Reliability of Qualitative Analytical Tests

In principle, there is no fundamental contrast between qualitative, (semi-quantitative), and quantitative analyses. The analytical signal is generated in the same way, only the detection and evaluation is done on the basis of a more rough scale, in qualitative analysis only in form of a yes/no decision. 

Today an increasing importance of qualitative analysis can be stated in certain fields. This is due to an increasing number of materials under study, especially active agents of interest on the one hand, and the many and diverse ways of synthesis (e.g., by combinatorial chemistry) on the other hand as well as the increasing demands on quality. Because analytical laboratories in research and routine control would be overtaxed in their capacity if full quantitative analyses were done generally, screening methods become more and more significant. 

Screening techniques can be understood as to be filtering procedures of samples. The principle of screening consists in giving an overview on constituents in certain samples, namely  

Although the term screening methods is often used as a synonym for qualitative analysis, it is mostly related to (i) the scheme of which is given in Fig. 4.8 see also Trullols et al. [2004]. 

Screening methods may be classified according to the detection of signals into  

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Both qualitative observations and quantitative measurements cannot be reproduced with absolute reliability. By reason of inevitable deviations, measured results vary within certain intervals and observations, mostly in form of decision tests, may fail. The reliability of analytical tests depends on the sample or the process to be controlled and the amount of the analyte, as well as on the analytical method applied and on the economical expenditure available. 

A number of recent analytical techniques have been used to improve the reliability of chemical analysis from a qualitative and quantitative point of view. The work of Tubino et al.13 should be mentioned as an example of using fiberoptic devices for spot tests of diffuse reflectance measurements. 

Reliability It is rather difficult to use the accuracy and precision concepts as these capital and basic properties are closely related in qualitative analysis. Their combination has produced a new property called reliability, which is defined as the proportion (percentage) of right yes or no answers provided by individual tests carried out on n aliquots of the same sample to identify an analyte or a family of the analytes. This definition represents the positive side of the errors in qualitative analysis false positives and false negatives. The reliability of the binary response is not an independent property as it strongly depends on the basic properties of sensitivity and selectivity. Moreover, it is in contradiction with productivity-related properties. Reliability is equivalent to certainty and, in quantitative analysis, the uncertainty of a result is a parameter associated with reliability. Indeed, the term is included in the definition of traceability as every experimental datum is affected by specific variations or doubts. As it directly affects the quality of an analytical result, it is necessary to find out an equivalent method to express vmcertainty in qualitative analysis. The term unreliabifity can be... 

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