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FIRE DEBRIS ANALYSIS THERMO DESORPTION GC/MS Jean-Marc BÉGUÉ, Véronique EUDES, Xavier ARCHER Laboratoire Central de la Préfecture de Police, 39bis rue de Dantzig 75015 PARIS, FRANCE - www.lcpp.fr Fire and Explosions Investigation Working Group Meeting, September 15th - 17th, 2010, Copenhagen, Denmark Fire debris analysis represent a major part of the tests performed in the forensic field at the LCPP. The great number of samples analyzed (around 2000 per year) leads us to search a fast, easy-to-use and automatized method. For these reasons, static headspace pre concentration followed by thermodesorption and GC/MS analysis was selected. Passive head space Sample preparation : • sealed glass or metal contener • metal diffusive sampling tube filled with Tenax Thermodesorption followed with a GC/MS analysis Correspondings authors : [email protected] [email protected] [email protected] • heating in an oven (90°C) • sampling time : 1 hour • cooling at ambiant temperature 30 min CONCLUSION : MOST ADVANTAGES OF THE METHOD • No solvent use (no solvent pic, no solvent extraction, no danger, low cost). • Ability to analyse both hydrocarbons or polar compounds (alcohols, ketones, acetates…). • Wide range of volatility (from methanol to n-C20). • Complete analysis in about 2 hours. O u t l e t s p l i t I n l e t s p l i t H é l i u m G C / M S Analysis in the lab : • Research of traces of unburned compounds • Identification of the flammable liquid (fuel, gazole, alcohol…) ANALYTICAL METHOD CRIRERIA RETAINED FOR IDENTIFICATION Identification of flammable liquid is achieved in two steps : 1. Examination of the total ion chromatogram (TIC) : Look for target compounds that enter in the composition of known ignitable liquid. Retention time and eventually pattern matching with a reference ignitable liquid TIC chromatogram. 2. Confirmation of the presence with extracted ion profiling : The extracted ion profile obtained for the unknown sample is compared against the corresponding extracted profile from the ignitable liquid reference obtained under similar conditions. Example of the use of our data base : Built with more than 300 references of commercial products. ANALYTICAL DIFFICULTIES Example 1 Ignitable liquid components may be present in adhesives used in some materials : Example 2 Loss of higher boiling point compounds with headspace concentration : • Chromatogram of mineral oil with headspace concentration (TD/GC/MS) • Chromatogram of mineral oil with direct injection of a diluted solution (GC/MS) METHOD VALIDATION Our internal method was validated on the basis of the French standard XP V 03-111 (protocol for the intra laboratory evaluation of an alternative method of qualitative analy- sis against a reference method). Reference method used was passive headspace concentration onto actived charcoal with desorption by carbon disulfide following the ASTM E1412 –07 standard practice. Example of a criteria used for the validation : ASTM E1618-06 test mixture analyzed using our internal method and the reference method. Comparison of the signal to noise ratio obtained for each compound. Injection of 5μL of the test mixture in a glass jar (370mL) containing a tenax tube (internal method) or a DFLEX device (reference method) TD/GC/MS (internal method) : thermodesorption and GC/MS. GC/MS (reference method) : desorption with 250μL of CS2 ; direct injection of 1µL in GC/MS. ASTM E1618-06 test mixture S/N ratio • Chromatogram obtained from a clean shoe (no ignitable liquid). n-C30 to n-C40 range • Chromatogram obtained from a piece of skipants stained with PVC glue used as an ignitable liquid in a fire case. n-C12 to n-C15 range n-C12 to n-C15 range
