Contamination, of samples

Suitable inlets commonly used for liquids or solutions can be separated into three major classes, two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile impurities in the liquid will have been mixed with the sample itself, and these impurities will appear in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the separate examination of levels of residues left by solvents in the absence of any sample. 

Patches are removed from the test subject immediately after the study replicate is completed. The Field Scientist should wear disposable surgical gloves to remove the patches and change gloves often to reduce the chance of cross-contamination of samples. Individual patches are wrapped in aluminum foil prior to placing them in a pre-labeled sealable plastic bag. Combined patches are placed exposed face to face, wrapped in aluminum foil, and stored frozen in sealable plastic bags. Patch samples may be placed in dry-ice in a cooler or directly into a freezer for storage. 

One alternative method for preparing field fortifications solutions/suspensions is to prepare each fortification sample of each matrix in a separate mini-vial in the analytical laboratory and ship the vials to the field for use. This procedure precludes the use of pipets in the field and may be useful when Field Scientists not experienced in the use of pipets are involved in the field fortification process. One disadvantage of this procedure is that the mini-vials, if not designed correctly, will be hard to handle in the field, and surface tension of the suspension or fortification solution will tend to leave unacceptable amounts of the solution/suspension in the vial or at the lip of the vial and not on the matrix in question. This procedure may lead to cross-contamination of samples as the field fortification liquid is forced from the top... 

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

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. 

Your list should have included at least some of the items mentioned above, but you may well have identified other sources of uncertainty. Remember that uncertainty is not about mistakes. The uncertainty estimate is intended to reflect the likely variation in results when a method is carried out correctly and operating under statistical control. Your list of sources of uncertainty should not therefore include any gross errors such as contamination of samples, mistakes in calculations or the analyst failing to follow the standard operating procedure correctly. 

The consequences of nonconformities are different for the different categories. Minor nonconformities will be noted and checked at the next assessment but will not normally appear in the written report. Major nonconformities can result in a total suspension of accreditation or suspension of accreditation for particular work. The intentional misuse of the accreditation body logo or mark is considered to be a serious nonconformity. If, after a previous warning, a laboratory continues to issue test reports showing the accreditation logo when the tests are outside the scope of the accreditation it can result in withdrawal of accreditation. Accreditation can be withdrawn for a particular area if it is found that accommodation is inappropriate and it is impossible to avoid cross contamination of samples. 

Because of the ubiquitous nature of FMs in consumer products, it is critical that any analytical chemistry laboratory measuring these compounds takes extra precautions to avoid laboratory contamination of samples. Several researchers [2,11,14-17] have pointed out that likely sources of FM contamination in the modern-day laboratory include the use of consumer products and fine fragrances by laboratory workers, fragrances in soaps used to clean glassware and the laboratory, and laboratory supplies such as gloves. 

It has been demonstrated that radial dispersion contributes more significantly to the dilution of the sample in the flow than does axial dispersion. This type of fluid movement, termed secondary flow by Tijssen [43], results in a washout effect accounting for the low mutual contamination of samples successively injected into a carrier stream. TTiis advantageous feature is a result of the use of low flow rates and small tubing bores, and results in decreased peak-width and hence to increased sampling rate. 

Care must be taken to avoid contamination of samples before analysis. Common causes of contamination are ... 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

The surface of a solid sample interacts with its environment and can be changed, for instance by oxidation or due to corrosion, but surface changes can occur due to ion implantation, deposition of thick or thin films or epitaxially grown layers.91 There has been a tremendous growth in the application of surface analytical methods in the last decades. Powerful surface analysis procedures are required for the characterization of surface changes, of contamination of sample surfaces, characterization of layers and layered systems, grain boundaries, interfaces and diffusion processes, but also for process control and optimization of several film preparation procedures. 

However, the nucleic acid-based assays for the detection of food pathogens show problems regarding the sensitivity of the polymerase enzyme to environmental contaminants, difficulties in quantification, the generation of false-positives through the detection of naked nucleic acids, non-viable microorganisms or contamination of samples in the laboratory, and may limit the use of PCR for the direct detection of microbial contamination. 

When moving the samples, place each Kraft bag in a 15 x 40 cm polythene bag and secure the top of the bag with a knot to prevent loss or cross contamination of samples during transport. 

Contamination of samples and loss of analytes are common sources of errors in environmental measurements. 

Equipment To determine contamination of Samples of final equipment... 

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