Preparing GSR analysis of different cartridges
To determine whether HSSE is a suitable extraction technique for the analysis of GSR, different types of cartridges were inserted into a semi-automatic pistol, discharged, and the spent cartridges placed in sealed HS vials with Gerstel Twisters suspended in the headspace above the sample. The cartridges contained propellant charges based on either nitrocellulose or a combination of nitrocellulose and nitroglycerine.
The results from the HSSETD-GC/MS analysis were compared with results from the analysis of a reference mixture based on 55 selected substances, identified as GSR compounds. This is how Gallidabino et al. approached the analysis: In the headspace above the spent cartridge to be analyzed, a perforated glass insert containing a conditioned Twister was positioned and the headspace vial was sealed.
Different Twister techniques enable different sampling approaches along with the use of different phases to collect a wider range of analytes. In this case, the analytes were extracted at 80 °C for 72 hours. The Twisters were removed from the vial and placed in individual sealed glass tubes ready for Thermal Desorption-GC/MS analysis. The glass tubes were fitted with individual transport adaptors, which seal the tubes and enable the Multi Purpose Sampler (MPS) gripper to transfer them to the Thermal Desorption Unit (TDU) for temperature programmed desorption.
The released analytes are focused in the Gerstel Cooled Injection System (CIS) PTV type GC inlet at -80 °C. The CIS is then heated using a temperature program and the analytes transferred to the GC column in splitless mode using a helium carrier gas flow of 1.3 mL/min for highly sensitive GC/MS determination. The CIS was installed in a gas chromatograph (Agilent GC 7890A) connected to an Agilent 5975 Mass Selective Detector (MSD). The column used was an Agilent HP-5MS (30 m x 0.25 mm x 0.25 μm). The GC oven initial temperature was 40 °C, the program progressing to a final temperature of 280 °C.
The GC run including cool-down and equilibration lasted 46 minutes. Mass Selective Detection (Agilent 5975C MSD) was performed in EI full scan mode across a range of m/z 40 to 500.
Headspace Solid Phase Micro-Extraction delivers
The results of their study clearly showed that by using HSSE, the scientists were able to determine and track the concentration degradation curves of 51 of the selected 55 GSR target analytes from discharged cartridges. HSSE proved to be significantly more sensitive while delivering better reproducibility than SPME. Among the four compounds not found were thermally labile compounds such as nitroglycerine that are decomposed during thermal desorption. Further studies are to be performed to optimize the analysis method parameters with the aim of increasing the number of target analytes that can be determined.
As part of their project, Gallidabino et al. generated aging curves in order to develop a method for determining the approximate time of discharge including determining whether the firearm was discharged at all. These aging curves are based on GSR compound concentrations on spent cartridges from handguns and their degradation curves.
The scientists report that based on the use of HSSE on aged samples, several GSR compounds have shown significant aging profiles. In addition, compound-to-compound ratios can be used to extend the time periods that can be covered. This approach also contributes to making the determination more rugged and to reducing variability, making the method more useful for the forensic scientist entrusted with the case. Gallidabino and his colleagues concluded that their results were very encouraging for the development of a new and complete forensic dating technique based on GSR.
 M. Gallidabino, F. S. Romolo, K. Bylenga and C. Weyermann, Development of a Novel Headspace Sorptive Extraction Method to Study the Aging of Volatile Compounds in Spent Handgun Cartridges, Analytical Chemistry 86 (2014) 4471-4478
* G. Deußing: Redaktionsbüro Guido Deußing, 41464 Neuss, Germany