On-site GC-QEPAS analysis on the scene of crime: The RISEN project

Abstract

The capability to provide timely forensic analytical information is of paramount importance in criminal investigations involving chemical substances of toxicological interest. Miniaturized Gas Chromatography (GC) systems are suitable sensors to be deployed on the scene of crime, allowing to analyse specimens there, providing timely information to protect security and safety of specialists on-site and saving the time needed to collect, transport and analyse them in a forensic laboratory. An innovative hand-portable point sensor has been recently developed using a fully Micro-Electro-Mechanical-System (MEMS)-based compact-GC platform of the size of a few cm3 coupled to a miniaturized detector based on InfraRed Quartz Enhanced Photo Acoustic Spectroscopy (QEPAS). Sensing is based on the piezoelectric transduction of the photo-acoustic signal that is generated when laser radiation is absorbed by molecules in vapour phase in the QEPAS cell. Signal intensity is proportional to laser intensity, to the absorption cross section of the substance, and to its concentration. QEPAS spectra are very similar to the spectra obtained by classical IR absorption spectroscopy (IRAS), but, unlike IRAS cells, QEPAS cells can be designed as very cheap components of miniature size. The GC-QEPAS prototype is built upon a patented compact-GC architecture integrating three silicon micro-machined chips for sample pre-concentration, injection and GC separation and a QEPAS sensor based on a new ultra-wide tunability Quantum Cascade Laser (QCL) source, designed and assembled into a package matching the needs of hand-portability by the operator. The system was equipped with a compact air sampler, which also contains a first stage of pre-concentration, to allow vapour analysis on-site. Preliminary tests were carried out in a 60 litres glass box, where vaporizing solutions (between 1 and 100 microlitres) using a syringe of target substances. Sampling time was less than 60 seconds. The GC-QEPAS sensor was used to successfully analyse ppm and sub-ppm amounts of two nerve agent simulants: dimethyl methyl phosphonate (DMMP) and dipropylene glycol methyl ether (DPGME). Methyl salicylate (a blister agent simulant) and sulfur mustard (a real blister agent sample) were also tested. Other substances of interest in forensic toxicology where tested, including precursors of drugs of abuse. The sensor showed also its capability of analyzing amphetamine types stimulants (ATS) and drug precursors both as pure compounds and as mixtures. LoDs measured were in the order of about 10 ng for many precursors, and ten times larger for target compounds containing an amino group. Ethanol, propanol, gasoline and paint vapours at much higher concentrations, up to saturation vapor pressure, compared to the target chemical substances of forensic interest were also analysed to study the selectivity of the system in a real scenario. The miniature size of the column has allowed the separation of substances in cycle times of 2–3 minutes, to be compared with chemical substances eluting after retention times in the order of tens of minutes with traditional GC systems commonly used in forensic laboratories for casework. The total absorption chromatograms and the photoacoustic absorption spectra related to chromatographic peaks of both pure compounds and mixture will be shown, to demonstrate how the approach developed in the RISEN project will be able to support an improved approach to crime scene activities in the near future.