Static headspace gas chromatography in the analysis of oils and fats

One of the weaknesses of HSGC is that quantitation is not as straightforward as in classical GC. Calibration of a HSGC system relies either on the availability of the matrix void of the analyte or on the composition of the matrix being known so that it can be simulated by individual ingredients. If neither option works, for instance if the matrix is not available and cannot be simulated, the so-called standard addition method has to be used. Even in situations where a standard addition method may lead to uncertainties, the multiple headspace extraction (MHE) technique allows a reliable quantification of trace constituents. 

In principle two versions of HS analysis have been developed to date, one dealing with vapour in a thermodynamic equilibrium with a liquid or solid sample (static HSGC), and one in which equilibrium is not reached (dynamic HSGC). Only the first approach, where a volatile substance partitions between the liquid or solid sample and the surrounding gas phase in a closed container, will be dealt with in this review. 

Fundamental principles concerning sampling and quantitation in HSGC have been covered in more detail by Hachenberg and Schmidt (1977), Kolb, Pospisil and Auer (1981), Kolb and Pospisil (1985) and Ioffe (1989). 

In HSGC - and this is also the case with other GC techniques - the property directly measured is the peak area. A, or the peak height, H, which is proportional to the partial vapour pressure, pi, of substance i in the HS. Given the substance concerned and the GC system used, this relation has the form  

The activity coefficient 7/ describes the intermolecular attraction forces between solutes and solvent in a mixture. It depends on the chemical nature of substance i but is also influenced by the concentration of all the other components making up the sample mixture. Moreover, the coefficient is temperature dependent and to a lesser extent pressure dependent. 

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Static headspace gas chromatography in the analysis of oils and fats... 

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