Classification of Gunshot Residue Using Laser Electrospray Mass Spectrometry and Offline Multivariate Statistical Analysis.

Nonresonant laser vaporization combined with high-resolution electrospray time-of-flight mass spectrometry enables analysis of a casing after discharge of a firearm revealing organic signature molecules including methyl centralite (MC), diphenylamine (DPA), N-nitrosodiphenylamine (N-NO-DPA), 4-nitrodiphenylamine (4-NDPA), a DPA adduct, and multiple unidentified features not observed in previous mass spectral measurements. Collision-induced dissociation measurements of unknown GSR signature ions reveals inorganic barium and derivatives BaOH, BaOHCH, BaCHCOO remaining from the primer. Both hydrophilic and hydrophobic signatures are detected using water-methanol electrospray solution. Offline principal component analysis and discrimination of the laser electrospray mass spectral (LEMS) measurements resulted in perfect classification of the gun shot residue with respect to the manufacturer. Principal component analysis of recycled and reloaded casings resulted in classification of the penultimate manufacturer with an accuracy of 89%.