Partition coefficient headspace

Where is the initial analyte concentration in the liquid phase, C( the concentration of analyte in the gas phase, K the gas-liquid partition coefficient for the analyte at the analysis temperature, V, the volume of liquid phase, and V, the volume of gas phase (318-321,324,325). From equation (8.3) it can be seen that the concentration of the analyte in the headspace above a liquid in equilibrium with a vapor phase will depend on the volume ratio of the geis and liquid phases and the compound-specific partition coefficient which, in turn, is matrix dependent. The sensitivity 1 of the headspace sampling method can be increased in some instances adjusting the pH, salting out or raising the... 

MHE can be used for substances of high volatility with a small partition coefficient. Method is based on a stepwise gas extraction at equal time intervals. Normal headspace chromatogram is run, a fraction of the gas phase exhausted, and a second headspace chromatogram is run. -The difference in petUc areas provides a measure of the total peak area of the analyte. 

Principles and Characteristics In boiling under reflux procedures a small amount of ground polymer (typically 3g) is placed in a headspace jar (typically 100 mL) and solvent (typically 30 mL) is added. After sealing, the jar is placed in an oven at a temperature where the solvent slowly refluxes. The solvent is, therefore, at the highest temperature possible without applying an external pressure. Consequently, reflux extractions tend to be much faster than Soxhlet extractions. Examples are Soxtec , Soxtherm , FEXTRA and intermittent extraction. Whilst, in theory, partitioning of the analyte between the polymer and solvent prevents complete extraction, this hardly ever constitutes a problem in practice. As the quantity of solvent is much larger than that of the polymer, and the partition coefficients usually favour the solvent, very low additive levels in the polymer result at equilibrium. Any solvent or solvent mixture can be used. 

Diterpenes require more than 80 min to reach equilibrium. This is expected for compounds that exhibit low vapour pressure in combination with a high partition coefficient between the fibre coating and the gaseous phase. During headspace SPME the amount of such compounds present in the gaseous phase is absorbed by the fibre coating at a much faster rate than their release from the matrix, thus the amount of mass in the headspace at any time is small and a long time is required to reach equilibrium [58]. 

Bakierowska, A.-M., Trzeszqzynski, J. (2003) Graphical method for the determination of water/gas partition coefficients of volatile organic compounds by a headspace gas chromatography technique. Fluid Phase Equil. 213, 139-146. 

Kolb, B., Welter, C., Bichler, C. (1992) Determination of partition coefficients by automatic equilibrium headspace gas chromatography by vapor phase calibration. Chromatographia 34, 235-240. 

While the gases used in stripping are usually air, nitrogen, or helium, electrolytically evolved hydrogen has been used as a collector for hydrocarbons [49]. In this technique, the gas is not passed through a column of adsorbent, but instead collects in the headspace of the container. Since the volume of seawater and of hydrogen are known, the hydrocarbon concentration in the headspace can be used to calculate the partition coefficients and the concentration of hydrocarbon in the seawater. 

Brachet, A. and Chaintreau, A. Determination of air-to-water partition coefficients using automated multiple headspace extractions. Anal Chem., 77(10) 3045-3052, 2005. 

Chaintreau, A., Grade, A., and Munoz-Box, R. Detertrrinationof partition coefficients and quantification of headspace volatile compounds, Anal. Chem., 67(18) 3300-3304, 1995. 

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]. 

In flavor analysis, the most frequent use of volatile traps is in analyzing the flavor compounds in foods using purge-and-trap or dynamic headspace, followed by GC-MS or GCO. Additionally, the traps can be used to measure static headspace and air-matrix partition coefficients where air is pushed out of an equilibrated cell containing the sample onto a volatile trap (Chaintreau et al., 1995). Volatile traps have been also used for flavor release measurements during eating (Linforth and Taylor, 1993) or simulated eating (Roberts and Acree, 1995). 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

Kawata et al. [ 128] have described the effects of headspace conditions on recoveries of volatile organic compounds from sediments and soils. Hewitt [129] compared three vapour partitioning headspace and three solvent extraction methods for the preparation of soil samples for volatile organic carbon determination in soils. Methanol extraction was the most efficient method of spiked volatile organic carbon recovery, which depended on the soil organic carbon content, the octanol-water partitioning coefficients of analytes and the extraction time. 

A total of 20 partition coefficients were measured using 1-pentanol, 2-methyl 2-hexanol, 2-methyl 3-hexanol, and 2,4-dimethyl 3-pentanol as the alcohols and C-6, C-8, C-10, C-12, and C-16 as the n-alkanes. For each alcohol, a 15-ml. aliquot of water with 1000 mg/L alcohol was placed in a 35 ml. vial with 15-ml. aliquot of NAPL and immediately sealed. The alcohol solutions were prepared quickly and immediately sealed with Teflon coated septa with no headspace to minimize volatilization. The samples were thoroughly shaken for 1 hour and allowed to separate for 24 hours. These mixing and separation times were sufficient for equilibration. Immediately following separation, the aqueous samples were analyzed for alcohol using a split injection Varian 3300 GC with FID detection. The partition coefficient was calculated by mass balance. 

