Headspace analysis, solubility determination

McNally, M.E. and Grob, R.L. Determination of the solubility limits of organic priority pollutants by gas chromatographic headspace analysis, J. Chromatogr. A, 260 23-32, 1983. 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

Sutton and Calder (9) have also measured the solubilities of several alkylbenzenes in distilled water and in seawater by a method based on GC. Saturated solutions were prepared by equilibrating water with aromatic vapor in an all-glass apparatus consisting of a 1-L Erlenmeyer flask with an insert tube. The insert tube was used to store the compound. It was capped with a ground-glass stopper. The liquid hydrocarbon did not come into contact with the water except through a perforation in the insert, which allowed hydrocarbon vapors to enter the headspace above the water in the flask. The flask was placed in a constant-temperature shaking bath controlled at 25.0 dt 0.1°C. The water was equilibrated for 48 hr prior to analysis. The solubilities were determined by solvent extraction of the saturated solutions with subsequent analyses of the extracts by GC. 

A better understanding of analysis of VOCs can be achieved by knowing the particular physicochemical properties of each analyte (Table 23.1). Vapor pressure and solubility provide an idea about the volatility, and in addition, if the compound of interest can be determined by headspace, purge and trap, solid-phase microextraction, etc. 

Method developments for food headspace SPME analyses have considered some, but not all, important issues related to equilibria and capacities of the various fibers. Dilutions for gas chromatography olfactometry (GCO) were monitored as a result of compound mass distribution between the headspace and liquid however, there was no indication of how to deal with liquid phase saturation [7]. The requirement for calibration of fibers by use of standard solutions has also been addressed, considering the various parameters needed to calculate supposed headspace concentrations for dilution analysis [8]. Comparing static headspace vapor sampling with SPME for citrus volatile compounds in standard solutions below the solubility limits [parts per billion (ppb)] of most of the compounds resulted in linear calibration curves however, curves were nonlinear at [parts per million (ppm)] concentrations above solubility limits [9]. This study [9] determined that there were analytical differences between actual juices, but these differences were nonquantitative. 

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