Estimation of discharge time for firearms or cartridges is an important part of firearm-related crime investigation. Accompanied by shooting, gunpowder in cartridges burns and reacts vigorously, and volatile nitrate organic compounds generated in the internal gunshot cases and barrels diffuse into air over time. Qualitative test was used to infer whether the cartridge cases or guns were fired and the discharge time ^[1]. Sinha summarized the dating methods of firing time. The gunshot case mouth was covered by filter paper treated with absorbent reagent consisted of L-naphthylamine and sulfanilic acid. The chemical reactions between reagent and nitrate organics synthesized during firing process presented as pink color on filter paper, which was measured and compared with time length in order to estimate the discharge time ^[2]. Sampling by wiping and washing of the fired barrels or cases, would destroy the original sample instead of improving the gaseous recovering rate ^[3].

The solid phase micro-extraction (SPME), a nondestructive sampling technique, was deployed by Andrasko et al. to retrieve volatile organic firing residues inside the barrel and cartridge^{[3, 4, 5, 6, 7]}. Gas chromatography-thermal energy analyzer (GC-TEA) or gas chromatography-flame ionization detector (GC-FID) was integrated to measure the lapsed time since a firearm or cartridge was fired. Nitrogenous compounds analysis with GC-TEA could achieve total nitrogen content, via high temperature splitting decomposition of nitrate organics coupled with chemiluminescence detection. One GC-TEA peak, decreased with time lapse, was considered to be closely related to the concentration of nitro- and nitroso- organic compounds. Naphthalene detection using GC-FID also presented the decay of organic gunshot residue over time, to verify the potential of the lapsed time assessment. Later, discharge time of rifles and pistols was further studied using different types of ammunition, while comparing GC-TEA to GC-FID. It was concluded that the analysis of total nitrate content was more reliable for discharge time estimation, due to the fact that naphthalene was quick to escape, at low content and over-relied on different types of cartridge, though under certain circumstances, naphthalene detection was more reliable and undisturbed than that of total nitrate content. Additionally, it was necessary for the separate measurement to draw variation curve of organic gunshot residue (GSR) on every type of firearm and cartridge.

Wilson JD et al. monitored naphthalene content decay in fired shotgun shells using gas chromatography-mass spectrometry (GC-MS) ^[8]. Airtight glass containers were used to preserve the shotgun shell, in order to prevent the escape of volatile organic of GSR. SEMP was carried out through cork of container, during which the non-contact operation could be achieved.

Weyermann et al^[9]. analyzed GSR of 9 mm Luger pistol ammunition using GC-TEA and 32 organic compounds were detected, 6 kinds of compound could be used to estimate the emission time. Among them, benzonitrile, phenol, 2-ethyl-1-hexanol and naphthalene decayed rapidly as result of quick volatilization, short residence time, and only extremely low content remained 2 hours after firing, while 1, 2-dicyanobenzene and diphenylamine both containing a great quantity of nitrogen containing groups, could be determined 32 hours later. This study indicated that GC-TEA was practicable for discharge time estimation by analysis of total volatile nitrate organic contents.

Qualitative analysis of discharge time is dominant in Chinese forensic community, such as identification of marks on cartridge and observation of gunshot residue particles with scanning electron microscope ^{[10, 11]}. There is barely relevant study on the quantitative/semi-quantitative analysis of time since discharge of Chinese homemade firearm using GC/MS, although it has already drawn more and more attention ^[12]. In the present study, SPME-TEA is employed to analyze volatile nitrate organic compounds in common cartridges made in China under simulant open and natural volatilization environment. With the content decay pattern monitored, the discharge time are discussed.

SPME sampling was undertaken to collect GSR from 7.62 mm model 51 pistol cartridge cases and 12/70 JIA-
LING shotgun shells 1 day after discharge, and the extracts were analyzed by GC-TEA. The result was shown in Fig.1. One marked double peak was observed on the chromatogram of 12/70 JIALING shotgun shells in the time segment from 1 to 2 min of GC-TEA analysis. Under nitrate pattern, the thermal energy analyzer was unable to determine the kind of organic compounds. Instead, the nitro-, nitroso- and nitric acid ester groups in volatile nitrate organics all decomposed at high temperature and generated NO and NO₂ which were available for the following photoelectric detection. As a consequence, the peak of 12/70 JIALING shotgun shell in Fig.1 should come from the nitrate groups of the volatile organic compounds extracted by SPME from cartridge case. The relative strength of the nitrate peak could be used to represent the relative contents of the nitrate organics.

