Good Calibration Practice in Trace Analysis

Accuracy of the obtained results is determined first of all by whether the measurements made for an analyte are vitiated by an interference effect. It is therefore of vital importance to judge well whether the sample contains components that could act as interferents with respect to the analyte. If there are any doubts in this matter, and they are not dispelled by the analyst s own experience or the proceedings of other researchers, analyses can be performed on the basis of the standard addition method (taking into account the limitations of the method). However, some additional efforts can also be made to examine the situation experimentally. 

Examination of the Interference Effect The most usual way of examining the interference effect is very simple it is based on performing measurements for the analyte alone at a given concentration, and for the analyte at the same concentration in the presence of an interferent. This enables detection, one by one, of the interactions of the analyte with all the substances composing the sample. Certainly, this approach is tedious, time consuming, and does not ensure that all components are taken into consideration. One should also remember that the chemical form of an individual component in a standard solution might be different from that present in a natural sample, which can result in them having different interactions with the analyte. 

The complementary dilution method (CDM) [10] is a development of the above-mentioned method for examination of the interference effect. It requires preliminary preparation of six solutions according to the scheme given in Fig. 3.14. Once measurements for the analyte in all the solutions have been completed, four 

It should be noted here that two of the four results are obtained by interpolation (as in the set of standards method) and two by extrapolation (according to the standard addition method). All the results are estimated values of the analyte concentration in the sample. Therefore, if the obtained results differ from each other there is a high probability that an interference effect occurs. Nevertheless, results obtained by extrapolation are closer to the real analyte content in the sample than those calculated interpolatively. 

In order to improve the procedure described above it was adapted to flow mode with the use of an original flow-injection manifold [11], Once a sample and a single standard solution are introduced to the system they can be gradually diluted, and interpolative and extrapolative estimations of analyte concentration in the sample can be obtained for every dilution degree. This procedure allows examination of possible interferences, and is, in effect, a sterling, integrated calibration method that combines in one calibration procedure the set of standards method and the standard addition method with the dilution method. Moreover, it has been demonstrated that any of the calibration methods described above can be integrated with the use of a constructed manifold. 

---