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Dynamic headspace & purge-and-trap

  • Page ID
    61203
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    Dynamic headspace sampling involves purging the headspace with a large known volume of inert gas which ultimately removes most of the volatile compounds. Exceptions would be those compounds showing a strong affinity for the sample matrix, such as polar compounds in aqueous samples.

    Schematic diagram of PTI

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    With regard to aqueous systems the stripping efficiency depends critically on the value of K, the partition coefficient. Non-polar or medium polar compounds which give medium or low K values strip from the sample very quickly, but polar compounds, such as alcohols and carboxylic acids, favor the aqueous phase and give very high K values. These compounds are retained preferentially in the aqueous phase and consequently they cannot be isolated in this way effectively.

    The exit gas from the sample is scrubbed by passing it through either a tube packed with a suitable absorbent material, or a cold trap. Tenax TA, a polymer based on 2,6-diphenyl-p-phenylene oxide is commonly used for this purpose. This material is stable to 375 °C and gives insignificant bleed of organics. It is suitable for the efficient trapping of low to medium polarity compounds and these can then be recovered either by solvent extraction or thermal desorption.

    Purge techniques are similar to dynamic headspace sampling except that the gas is passed through the sample. Clearly any apparatus used for dynamic headspace sampling could also be employed with slight modification for purge sampling.

    Dynamic headspace and purge methods are available as automatic systems, enabling concentrations in the ppb to ppt level of concentration to be measured. The principle of a system is shown which can be operated either for on-line thermal desorption or purge and trap analysis.This versatility is enabled by a system of automatic solenoid valves which select the appropriate gas flow paths.


    Dynamic headspace & purge-and-trap is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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