application of solid-phase microextraction to the recovery of explosives and ignitable liquid...
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Application of solid-phase microextraction to the recovery of explosives and ignitable liquid residues from forensic specimensK.G. Furton, J.R. Almirall,M. Bi, J. Wang, L. Wu
Solid-Phase MicroextractionSo what is solid-phase microextraction?A relatively new extraction technique known as solid phase microextraction (SPME) minimizes sample preparation and concentrates volatile analytes in a solvent-free manner. SPME is a sensitive, reproducible, cost efficient, solventless technique that incorporates extraction, concentration, and sample introduction into a single step.
How do you do it?A syringe-like device with an outer septum piercing needle and a plunger houses a fused silica fiber coated with a stationary phase. The fiber can be inserted into the sample matrix (aqueous samples) or the gaseous phase above the sample (headspace). The sample is the adsorped onto the fiber.
HeadspaceSo what is headspace analysis?Headspace analysis is the analysis of the vapor above a given solution.
ExperimentThe purpose of the experiment performed was to analyze explosives and ignitable liquids and identify which type of fibers worked the best for their analysis.
ExperimentThe fibers used to test for explosives included:Polydimethysiloxane (PDMS)Partially cross-linked PDMS-DVBPartially cross-linked PolyacrylatePartially cross-linked Carboxen PDMSPartially cross-linked Carbowax-divinylbenzene (CW-DVB)
Explosives SampleSamples were created by detonating explosives in soil and collecting soil samples. Soil samples were also taken prior to detonation.The samples were then washed with 5.0ml acetonitrile and shaken for 15 min and allowed to settle for 10 min and then filtered.A 75 l volume was added to 7.5 ml of water (containing 25% NaCl) and extracted by SPME.
Explosives AnalysisIt was found that the best results in analyzing the explosives were achieved by direct aqueous immersion of the fiber i.e. placing the fiber directly into the sample solution. The sample was then analyzed by GC-MS.
Absorption TimesCarbowax-DVBDirect immersion
Lower molecular mass molecules reach optimal recoveries at 50 min and then decrease.Higher molecular mass molecules continue to increase.
Explosives VariablesIn general, more volatile and more hydrophilic analytes are best recovered by headspace sampling.Less volatile more hydrophobic compounds are best sampled directly from aqueous solutions.Higher temperatures can increase recoveries by head space spme.Non-volatile and thermally unstable explosives cannot be recovered this way.
Explosives ResultsFor the explosives studied the PDMS-DVB fiber yielded the highest overall recoveries of the fibers tested.Analytes with significantly different polarities generally require different fiber chemistries (more polar fibers yield higher recoveries for more polar analytes).
Liquids SampleA mixture was prepared containing 30 major components of possible ignitable liquid types including those from light petroleum distillate which contain C4-C11 alkanes, medium petroleum distillate(C8-C12), high petroleum distillate (C10-C23), and aromatic hydrocarbons.
More Sample PrepDesorption studies were conducted by placing 1l of the mixture spiked on a Kimwipe inside a can. 200ml of water was then added into the can and sealed with a lid that contained a 2mm diameter septum.A PDMS fiber was inserted (by syringe) into the can and headspace for 15min, then the fiber was inserted into the injection port of the GC. The same procedure was followed using various fibers.
Ignitable Liquids ResultsFor ignitable liquids headspace sampling was the preferred method of extraction. PDMS fibers yielded uniformly high results. All other fibers tested except carboxen have lower recovery than PDMS.
VariablesSampling ModeIn general, more volatile and more hydrophobic analytes are best recovered by headspace sampling.Less volatile more hydrophobic compounds are best sampled directly from aqueous solutions.
VariablesTemperatureHigher temperatures can increase recoveries by head space SPME.Non-volatile and thermally unstable explosives cannot be recovered this way.