Vapor pressure headspace analysis

The following table provides the temperature dependence of the saturated vapor pressure and vapor density of water. This information is useful in gas chromatographic headspace analysis and for SPME sampling.12... 

Control of the vapor pressure in the headspace over a solid can also be used to maintain a relative humidity over a solid. As shown in Eq. (4), the relative humidity is directly correlated to the partial pressure of water in the vapor phase. To utilize this technique for relative humidity control, the headspace above the sample must be completely evacuated prior to analysis. Pure water vapor can then be carefully admitted to the vapor phase. Because only water vapor is present, the pressure measured over the system is directly related to the relative humidity over the sample. ... 

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. 

No peaks could be detected for samples with arsenic concentrations below 100 ppm (Thurow et al., 1998), since the concentration of the analyzed compound in the head-space is to low due to low vapor pressure. Thus headspace analysis is suitable for rapid screening procedures but can not be used for trace analysis. The reproducibility of the headspace results was about 2%. 

Static headspace analysis The sample for chromatographic analysis is taken from a closed vessel where the material under study reaches equilibrium with its vapor at a predetermined temperature. Experimental conditions that can influence the results are the temperature and the sample-withdrawal technique. The concentration of the analytes in the gaseous phase can be increased by raising the temperature, adding an electrolyte, and varying the pH. Solutes with low vapor pressures cannot be detected. 

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. 

Headspace GC can also be used to determine activity coefficients. Hussam and Carr (22) carried out a meticulous study that produced a device for automated measurements of both solute activity coefficients and vapor pressure (see also Section 12.4). The device allowed for rapid sample analysis, better than 0.01°C temperature control, the ability to work at low sample concentrations (mole fractions >0.01), minimal equilibrium perturbation by the sampling process, and the ability to vary solvent composition automatically. 

Headspace analysis involves chromatographing the vapors derived from a sample by warming it in a partially fQled vial sealed with a septum cap. After equilibration under controlled conditions, the proportions of volatile sample components in the headspace above the sample are representative of those in the bulk sample. The headspace vapors, which are under slight positive pressure, are sampled by a modified and automated injection system or gas s)uinge, and injected onto the column (Fig. 3(a)). The procedure is useful for mixtures of volatile and nonvolatile components, such as residual monomers in pol)uners, alcohol or solvents in blood samples (Fig. 3(b)), and flavors and perfumes in manufactured products, as it simplifies the chromatograms and protects the column from contamination by nonvolatile substances. 

Flavor release depends also on oil content, which affects the partition of aroma compounds during the different emulsion phases (lipid, aqueous, and vapor) [6]. In fact, lipids absorb and solubilize lipophilic flavor compounds and reduce their vapor pressures [7-8], as explained by mathematical models [9], headspace analysis [10], and sensory analysis [11 12]. 

How then do the techniques differ For this, the terms recovery and sensitivity must be defined. For both methods, the recovery depends on the vapour pressure, the solubility and the temperature. The effects of temperature can be dealt with because it is easy to increase the vapour pressure of a compound by raising the temperature during the vaporization step. With the P T technique, the term percentage recovery is used. This is the amount of a compound which reaches the gas chromatograph for analysis relative to the amount which was originally present in the sample. If a sample contains 100 pg benzene and 90 pg reach the GC column, the percentage recovery is 90%. In the static headspace technique, a simple expression like this cannot be used because it is possible to use a large... 

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