Errors in trace analysis

A final point is the value of earlier (old) validation data for actual measurements. In a study about the source of error in trace analysis, Horwitz et al. showed that systematic errors are rare and the majority of errors are random. In other words, the performance of a laboratory will vary with time, because time is related to other instruments, staff, chemicals, etc., and these are the main sources of performance variation. Subsequently, actual performance verification data must be generated to establish method performance for all analytes and matrices for which results will be reported. 

Most common sources of errors in trace analysis... 

Most metabolites, particularly those derived from hydrolysis, are polar and require deriva-tization prior to analysis by GC-MS. A disadvantage of this approach is that derivatization can be a major source of error in trace analysis. The metabolites normally require isolation from aqueous media before derivatization, and this can... 

Tolg G and Tschopel P (1994) Systematic errors in trace analysis. In Alfassi ZB, ed. Determination of trace elements, pp. 1-38, VCH Verlagsgesell-schaft mbH, Weinheim. 

For trace analysis, counting rates are low and peak-to-background ratios are generally poor. This means that the dominant error in trace analysis is normally due to counting statistics. Since peak-to-background ratios are low, it becomes important to measure and subtract the background contribution accurately. 

G. Tolg, P. Tschdpel Systematic Errors in Trace Analysis, in Z. B. Alfassi (ed.) Determination of Trace Elements. VCH, Weinheim, 1994. 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

Patterson CC, Settle M (1976) The reduction of orders of magnitude errors in lead analysis. In LaFleur PD (ed) Accuracy in trace analysis sampling, sample handling, analysis. NBS Special Publication 422, p. 321... 

Errors in trace analyses are usually hidden to all except those intimately involved in the sample collection and, later, in the bench analysis. In chromatography, especially, it is too easy to hide behind uncertain work because published research does not concern itself with exactly how the chromatographer makes his quantitative decisions. Today, with the advent of the microprocessor and with the use of black box instruments, the chromatographer knows even less about his calibration graph or line, or the error associated with it. In these instruments, a single point and the origin may determine the calibration graph. Similar problems exist in other modern instrumental analysis techniques. 

King, Donald E. "Detection of Systematic Error in Routine Analysis." pp.141-150 in. Accuracy in Trace Analysis ... 

Since trace analysis also includes air or gas samples, it is appropriate to point out that proper addition of an internal standard to this type of sample is difficult. This difficulty lies, not in the mechanical problem of transfer, but in the difficulty of knowing that the precisely intended volume has properly been transferred. However, the internal standard technique is still not widely used here for the same reason it is not generally used in trace analysis. This reason again is because the analyst normally has no prior knowledge of the variation in composition from sample to sample. The continual risk exists that any given sample in a series will have a component, not present in others, which elutes with the internal standard. This occurrence would introduce significant error into the quantitative calculations which result. 

In contrast with usual calibration (case B), modeling in trace analysis, and also of environmental relationships (case A), will probably fail to fulfil condition (2) because x is usually also subject to errors. Alternative linear models must then be considered. 

Versieck, J., Barbier, F., Cornelis, R. and Hoste, J. (1982) Sample contamination as a course of error in trace element analysis of biological samples. Talanta, 19, 973-978. 

Unfortunately, the method suffers several significant drawbacks, which should also be taken into account in trace analysis [2]. First, in principle, the method leads to greater random analytical errors than the set of standards method when both calibration approaches are performed under the same experimental conditions. Second, because the method is based on extrapolation, in some circumstances it can be also a source of serious systematic errors. 

The proper implementation of calibration is to a large extent determined by careful and correct preparation of calibration solutions and samples for measurements. This is especially important in trace analysis because even the smallest errors, random or systematic, at the laboratory stage of the calibration procedure can significantly influence the precision and accuracy of the obtained results. 

The most significant sources of error in trace element analysis are contamination or losses by adsorption or volatilization. The key property of radiotracers with respect to the investigation of the accuracy of analytical techniques is the emission of easily detectable radiation in any stage of an analytical procedure with extraordinarily high sensitivity. 

Contamination In trace analysis, this is the unintentional introduction of ana-lyte(s) or other species which are not present in the original sample and which may cause an error in the determination. It can occur at any stage in the analysis. Quality assurance procedures, such as the analyses of blanks or reference materials, are used to check for contamination problems. 

This first stage is as important as or even more so than the other two inasmuch as (a) they are a major source of a variety of errors —some of them so large as to decisively influence the final result—, (b) they are time-consuming, (c) they are complex and expensive and (d) the system is subject to human contamination, which is a high risk in trace analysis, for instance. 

Wurfels, M. and Jackwerth (1985). Investigations of the carbon balances in decomposition of biological materials with nitric acid, Fres. Z. Anal. Chem. 322,35-58 Yin, D. and Wang, Y. (1987). Analytical problems of trace elements In biological materials. Part 2. Sources of analytical errors in trace element analysis of biological materials, Zhong. Huan. Hexue 7(2), 67-71... 

The basic goal in trace analysis work Is to be able to first report the numbers. The second goal Is to Indicate the error of each number. The third goal Is for the user of the information, whether It be an agency, a newspaper reader, or the like, to accept the reports as having error, look for It, and then use those facts to give Information and to form knowledge (27). 

The split mode has the advantage that the injected zone is narrow and the small sample aliquot entering the capillary avoids overloading the column. Although very flexible in practice, this mode has a number of disadvantages. In many cases mass discrimination among analytes is observed, especially when their volatilities are very different, and this can lead to systematic errors in quantitative analysis. Another disadvantage, especially in trace analysis, arises from the (sometimes small) fraction of the analytes that is transferred into the capillary i.e. the majority exits via the split outlet. 

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