Fractionation methods artefacts

Methods to separate or fractionate biological and biomedical samples lie at the heart of a diverse range of scientific disciplines including biochemistry, cell biology and molecular biology. The choice and mode of separation is important to obtain the best results and avoid artefacts. There are a number of physical and chemical separation technologies routinely used by the bioanalytical chemist that are described in this chapter and various other chapters of this textbook. 

A low sodium concentration is sometimes reported in patients with. severe hyperproteinaemia or hyperlipoprotein-aemia. This is an artefact caused by the method used to measure. sodium concentration by some analytical instruments. These patients have in fact a normal sodium concentration in their plasma water. The increased amounts of protein or lipoprotein occupy a larger fraction of the plasma volutne than usual, and the water a smaller fraction. The analyser measures the amount of sodium in a small volume and assumes that the Witter content of that sample is normal. The result is an apparently reduced sodium concentration (Fig. 4). This so-called pseudohyponatraemia can be detected by measuring the scrum osmolality which will be nomial. 

In contrast to liquid chromatography which is the technique of choice when considering high molecular mass constituents, gas chromatography (GC) is well-suited for the fractionation of low molecular mass species which are volatile, thermally stable and preferably neutral. Some species that do not already possess the above properties can be converted into forms that are amenable to GC. Various derivatization procedures that can be used to perform such conversions have been described by Poole and Schuette (1984). However, it Is worth mentioning that hydride formation and alkylation are the two most commonly used derivatization methods that have found application in the study of the speciation of various elements like As, Bi, Ge, Hg, Pb, Sb, Se, Sn, Te and Tl. Two points have to be considered when derivatization is performed first, the specificity of the chemical conversion second, the percentage yield of the reaction. A specific reaction is desirable in order to avoid the introduction of artefacts. Moreover, if a quantitative estimate of the amount of the original species is required, then it is essential that the extent of the conversion is known. 

Many other filtration-related factors exist that may influence the concentration of constituents in the dissolved fraction. This was demonstrated by Horowitz et al. (1992) in a systematic investigation of membrane filtration artefacts when determining Fe and Al in synthetic and natural waters. They found that the dissolved concentration of these elements was significantly influenced by the filter type, filter diameter, method of filtration, concentration... 

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