Prevalence of explosives traces in cars: critical implications for perchlorate detection in forensic investigations

This work reports on the background levels of organic and inorganic traces relevant to forensic explosives in vestigations in 127 civilian cars from Germany, the Netherlands, the United Kingdom, Belgium, Switzerland, Norway, and the United States of America, and in 16 Dutch police cars. Extracts were analysed with liquid chromatography – tandem mass spectrometry to detect and quantify six organic explosives: RDX, HMX, NG, TNT, ETN, and PETN. Ion chromatography – mass spectrometry was used to identify and quantify sixteen inorganic ions: chloride, nitrite, thiocyanate, nitrate, chlorate, phosphate, sulfate, perchlorate, lithium, ammonium, magnesium, potassium, calcium, sodium, strontium, and barium. Organic explosives are generally absent in both civilian and police cars, similar to the broader environment. Background levels of most inorganic ions were also consistent with those previously found in the environment and on human hands. Perchlorate, however, was detected in more than half of the investigated cars (67%), in vehicles from each participating country, and at relatively high levels of up to 123.9 μg per sample. The traces may originate from PVC stabilisers used in the upholstery of the car. Driving a new car can transfer similar amounts of perchlorate to the hands (>10 μg) as the handling of a Super COBRA 6 flash banger. Suspect contamination of perchlorate from flash powder in a police car is also possible, albeit at low levels (<1 μg) in realistic scenarios. These results warrant careful investigative procedures and forensic interpretation, especially when evidence is retrieved from, or has been in contact with a car.