Although the majority of the residue particles formed are very small (
