Headspace conditioning time

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. 

The vial equilibration method is the most common in vitro method for determining partition coefficients for volatile or semivolatile materials and has been used most successfully for volatile organic solvents (Gargas et al., 1988). Tissues are harvested from the species of interest and incubated with the test compoxmd imtil equilibrium is reached between the tissue and the headspace in the vial. The blood/air or tissue/air partition coefficients are given by the ratio of the concentrations of the chemical in the blood or tissue relative to its concentration in the headspace. Tissue-blood partition coefficients are calculated from the respective tissue/air and blood/air values. A number of operational equations have been derived to calculate these ratios xmder specific experimental conditions. Time to steady state is critical and should be optimized for the test compoxmd. Metabolism of the compound in exposed tissue samples must be controlled. Analysis is performed by gas chromatography in a verified linear range. Human tissues can be obtained from tissue bank organizations to provide species specificity to models developed with human data. To estimate... 

Two basic methods are used for SPME direct immersion of the fibre into the sample and headspace sampling. Experimental parameters comprise the polarity of the sample matrix and coating material, solvent and salting-out. Other parameters for optimisation of SPME conditions include desorption time, injector port temperature and initial oven temperature. 

SPME can be >95% reproducible. However, the following conditions must be carefully controlled to obtain reproducible results sample temperature, exposure time to the headspace, sample equilibration time (if using a closed container), sample flow rate (if using a dynamic system), sample size (both food sample and container), stirring speed (if stirred), and composition of the sample. 

Reineccius and Liardon [207] studied volatiles evolved from heated thiamine solutions. Samples of 2% thiamine hydrochloride in various 0.2M buffers were heated under various conditions. A temperature of 40°C and a sampling time of 45 min were found to minimize artifact formation and yet produce sufficient volatiles for analysis. Nitrogen was used as the purge gas at a flow rate of 50 ml/min. Several materials were evaluated as absorbents, with graphite found to be the optimum. A microwave desorption system was used to rapidly desorb the trapped volatiles onto a fused silica capillary column. Twenty-five compounds were identified in the headspace of the heated thiamine solutions. 

Experiments were conducted in which the gas mixture flowing to the culture flask contained MJ as well as ethylene [in the optimum combination with 17% (v/v) 02 and 1.5 % (v/v) C02]. The results given in Fig. 7 demonstrate clearly that ethylene and MJ co-mediate the induction of paclitaxel biosynthesis. When MJ was supplied by passing an air stream over concentrated MJ in a flask, no product formation was noted. When the optimal gas mixture was applied to the headspace of the culture without first passing over the pure MJ in a flask, the level of paclitaxel accumulation was about 20% of that found in the culture to which both MJ and C2H4 had been applied by means of the vapor phase. The 8-day period of time before induction may represent the time required for the critical concentration of MJ to dissolve in the culture medium under the flow (40 ml total gas mixture min-1) and temperature (21 °C) conditions of the experiment. 

Data from the stoppered culture tests was also used to determine the effect of declining 02 concentration on the rate of metabolism of the cells. The specific 02 consumption rate was determined to depend linearly upon the 02 concentration throughout the range of headspace concentrations measured (0-31 %). This seems to contradict the experience of other researchers [31-33, 50], who report saturation kinetics in plant cell cultures whenever 02 concentration exceeded 4-5% (gas phase). In the stoppered culture tests, at the same time 02 concentration declined, C02 and C2H4 concentrations increased. Other unidentified compounds may also have been produced. The declining rate of metabolism observed may have been caused by conditions other than declining oxygen. 

Headspace GC-MS is the preferred method for the analysis of very volatile migrants. Practically the same GC conditions can be used as for GC-MS. Due to the coupling to MS, identification is also relatively easy. The heating time and temperature are the main experimental variables. The major drawback of headspace GC-MS is quantification. As a result of the principle of headspace GC-MS, i.e., partitioning of compounds between gas phase and liquid phase, the chemical properties will have a significant influence on the partition of each molecule between gas phase and liquid phase. Therefore, quantification is almost solely possible by using external standards of the same compound (Grob and Barry 2004). 

