Headspace conditioning temperature

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. 

Harsher conditions may employ pressurized oxygen headspaces, for which Parr type apparatus are employed. Incorporation of added metal impurities (commonly copper and iron salts) in conjunction with oxygen headspace has been reported (60). One research group utilized high temperatures (80°C), copper (II) salts, and oxygen gas headspace to study the... 

There are many factors involved in optimizing static headspace extraction for extraction efficiency, sensitivity, quantitation, and reproducibility. These include vial and sample volume, temperature, pressure, and the form of the matrix itself, as described above. The appropriate choice of physical conditions may be both analyte and matrix dependent, and when there are multiple analytes, compromises may be necessary. 

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. 

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). 

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). 

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... 

In the direct extraction mode, the SPME partitioning resembles liquid-liquid extraction and, besides the type of fiber coating used, parameters important for optimal recovery are. e.g. agitation technique and pH. Eor headspace SPME, the most important parameters are the vial volume, the headspace-to-sample voliune ratio and the equilibrium temperature, i.e. similar parameters as in traditional HS [24,25]. Although the theoretical treatment of SPME relies on extraction under equihbrium conditions, it is not necessary to estabhsh full equihbrium to perform quantitative analysis. It was shown by Ai that quantitative data can be obtained from both two-and three-phase systems imder non-equilibrium conditions [26-28]. This ex-... 

Quantitative or semi-quantitative determination of analytes by SPME requires working within the linear dynamic range of the SPME fiber. If the linear dynamic range is exceeded, the extracted amount of analyte will not reflect the amount of analyte in the sample. Figure 2 shows the normalized peak areas for the headspace extractions of different amounts of powdered polyamide 6.6 [67]. The extraction time and temperature were 45 min and 80 °C. Under the given conditions, the dynamic range of the PDMS/DVB fiber was linear if the polyamide sample size was between 1 and 100 mg. For the... 

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