In the same diagram, no prominent peak was observed during the test time frame for the model 51 pistol cases. It could be inferred that the amount of volatile nitrate organics was not synthesized enough for the detection with GC-TEA when the cartridge was fired. For model 51 pistol cartridges, it will not be possible to determine the time interval of GSR deposition. Therefore, the subsequent test of discharge time estimation was only performed on 12/70 JIAING shotgun shells with marked peak monitored, with the purpose to clarify the relationship between the discharge time and the GC-TEA peak value observed above.

Fig.1

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	Fig.1 GC-TEA chromatogram of GSR collected from 12/70 JIA-LING shotgun shell (up) and from 7.62 mm model 51 pistol cartridge (down) 1 day after shooting

SPME sampling was carried out from the same 12/70 JIALING shotgun shell 1, 3, 7, 9 and 11 days after shooting respectively, and followed with the GC-TEA analysis, of which the results were shown in Fig.2. All samples over the time frame of experiment had a double peak observed in the GC-TEA test time segment of 1 to 2 min. The shape of the double peak from earlier days was higher and sharper, while the peak shape from later ones became lower and broader. Under the same coordinate system, it was notable that the double peak representing nitrate organics revealed the decreasing pattern when the discharge time increased. On day 11, its chromatographic double peak became too weaker to be noticeable during the same test time range.

Fig.2

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	Fig.2 GC-TEA chromatograms of GSR collected from 12/70 JIA-LING shotgun shell 1, 3, 7, 9 and 11 days after shooting

It could be inferred from the repeated appearance of the nitrate peak in GC-TEA chromatogram that the volatilization and diffusion processes of nitrate organics inside the 12/70 JIALING shotgun shells existed over 11 days. As time went on, volatile nitrate organics adsorbed on the metallic interface of fired cases escaped, and less and less nitrate organics measurable with SPME-GC-TEA remained. Thus the above all well explains the decreasing trend of nitrate peak with the increase of discharge time.

The key point for the discharge time estimation is to find out a particular parameter, which will change significantly with time, and the estimated value of discharge time could be obtained by the measurement of this parameter. If the decreasing behavior of nitrate organics peak, demonstrates a certain regular pattern, as what observed in GC-TEA test of 12/70 JIALING shotgun shell, it would be possible to estimate the time since discharge of ammunition. With the GC-TEA data processed, the area and height of the nitrate peak of 12/70 JIALING shotgun shells were obtained, and the curve fitting lines were shown in Fig.3 and Fig.4.

Fig.3

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	Fig.3 Curve-fitting line between nitrate peak area and discharge time of 12/70 JIALING shotgun shell

Fig.4

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	Fig.4 Curve-fitting line between nitrate peak height and discharge time of 12/70 JIALING shotgun shells

No matter peak area or height, the variation tendency was the same, with the discharge time value decreased with the time. On day 11 after discharge of cartridge, the organic compound peak became too weak to be visible, and the peak area and height fell down to zero. The variety curve of peak area and peak height along with time could represent the variety of nitrate organic residue content in 12/70 JIALING shotgun shell after shooting, and the time estimation could be possible over 11 days since the similar type of shotgun shell was fired. As the peak area and peak height only stand for the relative content of nitrate organics, other than the absolute value, the direct comparison of nitrate organics measurement is not accurate, In addition, the decrease trend of nitrate compoundsis also closely related to the type of ammunition. Therefore, the standard curve describing nitrate organic content variety under similar firing condition has to be drawn, every time prior to estimation of cartridges discharge time.

In the present study, non-destructive SPME sampling coupled with GC-TEA was first employed in Chinese-made cartridges, to determine the decrease pattern of organic nitrate residue along with time. Nitrate organic shooting residue was detected from fired 12/70 JIALING shotgun shells, which generated one notable GC-TEA double peak in the time segment of 1 to 2 min. The peak decayed gradually as the discharge time extended, until the nitrate peak disappeared at all on day 11 after shooting. The curve-fitting line between discharge time of 12/70 JIALING shotgun shells and nitrate peak area or height was obtained, by which the shooting time of this type of cartridge could be estimated. There was no result from fired model 51 pistol cartridges, which indicates that the GC-TEA approach is not suitable for the discharge time estimation of the model 51 pistol cartridges. In general, the measureable concentration of volatile nitrate organic compounds retrieved from ammunition should be the prerequisite of discharge time estimation for different types of cartridges.