Potential sources of analytical error

The realisation that every laboratory determination that is carried out is associated with both random and systematic errors has had a major impact on laboratory medicine in the last thirty years. It is also at the heart of quality control and quality assurance procedures which are primarily concerned with understanding the sources of such errors and their suppression or minimisation (Whitehead, 1977 Aitio, 1981 Taylor, 1987). However, it has been pointed out by Broughton (1983) that all laboratories may carry out some form of quality control but this is often designed to give retrospective reassurance rather than provide prospective action. The dual concepts of bias and precision in laboratory medicine are well known, but not always appreciated even by users of reference materials (Taylor. 1985 Taylor, 1987). By definition an unbiased result should be the true result, but in practice this is hardly ever achieved. The nearest approach to a true value is generally obtained by using a certified reference material and a definitive method, but these ideals are unobtainable in the case of most trace metal analyses. 

The potential sources of error in carrying out the determination of trace metals in biological material are legion and should never be underestimated. Some of these sources of error have been discussed by previous authors and have also been the subject of extensive reviews (Nieboer and Jusys, 1983 Versieck et al., 1982 Versieck, 1984 Versieck and Cornelis, 1989). Some of the most important potential sources of error in trace metal determination are discussed in outline as follows  

As pointed out by Yeoman (1983), the simplest yet one of the most important sources of random error is in the volumetric handling of liquid samples or standards. Differences in the surface tension and viscosity of specimens, calibration solutions or controls may cause major problems in analytical performance. The usefulness of lyophilised control material can often be abrogated by inaccurate reconstitution or unforeseen contamination. Any pipettes (manual or automatic devices) used in volumetric measurements should be of grade A quality and regularly checked for bias they must be carefully washed and kept clean, free of contamination. 

The expression of results for cellular components is particularly difficult to resolve. It is possible to express results in terms of cell number, dry weight, total protein or DNA content. Each method of expression may be prone to certain errors, particularly when assessing the influence of disease states or malnutrition (Patrick and Dervish, (1984). 

Although a routine aspect of any analytical procedure, the method of calibration is a major potential source of systematic error. Error in the preparation of calibration standards or their working dilutions is one obvious source of discrepancy. The recovery of an 

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