The following table provides the partition coefficients (or distribution coefficients), K = Cs/Cv (solid/vapor), at various temperatures, for application in gas chromatographic headspace analysis.1,2 The values marked with an asterisk were determined from a linear regression of experimental data. 

The major factors that control headspace sensitivity are the analyte partition coefficient (K) and phase ratio (/ ). This was demonstrated by Ettre and Kolb [14] ... 

Figure 4.4. Influence of temperature on headspace sensitivity (peak area values, counts) as a function of the partition coefficient K from an aqueous solution with /i = 3.46. The volatiles plotted above are ethanol (1), methyl ethyl ketone (2), toluene (3), u-hexane (4), and tetra-chloroethylene (5). [Reprinted with permission from Ref. 15 (p. 26). Copyright John Wiley Sons.l...

The advantage to MHE is that sample matrix effects (which are mainly an issue only with solid samples) are eliminated since the entire amount of analyte is examined. This examination is done by performing consecutive analyses on the same sample vial. With the removal of each sample aliquot from the vial, the partition coefficient K will remain constant however, the total amount of analyte remaining in the sample will decline as each analysis is performed and more of the analyte is driven up into the vial headspace for removal and analysis. Chromatograms of each injection of sample show... 

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. 

Keymeulen, R., De Bruyn, G., and Van Langenhove, H., Headspace gas chromatographic determination of the plant cuticle-air partition coefficients for monocyclic aromatic hydrocarbons as environmental compartment, J. Chromatogr. A, 774, 213-221, 1997. 

Attention should be given when determining odor or taste threshold levels for a substance in a food or other testing medium, that during the taste test the compound studied can be detected in the gas headspace in contact with the food where the partition coefficient KG/F plays an important role. 

Solid phase micro-extraction (SPME) allows isolation and concentration of volatile components rapidly and easily without the use of a solvent. These techniques are independent of the form of the matrix liquids, solids and gases can be sampled quite readily. SPME is an equilibrium technique and accurate quantification requires that the extraction conditions be controlled carefully. Each chemical component will behave differently depending on its polarity, volatility, organic/water partition coefficient, volume of the sample and headspace, speed of agitation, pH of the solution and temperature of the sample (Harmon, 2002). The techniques involve the use of an inert fiber coated with an absorbant, which govern its properties. Volatile components are adsorbed onto a suitable SPME fiber (which are usually discriminative for a range of volatile components), desorbed in the injection chamber and separated by a suitable GC column. To use this method effectively, it is important to be familiar with the factors that influence recovery of the volatiles (Reineccius, 2002). 

There are different experimental methods for determining the gas-liquid partition coefficients leading to the determination of activity coefficients at infinite dilution y . The most frequently used methods are dynamic and static headspace methods. 

Most of the static headspace methods determine the partition coefficient by quantifying volatile concentration above a sample by gas-chromatography. The vapour phase calibration method (VPC) uses an external vapour standard for calibration. One must assure that the pure component is completely vaporized before injection. A widely employed alternative is the Liquid calibration static headspace (LC-SH) method (YoiWey et al. 1991 Nedjma 1997). A third approach uses HS-SPME. SPME may be used to determine partition coefficients if short sampling times are applied the process must only sample the headspace and not disrupt the equilibrium (Jung and Ebeler 2003). This method has become very popular to study the effect of wine macromolecules on the liquid-vapor equilibrium, (Whiton and Zoecklein 2000 Escalona et al. 2002 Hartmann et al. 2002 Aronson and Ebeler 2004). 

Some static headspace methods do not require an external calibration and are based on measurements performed at thermodynamic equilibrium between liquid and gas phase. In the phase ratio variation method (PRV) described by Ettre and Collaborators (1993), the partition coefficient calculation is based on the fact that the headspace concentration changes as a function of the phase volume ratio (gas and liquid phases), while the partition coefficient remains constant. This method has been recently applied to study the interactions between aroma compounds and macromolecules in different food systems (Savary et al. 2006, 2007) but so far not to the wine. 

Athes et al. (2004) compared the data from three static headspace methodologies (VPC, PRV and LC-SH) for determining gas/liquid partition coefficients of two aroma compounds in hydroalcoholic, multicomponent solutions at infinit dilution. They found that PRV was a simpler method compared to VPC and LC-SH and that VPC and PRV were more accurate than LC-SH since errors due to gas leaks and adsorption in gastight syringes are avoided. They suggested that these issues could be responsible for significant bias (50% lower values) obtained when using the LC-SH method. Nevertheless, all three methods were able to find an effect of ethanol (up to 20%) on the release of aroma compounds from their model system (Fig. 8F.1). 

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