Costs static sampling systems

The advantages of static sampling systems are their portability, convenience, reliability, and low cost. The systems are lightweight and can be attached directly to individuals. Nonstatic sampling systems can, of course, also be attached to individuals, but are less convenient because the person must carry a battery-powered pump and its batteries. Static sampling systems are very reliable, and the materials used limit the costs to acceptable levels. 

In both cases, either conventional FTIR transmission or diffuse reflection detection may be used. Because TLC and the postspectroscopic evaluation are not linked directly, few compromises have to be made with regard to the choice of the solvent system employed for separation. Chromatographic selectivity and efficiency are not influenced by the needs of the detector. The TLC plate allows the separation to be made in a different site from the laboratory where the separated analytes are evaluated. The fact that the sample is static on the plate, rather than moving with the flow of a mobile phase, also puts less demand on the spectrometer. The popularity of TLC-IR derives in part from its low cost. 

The reference methods usually require wet samplers. Sampling trains have been developed that allow the sampling of five or more gases simultaneously in separate bubblers. These static methods of sampling can be accomplished with a modest initial investment, but the manpower required to distribute and pick up the samples and to analyze them in the laboratory raises the total cost to a point where automated systems may be more economical for long-term studies. 

In another type of membrane extraction devices, porous polypropylene hollow fibers are used, often in a disposable way, which minimizes carryover problems and reduces costs [26-33]. On the other hand, manual manipulations are needed, limiting the possibility for automation. With these devices, the extraction can be carried out in a static mode, either in large sample volumes, where the extraction is not intended to be complete, or in small volumes aiming for complete extraction. Usually, stirring is applied to increase the speed of mass transfer. Some typical practical arrangements are shown in Figure 12.2. This type of SLM extraction is often called hollow fiber liquid phase microextraction, or three-phase liquid phase microextraction or two-phase liquid phase microextraction but the terminology in this active field of research has not been settled. Also hollow fibers can be connected in flow systems [34,35]. 

Method 5021 describes the automated static-HS technique. Static HS has been introduced in this book from a theoretical viewpoint. A soil sample is placed in a tared septum-sealed vial at the time of sampling. A matrix modifier containing internal and/or surrogate standards is added. The sample vial is placed into an automated equilibrium headspace sampler. The vial s temperature is elevated to a fixed value that does not change over time and the contents of the vial is mixed by mechanical agitation. A measured volume of headspace is automatically introduced into a GC or a GC-MS. The method is automated and downtime is minimal. However, the cost of the automated system is appreciable. Contamination of the instru-... 

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