Evaluation of interferers in sampling materials used in explosive residue analysis by ion chromatography

Analysis of explosive residues is of special interest in forensic science. It can be crucial for investigating acts of terrorism or robberies. Ion Chromatography (IC) is a standard technique used by many forensic labs in these cases. Due to the common nature of many explosives and the inherent dirtiness of samples from explosion scenes, sometimes inconclusive or false positive results are obtained by IC, making the identification of interferers extremely important. One possible source of interferer can be the sampling materials themselves, as investigated in this work. First of all, interferers from forensic swabs and syringe filters were evaluated, as these are commonly employed in explosive residue IC analysis. Among the 6 tested samples, only two syringe filters were considered free of interferers. Significant amounts (>0.2mg/L) of Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, and SO₄^2- were found in all forensic swabs and two syringe filters. Then, some ordinary commercial materials, such as cotton swabs, cotton balls, and cotton discs were also tested, as they are often employed in a forensic routine, also. These also showed significant amounts (>0.2mg/L) of Na⁺, K⁺, Cl^-, NO₃^-, and SO₄^2-. However, the total concentration of interferers in ordinary commercial materials was lower than in the forensic swabs. Then the effectiveness of removing interferers by washing sampling (swabs, and cotton balls, rolls and disk) and pre-processing (syringe filters) materials was evaluated. This revealed that the interferers could be removed with at least three washes. The results indicated that the sampling materials used for explosion scene analysis should be carefully addressed in forensic analysis. First, by making a previous analysis of the sampling material, preferably prior to use, to evaluate the presence of interferers to ensure that only material free of interferers is used. When it is not possible to perform a prior analysis of sampling materials, they should be analyzed concomitantly with the samples from the post-blast residue in order to estimate the presence of interferers. This would reduce the risk of false positives. Additionally, when interferers are detected, washing can be employed for more reliable routine analysis. We suggest at least 3 wash cycles. This method could be used for ordinary commercial materials, which are cheap and easily obtained.