Internal static headspace 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. 

The characteristics of solid headspace sampling with an internal standard for the determination of residual volatiles in polymers are given in Table 2.45. Generation of a headspace sample is an equilibrium process that limits the amount of a specific component available for analysis within the practical restraints of time and temperature. Static headspace sampling in atmospheric conditions is limited to about 210°C (oxidation and thermal decomposition of polymers) an alternative is thermo desorption in inert conditions. Sensitivity is enhanced by 100 times using LVI with on-column cry-ofocusing. Solid headspace provides about 10-fold more sensitivity than solution headspace. HS-GC does not suffer from interference fl om the solvent peak or from impurities. Typical detectors used in SHS-GC are FID, BCD and MS. Determination of... 

The static headspace method is therefore an indirect analysis procedure, requiring special care in performing quantitative determinations. The position of the equilibrium depends on the analysis parameters (e.g. temperature) and also on the sample matrix itself. The matrix dependence of the procedure can be counteracted in various ways. The matrix can be standardized, for example, by addition of Na2S04 or NajCOj. Other possibilities include the standard addition method, internal standardization or the multiple headspace extraction procedure (MHE) as published by (Kolb and Ettre, 1991 Zhu et al., 2005) (Figure 2.11). 

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