Headspace modes static

Although the static and multiple headspace modes use similar equipment, the two rely on rather different principles. On the other hand, purge and trap, and dynamic headspace, possess the same foundation, the only difference between them being the location of the tubing used to transfer the carrier gas to the sample container. 

Soil spiked with trichloroethylene and toluene was analysed using a gas chromatograph equipped with a PT concentrator that was found to be replaceable by a headspace unit in order to simplify the overall assembly. The headspace analysis of soil samples was found to be restricted by incomplete desorption of the contaminants in soil-water mixtures this shortcoming, however, was effectively overcome by the addition of methanol. Henry s law constants for volatile organics in methanol must previously be determined if the method is to be applied to soils [142]. A comparison of the performance of static and dynamic (PT) headspace modes in the determination of nine VOCs in five different soils revealed poor PT recoveries in relation to those of static headspace (which ranged from 68 to 88%) the latter, however, required longer development times [143],... 

A sampling technique known as headspace , of which there are two modes, static and dynamic, is very widespread in GC for the qualitative and even quantitative analyses of volatile constituents present in some samples (cf. Chapter 21). 

It should be stressed that the use of a general pharmacopeial method is not a reason not to validate the latter when analyzing a particular substance. The matrix effect, in particular, has to be investigated when using the static headspace mode of injection. 

Static mode the sample (liquid or solid matrix) is placed in a glass phial capped with a septum such that the sample occupies only part of the phial s volume. After thermodynamic equilibrium between the phases has been reached (1/2 to 1 h), a sample of the vapour at equilibrium is taken (Fig. 20.4). Under these conditions, the quantity of each volatile compound present in the headspace above the sample is proportional to its concentration in the matrix. The relationship between the amount of sample injected into the gas chromatograph and its concentration in the matrix can be obtained by calibration (using internal or external standards). 

Figure 20.4—Static mode of headspace sample analysis. The sampling phial is pressurised with the carrier gas of the chromatograph. After equilibrium, a small volume of the gas containing the volatile compounds is inserted into a sample loop. Rotation of the six-way valve allows introduction of the sample into the injector of the chromatograph. Consequently, this set-up combines sample preparation with sample introduction into the chromatographic column. (Reproduced by permission of Tekmar.)...

The basic set-up for headspace analysis comprises an HS element — the characteristics of which depend on the particular mode used for pretreatment and a gas chromatograph or, less often, an alternative detector for measurement. Static and dynamic headspace (purge and trap included) differ in the type of equipment required multiple headspace uses the same automated device as static headspace. 

The use of pervaporation as an alternative to the headspace technique is worth separate discussion. This is, in fact, one of the most promising uses of this approach, as revealed by two existing methods for mercury speciation and VOC analysis in solid samples that exemplify the advantages of pervaporation over static and dynamic head-space modes. Both methods were developed by using the overall assembly depicted in Fig. 4.24A, by which the analytical process was developed in the following four steps ... 

In the static mode, the sample is placed into an extraction vessel, filled with a supercritical fluid at the appropriated temperature and pressure, and allowed to stand for a period. When the extraction is complete, the supercritical fluid is released through a trap to collect the analytes. Static extraction allows analytes with slow mass transfer time to be solvated by the SF. In addition, the use of a known concentration of modifier is possible by direct addition of the modifier to the extraction cell. The main limitation of static extraction is its inability to perform an exhaustive extraction. As in static headspace GC, and the traditional liqnid-liquid extraction, as a result of the equilibrium of the analyte between the matrix and SF, one extraction can not exhaustively extract the analyte from the matrix. Consequently, it is often necessary to perform multiple static extraction. The use of SFE has been decreasing over the years in part due to the growth of accelerated solvent extraction (ASE), which employs much of the same instrumentation and methodology of Sra. 

What would constitute the ideal MS-based e-nose system It would likely (a) incorporate a more sensitive technique than static headspace to deliver volatiles to the sensor array (b) be less expensive than most e-nose instruments currently on the market (c) be easy to use and (d) provide results in less than 10 minutes per sample. An optimum instrument configuration would also allow the same instrument to be used in a rapid e-nose mode but would also permit investigation of sample anomalies using conventional GC/MS methods. Such an instrument could be used as a rapid screening tool and also as a research tool for uncovering further chemical information about suspect samples. Switching from one mode to the other should not require any hardware modification or even instrument shutdown to change columns. 

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