Literature Review: Hand-Held Portable Desorption Atmospheric Pressure Chemical Ionization Ion Source for in Situ Analysis of Nitroaromatic Explosives

This study presents a novel handheld device used for “in situ” monitoring of nitroaromatic explosives (e.g. TNT) using desorption atmospheric pressure chemical ionization (DAPCI) coupled to a tandem mass spectrometer. The need for portable and relatively fast methods of detection for explosive compounds is important for security checkpoints where there is not time for samples to be sent to a lab for analysis. Because of the time constraints, sample preparation and preconcentration steps are not possible. Samples must be ionized directly from a solid support under atmospheric pressure because the apparatus and time needed to establish a vacuum decrease portability and increase analysis time. DAPCI is useful under these constraints because it does not require a solvent (liquid or gas) or produce waste. The method of detection used in this study was a benchtop mass spectrometer, but a miniaturized instrument would be required for implementation in field measurements.

The proposed handheld DAPCI device was able to ionize and detect the nitroaromatics 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB), 2-amino-4,6-dinitrotoluene, and 2,4,6-trinitrophenylmethylnitramine (Tetryl) at levels on the order of 10 pg. All of the compounds were able to be resolved under a negative ion mode which was favorable because of the electronegative nitro and amine groups present. The peaks were identified using tandem mass spectrometry and collision-induced dissociation (MS/MS CID) via a linear ion trap mass spectrometer. The conditions used for the second mass spectral dimension and collision chamber were not address but were likely quadrupoles. Analysis times were approximately 1 minute which was optimal for eventual field measurements. However, analysis time at a security checkpoint would depend on the miniaturized mass spectrometer in use.

This work has direct applications in real world scenarios and will likely continue to be funded by security, forensics, and environmental agencies. The authors address the miniaturized mass spectrometer as a source of future work but do not address complex sample matrices beyond their sample mixture of various nitroaromatics. Interfering compounds could potentially cause problems in the detection of the nitroaromatics. A true field sample that would test positive for nitroaromatics could also contain other explosive component residues. Overall, this study was thorough and quantitative as the limits of detection for the nitroaromatics using the handheld DAPCI ionizer were either better or comparable to a previously studied miniaturized version of a low-temperature plasma (LTP) ionization source.

Jjunju, F. P. M.; Maher, S.; Li, A.; Syed, S. U.; Smith, B.; Heeren, R. M. A.; Taylor, S.; Cooks, R. G. Analytical Chemistry 2015, 87 (19), 10047–10055.

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