Capabilities and limitations of GC–MS and LC-MS/MS for trace detection of organic gunshot residues from skin specimens

This study assesses the capabilities and limitations of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and gas chromatography-mass spectrometry (GC–MS) for the trace detection of organic gunshot residue (OGSR), at a time when forensic scientific organizations in the U.S are developing standards to facilitate their adoption. Here, recommendations are provided to aid managers and practitioners with strategies for incorporating OGSR in their workflow. Limits of detection (LODs), limits of quantitation (LOQs), and measurement variability are reported for top eight analytes of interest: nitroglycerin, 2,4-dinitrotoluene, akardite II, methyl centralite, ethyl centralite, diphenylamine, 2-nitrodiphenylamine, and 4-nitrodiphenylamine. Both techniques detected the targeted analytes with LODs as low as 0.3 ppb for LC-MS/MS and 40 ppb for GC–MS. The methods’ performance rates were evaluated using 50 shooter sets, including four different ammunition types. Background specimens were collected from individuals who had not handled a gun in the last 24 h. The solvent extracts were split in half for a direct comparison of the methods on the same specimens. For the ammunition used here, enhanced sensitivity was observed for the triple quadrupole configuration (LC-MS/MS), with accuracies as high as 80 %. However, under the extraction and EI-quadrupole instrumental configuration used in this study, GC–MS could not detect enough characteristic compounds to identify OGSR, especially in lower calibers. As a result, the GC–MS analysis displayed accuracies only as high as 35 %, raising a flag that GC–MS alone is not recommended for OGSR detection unless combined with LC-MS/MS, or used with alternative pre-concentration methods or high-resolution configurations.