Swab touch spray mass spectrometry for rapid analysis of organic gunshot residue from human hand and various surfaces using commercial and fieldable mass spectrometry systems

Abstract

Organic gunshot residues, specifically methyl centralite (1,3-dimethyl-1,3-diphenylurea) and ethyl centralite (1,3-diethyl-1,3-diphenylurea), are characteristic compounds for which forensic analysts test when determining if an individual has discharged a firearm. These distinctive compounds have long been analyzed by several instrumental techniques, many of which involve extensive sample preparation or have lengthy analysis times. Presented here is an ambient ionization method that requires no sample preparation, offers real-time analysis, and can be paired with a portable ion trap mass spectrometer for in-situ analysis. Swab touch spray ionization utilizes a rayon-tipped swab that has an aluminum wire handle which can simply be swabbed over the area of interest. After contacting the hands of the shooter, or other surfaces, the swab simply has solvent applied to the rayon tip and a high voltage applied to the aluminum handle. This process generates ions that the mass spectrometer will use to determine if the organic gunshot residues are present. Mass spectrometry allows for the direct confirmation of organic gunshot residue on suspects hands, confirmed through tandem mass spectrometry. Additional benefits of swab touch spray ionization are that the swabs are commercially available, they are sterile, and they are individually packaged to prevent contamination. The swabs are also forensically feasible because they are sealed prior to delivery by the vendor with a tamper-proof label. Here we have shown that this ambient ionization method allows for the direct swabbing of suspected shooters hands for trace residues and confirm the presence of the two organic gunshot residues investigated.