Temperature dynamic headspace extraction

Quantitative or semi-quantitative determination of analytes by SPME requires working within the linear dynamic range of the SPME fiber. If the linear dynamic range is exceeded, the extracted amount of analyte will not reflect the amount of analyte in the sample. Figure 2 shows the normalized peak areas for the headspace extractions of different amounts of powdered polyamide 6.6 [67]. The extraction time and temperature were 45 min and 80 °C. Under the given conditions, the dynamic range of the PDMS/DVB fiber was linear if the polyamide sample size was between 1 and 100 mg. For the... 

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. 

Heat extraction techniques for solid sample preparation in GC are static and dynamic headspace analysis (SHS, DHS, HS-SPME and HSSE), thermal desorption (TD-GC, TD-GC-MS), pyrolysis and thermochromatography. Nomenclature is not unambiguous as to DHS, TD and PT. The terminology purge-and-trap is usually preferred for the simplest dynamic technique in which it is not necessary to subject the sample to either solvents or elevated temperatures. Scheme 2.7 shows the family of headspace sampling techniques. Headspace sorptive extraction (HSSE) and HS-SPME represent high capacity static headspace. 

Ezrin et al [980] developed a direct dynamic headspace device for use in GC-MS in which the sample (5-25 mg) is placed in the hot zone directly on the GC column head. The method is adequate even for very high boiling compounds. The design of the DHS-GC-MS device probably contributes to this capability, there being no transfer lines and the sample tube being located directly at the head of the GC column. It is capable of isolating trace level compounds that would have been much more difficult to determine by extraction methods. Analysis time is much shorter than by extraction. Identification of compounds is based on GC retention times, mass spectrometry and reference compounds. Alternate methods of analysis, such as SFE and SFC, which use CO2 for extraction and as the carrier in SFC, operate at considerably lower temperature than in DHS-GC-MS. Possibly the aromatics can be extracted more readily and completely by SFE than by heat alone (headspace). 

Analyte recoveries in P T experiments can vary widely due to matrix effects, purging efficiency, volatiUty, purge ceU design, choice of adsorbent, isolation temperature, and many other factors. Quantification with the various headspace techniques always requires method development in terms of extraction time and temperature in order to avoid degradation. With dynamic headspace (DHS) nearly 100% recovery of volatiles is possible provided headspace temperature is appropriate to remove most of the analyte in a reasonable time. Kolb et al. [32] have outlined the prospects of quantitation by means of headspace techniques. 

The sample chamber is set to a temperature of 40 C for pre-heating and extraction. This temperature was chosen for a good representation of the released VOCs of the cheeses as a mock-up of the typical mouth feeling. In this dynamic headspace sampling step the volatile headspace is flushed from the cheese samples by using the inert gas nitrogen for collection onto the sorbent packed tubes. After collection the tubes had been transferred to a thermal desorption unit. 

Dynamic-HeadSpace (D-HS) is a highly sensitive sampling technique which is commonly used in this context. However, due to its cost (requires using a thermal desorber and a Programmed Temperature Vaporizing (PTV) injector) and complexity of implementation, the use of passive techniques have been developed, in particular, HeadSpace-Solid-Phase Micro-Extraction (SPME). 

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