Errors trace analysis

Heydorn, K. Detecting Errors in Micro and Trace Analysis by Using Statistics, Anal. Chim. Acta 1993, 283, 494M99. 

Manganese, determination by x-ray emission spectrography, 328 in domestic ores, 200, 202, 203 trace analysis by x-ray emission spectrography, 228, 229, 231, 232 Manipulative errors, standard counting error comparable with, 285-287 Mass absorption coefficient, additivity, 15... 

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. 

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... 

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

In summary, when using the indirect" technique for optical trace analysis, all of the points mentioned above have to be considered and have to be validated when setting up a standard operation procedure" (SOP), in order to conform to good laboratory practice (GLP) analysis methods. This means an extra work load compared with validation of the direct optical trace analysis procedure. Sometimes there is no way of getting round this so-called less elegant , more cumbersome and more error prone indirect technique. However, if it is performed correctly and judged critically, it is still a good method and should easily allow optical trace analysis down to 0.1% and lower. 

Trace element concentrations are obtained in the semi-quantitative mode by ICP-MS, LA-ICP-MS, SSMS or SNMS with an error factor of about 0.3-3 for most elements. The results of semi-quantitative trace analysis, e.g., for high purity materials, are sometimes sufficient to estimate the purity of the matrix investigated. 

Any presentation such as this, which is intended to guide the analyst through trace analysis, must begin with describing those factors that can introduce errors so that the application of a general approach can be properly exploited by the analyst with his particular sample problem. However, before he can evaluate which of these factors must be of concern to him, he must have... 

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. 

Synchrotron storage rings, for instance, are able to provide an extremely high flux of nearly monochromatic X-radiation on a small sample area. They could form the basis of XRF set-ups and enhance other microana-lytical methods to provide accurate determinations. In the future they could serve as a reference method for elemental trace analysis on the microscopical level (with the quality of the random number generator, a non-SI concept, as the prime source of error). 

The values of errors and uncertainty strongly depend on the level of analyte content (concentration). High values of these parameters, unacceptable in the case of an analyte content at the percentage level, can be satisfying in the case of trace analysis. 

Risk of occurrence of the interference effect increases with an increasing number of sample components as well as with a higher ratio of the concentrations of interferent and analyte. Because a specific feature of trace analysis is the excess of some sample components accompanying the analyte, the interference effect in these circumstances can potentially lead to great analytical errors. 

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. 

As shown, various calibration methods can be applied in chemical analysis. The choice of method depends on the kind of analytical problems and sources of random errors expected in the course of analysis. Nevertheless, it is hard to say that any of the discussed methods is especially adapted to trace analysis. However, because of its specificity, trace analysis does require special attention in the choice of calibration method, as well as special care in realization of the selected method at every step of the calibration procedure. 

Table 4.2 Sources of errors in inorganic trace analysis [15]...

Most common sources of errors in trace analysis... 

Oehme, M., Berger, U., Brombacher, S., Kuhn, F., Koliker, S. Trace analysis by HPLC-MS contamination problems and systematic errors. Trends Anal. Chem. 21, 322-331 (2002)... 

Once a method is established, precision may be determined by suitable replicated experiments. However it is in inter-laboratory trails that the problems with environmental methods often show up. It is accepted that for trace analysis RSD values of tens of percent are likely. In studies conducted in Western Australia on pesticide residues in bovine fat RSD values for dieldrin were 12% and for dia-zonium were 28%. It is typical to see a quarter of the laboratories in such a trial producing values that could be termed outliers. In the previously mentioned study, 5 laboratories out of 26 had z> 3 for aldrin. In a parallel study RSD values for petroleum in dichloromethane and water were 40% and 25%, respectively. The conclusions of these studies was that there was poor comparability because of the different methods used, that accreditation apparently made no difference to the quality of results, and that a lack of understanding of definitions of the quantities to be analysed (for example gasoline range organics ) caused non-method errors. In relation to methods, this is contrary to the conclusion of van Nevel et al. who asserted that the results of the IMEP round of analyses of trace elements in natural and synthetic waters showed no dependence on method [11]. If properly validated methods do yield comparable results, then one conclusion from the range of studies around the world is that many environmental methods are not validated. It may be that validated methods are indeed used, but not for exactly the systems for which they were validated. 

The concentration of metals that are detrimental to catalysts added can vary between 20.0 ppm for Fe to 100 ppm for Ni and lOOOppm for V. The presence of these metals necessitates the need for analysis of these metals to determine their concentrations prior to the cracking process. The best method to analyse these oil samples needs to be rapid and accurate. Careful selection of the method either from experience or by trial and error may be applied depending on the metal and the concentration. Sample dissolution in a solvent or solvent mixture is considered the easiest but may not be suitable for low limits of detection. Destructive sample preparation methods, i.e. oxygen bomb combustion, microwave acid digestion followed by pre-concentrating may be required for trace analysis and/or with the aid of a hyphenated system, e.g. ultrasonic nebuliser. Samples prepared by destmctive methods are dissolved in aqueous solutions that have very low matrix and spectral interferences. 

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