Experimental Sample Preparation and Measurement The GSR samples were obtained on an electrically conductive adhesive that was adhered to the back of a shooter’s hand while a handgun was discharged. 2, 5-Dihydroxybenzoic acid (DHB) was dissolved in MeOH at a concentration of 30 mg/mL. Two milliliters of this DHB matrix solution was sprayed onto the GSR using an air-brush. Polypropylene glycol (PPG) was used for the external calibration standard. After the samples were dried, they were measured using the JMS-S3000 SpiralTOF MS system.