Well purging techniques

Remove the pump from the well. (If not sampling with a bailer, go to Step 5 of low-flow micro-purge technique, described below in Chapter 3.6.2.4.)... 

Space does not permit a long discussion of the well-established technique, but one interesting experiment illustrates the wealth of information accessible. Figure 7,35 shows the determination of the carbon content of a black-filled SBR. The first ramp to 650°C in flowing nitrogen decomposes the polymer, and the breakdown pattern could be studied if desired. The purge is change to... 

Unusual circumstances, however, may defeat any of these gas-purging techniques. It should also be remembered that batteries generate a significant amount of explosive gas during the maintenance procedure, and therefore maintenance should always be performed in a well-ventilated shop. 

The accuracy of the injection volume measurement can be very important for quantitation, since the amount of analyte measured by the detector depends on the concentration of the analyte in the sample as well as the amount injected. In Section 12.8, a technique known as the internal standard technique will be discussed. Use of this technique negates the need for superior accuracy with the injection volume, as we will see. However, the internal standard is not always used. Very careful measurement of the volume with the syringe in that case is paramount for accurate quantitation. Of course, if a procedure calls only for identification (Section 12.7), then accuracy of injection volume is less important. See Workplace Scene 12.1 for an example of a purge-and-trap procedure for injecting a GC sample. 

Headspace analysis (EPA 3810, 5021) also works well for analyzing volatile petroleum constituents in soil. In the test method, the soil is placed in a headspace vial and heated to drive out the volatiles from the sample into the headspace of the sample container. Salts can be added for more efficient release of the volatile compounds into the headspace. Similar to water headspace analysis, the soil headspace technique is useful when heavy oils and high analyte concentrations are present, which can severely contaminate purge-and-trap instrumentation. Detection limits are generally higher for headspace analysis than for purge-and-trap analysis. 

Oxidation, as is well known, leads not only to the formation of heavy compounds but also of volatile compounds that are responsible for off-flavors. Usually they are evaluated by capillary GC, through several sampling techniques are available (purge and trap, head space solid phase microextraction (SPME)). Rovellini et al. [26] recently proposed the application of HPLC for identifying... 

Purge-and-trap collection is well adapted to biological samples such as blood or urine that are soluble in water (Pellizzari et al. 1985a Peoples et al. 1979), and is readily adapted from techniques that have been developed for the analysis of carbon tetrachloride in water and wastewater. For water- insoluble materials, the purge-trap approach is complicated by uncertainty of partitioning the analyte between sample slurry particles and water. 

Although the first two techniques have the same basic steps (the well is purged, stabilized, and samples are collected), we implement them using different tools and procedures. Common to these techniques are the measurements of well stabilization parameters, which allow us to judge whether the water in a purged well represents the formation water. 

A technique for the determination of methylmercury in aqueous samples (natural and seawater) involved the conversion of methylmercury to gaseous methyl-ethylmercury by reaction with sodium tetraethylborate (Bloom, 1989 Bloom and Watras, 1989). The volatile derivative was purged from the solution and concentrated on a graphitic column at room temperature. The derivative was thermally desorbed from the column, and then analysed by cryogenic gas chromatography with cold vapour atomic fluorescence detection. In addition to methylmercury, labile Hg11 species could be determined (as diethylmercury) as well as dimethylmercury (which is not ethylated). The detection limit for... 

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