Headspace extraction static

In this chapter, techniques for the extraction of volatile compounds from various matrices are described. Details are provided on the basic theory and applications of each technique with a focus on providing useful information to the analyst working on the analysis of volatile analytes from difficult matrices. Since the analytes are volatile, most of the techniques are geared toward preparation of samples for gas chromatography, although they are appropriate for many instrumental methods. The chapter is heavily referenced and the reader should refer to the appropriate references for more details on a particular technique or application. 

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The principles behind MAP liquid-phase and gas-phase extractions are fundamentally similar and rely on the use of microwaves to selectively apply energy to a matrix rather than to the environment surrounding it. MAP gas-phase extractions (MAP-HS) give better sensitivity than the conventional static headspace extraction method. MAP-HS may also be applied in dynamic applications. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix [477]. 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

Because SPME extracts compounds selectively, the response to each compound must be calibrated for quantification. A specific compound can be quantified by using three GC peak area values from solvent injection, static headspace (gas-tight syringe), and SPME. The solvent injection is used to quantify the GC peak area response of a compound. This is used to quantify the amount of the compound in the headspace. The SPME response is then compared to the quantified static headspace extraction. These three stages are necessary because a known gas-phase concentration of most aroma compounds at low levels is not readily produced. A headspace of unknown concentration is thus produced and quantified with the solvent injection. Calibration must be conducted independently for each fiber and must include each compound to be quantified. 

For a compound to contribute to the aroma of a food, the compound must have odor activity and volatilize from the food into the head-space at a concentration above its detection threshold. Since aroma compounds are usually present in a headspace at levels too low to be detected by GC, headspace extraction also requires concentration. SPME headspace extraction lends itself to aroma analysis, since it selectively extracts and concentrates compounds in the headspace. Some other methods used for sample preparation for aroma analysis include purge-and-trap or porous polymer extraction, static headspace extraction, and solvent extraction. A comparison of these methods is summarized in Table Gl.6.2. 

Static headspace extraction is also known as equilibrium headspace extraction or simply as headspace. It is one of the most common techniques for the quantitative and qualitative analysis of volatile organic compounds from a variety of matrices. This technique has been available for over 30 years [9], so the instrumentation is both mature and reliable. With the current availability of computer-controlled instrumentation, automated analysis with accurate control of all instrument parameters has become routine. The method of extraction is straightforward A sample, either solid or liquid, is placed in a headspace autosampler (HSAS) vial, typically 10 or 20 mL, and the volatile analytes diffuse into the headspace of the vial as shown in Figure 4.1. Once the concentration of the analyte in the headspace of the vial reaches equilibrium with the concentration in the sample matrix, a portion of the headspace is swept into a gas chromatograph for analysis. This can be done by either manual injection as shown in Figure 4.1 or by use of an autosampler. 

The ease of initial sample preparation is one of the clear advantages of static headspace extraction. Often, for qualitative analysis, the sample can be placed directly into the headspace vial and analyzed with no additional... 

In static headspace extraction, sample preparation for liquid samples is usually quite simple—most often, the sample can just be transferred to the headspace sample vial and sealed immediately following collection of sample to minimize storage and handling losses [13],... 

Optimizing Static Headspace Extraction Efficiency and Quantitation... 

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. 

For the analysis of trace quantities of analytes, or where an exhaustive extraction of the analytes is required, purge and trap, or dynamic headspace extraction, is preferred over static headspace extraction. Like static head-space sampling, purge and trap relies on the volatility of the analytes to achieve extraction from the matrix. However, the volatile analytes do not equilibrate between the gas phase and matrix. Instead, they are removed from the sample continuously by a flowing gas. This provides a concentration gradient, which aids in the exhaustive extraction of the analytes. 

There are many techniques available for the preparation of volatile analytes prior to instrumental analysis. In this chapter the major techniques, leading primarily to gas chromatographic analysis, have been explored. It is seen that the classical techniques purge and trap, static headspace extraction, and liquid-liquid extraction still have important roles in chemical analysis of all sample types. New techniques, such as SPME and membrane extraction, offer promise as the needs for automation, field sampling, and solvent reduction increase. For whatever problems may confront the analyst, there is an appropriate technique available the main analytical difficulty may lie in choosing the most appropriate one. 

The equipment for static headspace extraction consists of a container, where equilibrium takes place, a device that heats the container at a constant temperature and an injection device, which transfers a portion of the headspace gas to the gas chromatograph. The container is a glass vial of between 5 ml and 25 ml capacity, which is sealed with a septum coated with polytetrafluoroethylene (PTFE) and an aluminum cap, using a crimp. Injection... 

One of the main advantages of static headspace extraction is how easy the sample preparation is in the case of qualitative analysis, it suffices to place the sample in a vial and seal it with a PTFE septum and an aluminum lid however, for quantitative analysis, it is necessary to xmderstand and optimize the effects of the matrix, in order to obtain good sensitivity and, above all, accuracy. 

Several factors must be optimized in a static headspace extraction in order to obtain a method with the desired extraction sensitivity, reproducibility and efficiency. These factors include the volume of the used vial, the temperature and pressure levels, and how the sample is to be prepared. 

The main factors controlling the sensitivity of the static headspace extraction are the analyte partition coefficient (K), which is the ratio between the analyte concentrations in the liquid phase with the concentration in the gas phase ... 

Where A is the area of the chromatographic peak obtained for the analyte, is the concentration of analyte in the headspace, C° is the concentration of analyte in the liquid sample, K is the partition coefficient, and P is the ratio volume of the phases. The partition coefficient depends on the extraction temperature, while P is determined by the relative volume between the two phases In static headspace extraction the sensitivity depends on the solubility of the analyte in the matrix for analytes with a high partition coefficient, the most important parameter is the extraction temperature, since most of the analyte is in the liquid phase and it can only be passed into the headspace by heating the vial on the other hand, for analj es with low partition coefficients, they are already present in the headsp>aoe even without any heating, so in this case, the most important parameter is the volume relation between the phases. That is, increasing the extraction temperature is only effective in polar volatile analytes, while the sensitivity of non-pelar analytes remains essentially unchanged by the increase of the extraction temperature (Slack, et aL, 2003). 

Hgure 1 Comparison of purge and trap with static headspace extraction. (A) Static head space and (B) purge and trap. 

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