Partitioning of Volatile Compounds

Volatilization (also referred to as vaporization or evaporation) is the conversion of a chemical from the sohd or hquid phase to a gas or vapor phase. The partitioning of a volatile compound in the subsurface environment comprises two distinct patterns volatilization of contaminant molecules (from the liquid, sohd, or adsorbed phase) and dispersion of the resulting vapors in the subsurface gas phase or the overlying atmosphere by diffusive and turbulent mixing. Even though the two processes are fundamentally different and controlled by different chemical and environmental factors, they are not wholly independent under natural conditions only by integrating their effects can volatilization be characterized. 

The volatilization process changes the contaminant from a sohd or hquid state, where the molecules are held together by intermolecular forces, into a vapor phase. The molar heats of fusion (A//p, volatilization (AH), and sublimation (AH) are related according to the Bom-Haber cycle by 

Even at low temperature some molecules may overcome the energy barrier of the cohesive forces and escape from the sohd or hquid state into the gaseous phase. 

The vapor dispersion process is described mathemahcally as a vapor flux (7) through any plane at a height z, and is expressed by 

Taylor and Spencer (1990) pointed out that a laminar layer can be regarded as the limiting distance above the soil surface to which the smallest eddies of the overland turbulent flow can penetrate. Therefore, above this layer, transport takes place through eddy diffusion, and the corresponding vapor flux can be described by 

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Pollien, P., Jordan, A., Lindinger, W., and Yeretzian, C. Liquid-air partitioning of volatile compounds in coffee dynamic measurements using proton-transfer-reaction mass spectrometry, Int. J. Mass Spectrom., 228(l) 69-80, 2003. 

Static headspace GC/MS. The partitioning of volatile and semivolatile compounds between two phases in a sealed container. An aliquot of the headspace gas generated is injected onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

Vejrosta, J., Novak, J., Jonsson, J. (1982) A method for measuring infinite-dilution partition coefficients of volatile compounds between the gas and liquid phases of aqueous systems. Fluid Phase Equil. 8, 25-35. 

Thoms, S.R., Lion, L.W. (1992) Vapor-phase partitioning of volatile organic compounds a regression approach. Environ. Toxicol. Chem. 11, 1377-1388. 

Dewulf, J., van Langenhove, H., Grare, S. (1999) Sediment/water and octanol/water equilibrium partitioning of volatile organic compounds temperature dependence in the 2-25°C range. Water Res. 33, 2424—2436. 

Abraham, M.H., Ibrahim, A., Zhao, Y., Acree, W.E. A data base for partition of volatile organic compounds and drugs from blood/plasma/serum to brain, and an LFER analysis of the data. J. Pharm. Sci. 2006, 95, 2091-100. 

Headspace-GC-MS analysis is useful for the determination of volatile compounds in samples that are difficult to analyze by conventional chromatographic means, e.g., when the matrix is too complex or contains substances that seriously interfere with the analysis or even damage the column. Peak area for equilibrium headspace gas chromatography depends on, e.g., sample volume and the partition coefficient of the compound of interest between the gas phase and matrix. The need to include the partition coefficient and thus the sample matrix into the calibration procedure causes serious problems with certain sample types, for which no calibration sample can be prepared. These problems can, however, be handled with multiple headspace extraction (MHE) [118]. Headspace-GC-MS has been used for studying the volatile organic compounds in polymers [119]. The degradation products of starch/polyethylene blends [120] and PHB [121] have also been identified. 

Platts, J.A. and Abraham, M.H., Partition of volatile organic compounds from air and from water into plant cuticular matrix an LFER analysis, Environ. Sci. Tech., 34, 318-323, 2000. 

Partitioning of volatile substances between the liquid and gas phases is mainly governed by aroma compound volatility and solubility. These physicochemical properties are expected to be influenced by wine constituents present in the medium, for instance polysaccharides, polyphenols, proteins among others. Consideration of the physicochemical interactions that occur between aroma compounds and wine constituents is necessary to understand the perception of wine aroma during consumption. The binding that occurs at a molecular level reflects changes at a macroscopic level of the thermodynamic equilibrium, such as volatility and solubility, or changes in kinetic phenomena. Thus, thermodynamic and dynamic approaches can be used to study the behaviour of aroma compounds in simple (model) or complex (foods) media. 

Nevertheless, the most studied ethanol effect is related to its capacity to modify solution polarity, thus altering the gas-liquid partition coefficient. An increase in ethanol content has been shown to decrease the activity coefficients of many volatile compounds in wine because of an increase in solubility (Voilley et al. 1991). Hartmann et al. (2002) showed a decrease in the recovery of 3-alkyl-methoxy-pyrazynes extracted with a divinylbenzene/carboxen SPME fibre from wine model systems when the ethanol content increased from 0% to 20%. Similarly, Whiton and Zoecklein (2000) reported that a small increase in ethanol content (from 11 % to 14%), in general, reduced the recovery of typical wine volatile compounds. Both of these studies suggest that increasing the alcohol content will reduce the release of volatile compounds from wines. 

Aroma compounds selected were isoamyl alcohol (lOOpL/L), isoamyl acetate (lOOpL/L), ethyl hexanoate (lOOpL/L), ethyl octanoate (40pL/L), ethyl decanoate (lOpL/L), octanal (lOOpL/L), P-ionone (lOOpL/L), y-decalactone (lOOpL/L), supplied by Aldrich (Steinheim, Germany). They are all slightly soluble in water except for isoamyl alcohol which is soluble in water. The hydrophobic constants of volatile compounds are expressed by Log P where P is the partition coefficient of the compound in water/octanol system. 

Activity Coefficients of Volatile Compounds. The headspace technique was used to determine the activity coefficients of volatile compounds as described previously (11). The headspace system flask contained 10 to 20 mL of the model wine with the diluted volatile compoimd, at 25°C. The flow rate of nitrogen gas in the flask was 5 to 10 mL/min. The concentration in volatile compound in the vapor phase was analysed by gas chromatography. The conditions were reported in a previous paper (11). The relative volatility of the volatile compound can be expressed as a partition coefficient K and activity coefficient y... 

Yeast Cell Walls. Interactions between aroma substances and yeast walls induce to a modification of the volatility of some aroma compounds in the model wine (76/Yeast walls do not bind a specific chemical class of volatile compounds (Table II). The volatility of octanal, an aldehyde and of ethyl hexanoate, an ester, decreases by 14% with yeast walls at 1 g/L. The effect of walls is greater on the volatility of ethyl octanoate than that of the other aroma compounds the partition coefficient decreases by 45% for ethyl octanoate in the presence of 1 g/L yeast cell walls. 

The sensory effects of volatile compounds depend on the chain length, type of functional group, and positional and geometric isomerism. The same aldehyde can produce different odors and flavors in aqueous and oil solutions, the threshold being much lower in the former medium. In oil-in-water emulsions such as mayonnaise, the oil-water partition coefficient (Pq/ ) of a given compound affects its sensory impact on the emulsion (Jacobsen, 1999). 

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