Headspace analysis, solubility

Advantages and disadvantages of HS-GC over regular GC are summarised in Table. 4.26. HS-GC fingerprinting chromatograms obviously include only the volatile components present and do not provide a complete picture of sample composition on the other hand, when solvent extraction is used, all the soluble sample constituents are removed, including also those having no appreciable vapour pressure at the equilibration temperature. Headspace analysis enhances the peaks of volatile trace components. 

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. 

Surfactants can act like lipids or emulsifiers in solubilizing flavor materials in surfactant micelles. Headspace analysis techniques were used to follow the release of several common dentifrice flavorants from a solution containing the surfactant sodium lauryl sulfate. Water/micelle partition coefficients were derived to describe the solubilization of the flavorants in tiie surfactant micelle (76). Initially, the flavor is solubilized in the surfactant micelle. As both the micelle and flavor concentration decrease on dilution, flavor compounds, which are highly soluble in the micelle, preferentially increase in the headspace [HGURE11]. 

FIGURE 11. Headspace analysis of solutions of menthol [- -], carvone [- -] and cineole [-A-] in SLS (initial concentration, SLS=1.0%, flavorant=0.0075%) showing differences in release of flavor compounds according to their solubility in SLS micelles [Adapted from ref. 77]. 

The following table provides data on the common salts used for salting out in chromatographic headspace analysis, as applied to direct injection methods and to solid phase microextraction.1 2 Data are provided for the most commonly available salts, although others are possible. Sodium citrate, for example, occurs as the dihydrate and the pentahydrate. The pentahydrate is not as stable as the dihydrate, however, and dries out on exposure to air, forming cakes. Potassium carbonate occurs as the dihydrate, trihydrate, and sesquihydrate however, data are provided only for the anhydrous material. The solubility is provided as the number of grams that can dissolve in 100 ml of water at the indicated temperature. The vapor enhancement cited is the degree of increase of the concentration of vapor over the solution of a 2% (mass/mass) ethanol solution in water at 60°C.3... 

Headspace analysis, 151 Hildebrand s solubility parameter, 49, 159 History of chromatography, 1-4 Homologous series plots, 78, 80 Hydrogen bonding, 45, 141... 

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. 

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. 

Gas evolution from the hydrolysis of Grignard reagents can be used for the activity analysis. However, care must be taken in the calibration of standards, because the resulting gas will have some solubility in the solvent. Typically, the gas is analyzed by GC, taking the gas from the headspace of a closed system. The obvious limitation in this method is that only a selected amount of Grignard reagents (C4 or less) can be used, owing to the volatility of the hydrolysis products. 

Dissolved gases must be extracted from the aqueous system before analysis. This is usually accomplished by a simple gas-water partition into a vapour phase followed by standard headspace measurement techniques (McAuliffe, 1966). Alternatively a so-called stripper continuously partitions the dissolved gases into a carrier gas which is then sent to a gas chromatograph for analysis (Mousseau and Williams, 1979 Aldridge and Jones, 1987). These separations are aided by the very low solubility of the light hydrocarbon gases. 

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. 

For GC analysis, HS is a preconcentration technique particularly suitable for the sampling of volatile organic compounds in air, water, and solids. Few reports have been published on the use of static headspace in the analysis of free amines in aqueous samples because of the high polarity and solubility in water of these compounds." In one experiment," static headspace preconcentration was developed for the gas chromatographic analysis of aliphatic amines in aqueous samples. A liquid-gas ratio of 1, an incubation temperature of 80°C (15 min), a pH of 13.7, and a mixture of salts (NaCl and K2SO4) at saturation concentration gave a maximal headspace amine concentration (Table 11.4). 

This simple technique permits the quantitative analysis of volatile compounds in various liquid, semi-liquid or solid foods, in biological fluids and tissues, and environmental contaminants in water, air and soils. The method is very sensitive to the equilibrium solute distribution between phases at the temperature selected for the analysis. Equilibration is greatly dependent on the solubility and viscosity of the samples. This method is particularly suited to highly volatile compounds because they have a favorable equilibrium between liquid (or solid) phase and its headspace, producing a higher concentration of volatile compounds in the headspace. 

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. 

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... 

For the analysis of volatile compounds the sample material, for example rice or leaves are weight in equal amounts into headspace vials and capped. For the analysis of the extracted metabolites plant material (leaves) gets homogenized under liquid nitrogen. About 50 mg are applied to extraction with a water/chloroform/methanol mixture to extract water soluble metabolites. The polar phase is dried in a vacuum centrifuge. A 2-step derivatization can be applied First a methoxyamination (methoxyamine hydrochloride in pyridine)... 

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