Early experiments in liquids were quite variable for many reasons. The conductivity technique, which was used in the gas phase to measure dose, was not applicable to the liquid phase. Reactions were measured using dissolved radium salts or radon gas as the ionization source. Some thought the chemistry was due to the reactions with radium however, it was soon recognized that it was the emitted rays that caused the decomposition. Both radium and radon could cause radiation damage. Because the radon would be partitioned between the gas and liquid phase, the amount of energy that was deposited in the liquid depended critically on the experimental conditions such as the pressure and amount of headspace above the liquid. In addition, because the sources were weak, long irradiation times were necessary and products, such as hydrogen peroxide, could decompose. 

Several temperature-catalyzed stability tests are used in evaluating the oxidative stability of oils and fats. The oldest method is the Schaal oven test (39). It is inexpensive but subjective, because it uses organoleptic and odor intensities in the procedure and still requires days to obtain the result. This approach has been standardized into a recommended practice (AOCS method Cg 5-97). In the active oxygen method (AOM) (39), the development of peroxide is measured with time. As the formation and decomposition of peroxides are dynamic processes, the results obtained by this method do not correlate well to the actual stability of the oils and fats observed under practical application conditions. Other methods that have been based on oxygen absorption are the gravimetric (59) and the headspace oxygen concentration measurement (60, 61). 

Fig. 2 An example chromatogram illustrating the determination of headspace oxygen by GC using a PLOT molecular sieve column with thermal conductivity detection. Chromatographic conditions were carrier gas helium (2mLmin ) oven temperature 26 C inlet 160 C, split mode, 10 1 split ratio, split flow of 20mLmin injector 160°C run time lOmin TCD detector 160 C. (From Ref. p. 41. Copyright 2002 Advanstar Communications Inc.)...

Automated Procedures Some of the difficulties associated with manual procedures can be eliminated with an automated headspace sampler. Such a device has been described in the literature (14), and is commercially available. The schematic diagram of such a semi-automatic headspace analyzer is shown in Figure 3. Precise control of times and temperatures, as well as the capability to hold samples at high temperatures, produces better chromatographic reproducibility. We have been able to analyze ppm levels of ethyl dodecanoate in aqueous solution with this system. Significant amounts of water vapor are introduced into the gas chromatographic column under these conditions. For this reason, columns with non-polar liquid phases or bonded Carbowax-type liquid phases... 

Concentrations of methanethiol measured in headspace samples of the experimental cheeses are summarized in Table II for the analysis times of 1 day, 21 days and 4 months for each of the two ripening conditions employed. Notably, the cheese made with only encapsulated buffer did not contain methanethiol after 1 day at either temperature. However, the encapsulated methioninase system yielded significant amounts of methanethiol at 1 day, and continued to increase through 4 months. Generally, the final concentration of methanethiol in the encapsulated-buffer control... 

Thus, for many of the compounds that are extracted by this method, the equilibrium state does not quantitatively remove the analyte from solution. Rather from 2 to 20% of the analyte may be removed (Louch et al., 1992), although the application may be quantitative if standards are treated in an identical way. Another important point brought out in this study is that at high K values (>1000) the time to equilibrium is much longer than at lower K values. This effect is the result of the slow diffusion of the nonpolar analytes into the coated phase (Louch et al., 1992). Thus, for compounds with high K values (>1000) a thin film is used, 7 pm (Supelco) in order to attain quick equilibrium conditions. In the case of headspace analysis where sorption occurs from the gaseous state rather than the liquid state, the rate of equilibrium is much faster because of faster diffusion rates around the fiber and the lack of the water boundary layer. Thus, equilibrium for headspace analysis is approximately 10 times faster than the liquid-state sorption. 

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