Thermal extraction and static headspace

Thermal extraction techniques are usually performed in conjunction with gas chromatography. Petroleum hydrocarbons can be thermally desorbed from soil matrices at elevated temperatures. The eluting compounds are trapped in an absorbent phase such as Tenax and subsequently desorbed directly onto the column of the gas chromatograph. Whilst this technique is regarded as the closest to producing a real TPH value (C4-C35) it suffers from low sample size requirements (typically 1-10 mg) and is unlikely to be representative of the whole sample. Nevertheless, it can be used as a quick qualitative screening analysis. 

The detection of low level concentrations of volatile petroleum hydrocarbons in either soil or water can be performed by static headspace analysis. In this technique, the gas phase in thermodynamic equilibrium with the matrix is analysed. The soil is placed in a headspace vial to which water and soluble salts such as sodium chloride are added to aid the transfer of hydrocarbons into the headspace. Internal standards and surrogate spikes can also be introduced. The vial is heated and an aliquot of the static headspace vapour is directly injected onto the column of the gas chromatograph. The advantages of this technique for volatiles such as gasoline range organics are less sample handling which minimises losses, no introduction of solvents which can interfere with the compounds of interest (MTBE), and the technique can be easily automated. 

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