Calibration Problems in Trace Analysis

Pawet Koscielniak, Marcin Wieczorek, and Joanna Kozak 

An entire analytical procedure (i.e., a group of activities leading to information on either the kind or quantity of a component in the sample assayed) consists of several separate stages. If the final goal of this procedure is a quantitative result, one of the essential stages is analytical calibration. 

Calibration is necessary because the quantity (content, concentration) of a given component in a sample cannot be found directly, but only through the signals produced by the measurement instruments dedicated to analytical examination of this component. Under well-defined chemical and instmmental conditions, a definite relationship (calibration dependence) between the analytical signal and the concentration of a component (analyte) exists. The crucial point of calibration is to define this dependence and exploit it for determination of analyte in a sample. 

Accurate description of the calibration dependence in a theoretical or even semi-empirical way is extremely difficult, perhaps impossible, to achieve. Therefore, the conventional and generally accepted approach is empirical, involving experimental reconstruction of the calibration dependence in the form of a calibration graph. For this purpose, the signals for known concentrations of analyte in standard solutions (i.e., for solutions of known concentrations of analyte) are measured. Then, the signal measured for the analyte in the sample is related to the calibration graph and the analyte concentration calculated. 

The problem is that calibration dependence is not a general feature of the analyte and measurement system used, but depends on the chemical environment of the analyte in the sample analyzed. Some of the sample components might influence (interfere with) the analytical signal, causing the so-called interference effect. In particular, interferents can react with the analyte and thus change the angle of the 

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Perhaps the greatest problem in trace analysis is assurance of the accuracy of the results (i.e., the avoidance of systematic errors). Systematic sources of error are possible in every step of an analytical process. The most reliable method for detecting systematic errors is continuous and comprehensive quality assurance, particularly by occasional analysis of (certified) standard reference materials. Strictly speaking, an analytical method cannot be calibrated if suitable (i.e., representative) standard reference materials adequately representing the matrix of the expected test samples are not available. However, internal laboratoiy reference materials can then usually be prepared, whose matrix largely resembles the matrix of the test portions expected. If problems occur in the preparation of such reference samples, the standard addition method (SAM) can be applied, in which internal laboratory standards are added stepwise to the test sample (analyte and matrix)... 

Because of the problems described above, the standard addition method can only be recommended in trace analysis with serious reservation. However, the possibility of compensating for the interference effect, even when unexpected or caused by unknown sample components, is so great an advantage that this calibration approach deserves greater interest in analytical practice than it is given at present. 

The techniques used in flow analysis can help overcome these problems. The natural peculiarities of flow techniques are conducive to proper and efficient preparation of calibration solutions and, therefore, in obtaining reliable analytical results in trace analysis. These techniques have been successfully examined and are widely employed for a range of analytical purposes. They are also widely used to perform calibration by a variety of procedures, implementation of which in the traditional way is not possible. 

Danzer K (1990) Problems of calibration in trace-, in situ-micro- and surface analysis. Fresenius J Anal Chem 337 794... 

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. 

For analysis of minor components down to trace amounts or when continuous calibration curves are necessary, it is possible in certain cases to use specially adapted, mathematically simplified and hence speeded-up algorithms. Anyhow, here, even more than with classification tasks, the performance depends very much on the selection of a suitable algorithm and the careful adaptation towards the specific problem. In any case, such algorithms frequently are not overly stable against unforeseen spectral interferences. 

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. 

The second consideration for quantitative analysis is that if the elements are present in trace quantities the problems of multiple sample preparation by ashing increase. A vicious circle is endured when the quantity and percentage recovery of the element cannot be determined until the quantity of the element present is known for dilution limits and range of standard calibration curve required. If the quantity is known, the percentage recovery may be determined by calibration curve or by standard addition. The percentage recovery results should be 100% 2% to allowing for errors and loss. Drawbacks to this method are ... 

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