Contamination samples

Sample contamination from sampling apparatus, storage media or by introduction of foreign elements during analytical procedures must be given consideration. This is a particular pro Hem in those determinations carried out using electrothermal atomisers, due to the high sensitivity of these devices. 

It is not possible to precisely define sampling conditions and storage media for all situations. Certain general recommendations can, however, be made. Solder seams in metal containers often used for sampling are to be avoided. Exposure of samples to metal surfaces may be reduced by coating 

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One measure of lire significance of coniaininants in soil or solid waste samples may be determined by comparing the levels with reported naturally occurring concentrations. Provided levels are within the range that may occur naturally, one might conelude that the sample contaminant levels are of little consequence. 

Figure 6.6 SPE-SFE-HRGC(PID) cliromatograms obtained for (a) a mixture of analytical standards of selected aromatic compounds in water, and (b) the analysis of a water sample contaminated with various aromatic compounds.

A practical application of SEE-CEST coupling is shown in Eigure 6.13, which displays the electropherogram obtained for a tomato sample contaminated with a pesticide, i.e. carbaryl. The sample was placed in the SEE cell, extracted with CO2... 

Precision. The choice of 400 mg of soil is arbitrary. It was chosen in order to keep the entire cleanup step within a 2 cc vial. A test of replication was done on one field sample contaminated with about 20,000 ppm of 1254. Three samples of 50, 54, and 54 mg were weighed into separate vials, extracted, and then diluted 1 1000 into hexane in a separate vial. The three chromatograms are shown in Figure 7. Two peaks were quantified to demonstrate how reproducible a measurement can be, even in a field sample. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

Vacuum Distillation Hater is evaporated at reduced pressure and at or near ambient temperature. Nonvolatile organics Slow process when sample volumes are large. Inorganics also concentrated. Sample contamination is low but sample may be modified due to thermal degradation or chemical and microbial reactions. 

Menger 2001, Assement of Sample Contamination by Downhole NMR Fluid Analysis, SPE 71714, presented at the 2001 SPE ATC E, New Orleans, 30 September-3 October, 2001. 

This layer is then analysed directly by internal reflectance infra-red spectroscopy. Since there is no handling of the sample, contamination is reduced to a minimum. However, only infra-red spectral analysis is possible with this system since the material absorbed on the germanium prism is always a mixture of compounds, and since the spectrophotometer used for the production of the spectra is not a high-precision unit, the information coming from this technique is limited. While identification of specific compounds is not usually possible, changes in spectra, which can be related to the time of day, season, or to singular events, can be observed. 

The use of a special device that allows filtration under nitrogen, the direct introduction of sample into containers for storage during filtration and, the use of these containers as analysis cells are all improvements that minimise external sample contamination and improve between-sample reproducibility. 

In contrast to infrared spectrometry there is no decrease in relative sensitivity in the lower energy region of the spectrum, and since no solvent is required, no part of the spectrum contains solvent absorptions. Oil samples contaminated with sand, sediment, and other solid substances have been analysed directly, after being placed between 0.5 mm 23-reflection crystals. Crude oils, which were relatively uncontaminated and needed less sensitivity, were smeared on a 2 mm 5-reflection crystal. The technique has been used to differentiate between crude oils from natural marine seepage, and accidental leaks from a drilling platform. The technique overcomes some of the faults of infrared spectroscopy, but is still affected by weathering and contamination of samples by other organic matter. The absorption bands shown in Table 9.1 are important in petroleum product identification. 

Kelly et al. [ 372 ] has described a sampling apparatus constructed to collect 28 litre samples of seawater designed to mimimise sample contamination derived from the ships environment. Its utility in the study of polychlorobiphenyls, pentachlorophenols, and organochlorine pesticides was investigated. 

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