Passive headspace procedure

Samples collected on adsorbents can be desorbed by heat (thermal desorption) or by solvent extraction. Thermal desorption of samples from charcoal is not efficient however, because of the high temperature needed (950°C) to remove hydrocarbons from the charcoal (192). For this reason, most ACS passive headspace procedures use carbon disulfide to extract the adsorbed liquid residues. In 1967 Jennings and Nursten (193) reported concentrating analytes from a large volume of aqueous solution using activated charcoal as the adsorbent and extracting with carbon disulfide. Since then many adaptations of this method have been used to detect accelerants in fire debris, but currently dynamic headspace methods are seldom used because of the inconvenience of sampling and possible contamination issues with equipment. 

Passive headspace method A sample preparation procedure in which vapors from a headspace are withdrawn for analysis. 

Solid-phase microextraction (SPME) is well documented with respect to its convenience and applicability to sampling volatiles and as an extraction technique to detect ignitable liquid residues when coupled with GCMS (185-188). Nonetheless, fire debris analysts have yet to widely adopt SPME as a viable alternative to the activated charcoal passive headspace technique. SPME is a simple, solventless extraction procedure in which a phase-coated fused-silica fiber is exposed to the headspace above the fire debris packaged in a closed container. A drawback to the procedure requires a rubber sleeve septum be placed at the opening of the container for maximum recovery of analytes. The technique has been applied successfully for the detection of flammable and combustible liquid residues on human skin (189). 

The oxygen level in the milkfat may be limited by either active or passive actions. For passive control, processing procedures and plant design are established to minimize air exposure. Deaeration devices (74), vacreation (60), the use of antioxidants (72), effective destmction of lipases (75), and nitrogen spanning of container headspace are examples of active control of product quality. 

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