Performance criteria of methods

In principle, the quality of an analytical method can be expressed through two groups of basic and secondary characteristics or figures of merit, which must be quantified by the user of the method before using it to deliver results. The determination of these quantified figures is achieved in the method validation process (see section 2.3). ba.sic characteristics.  

The specificity or the selectivity guarantees that the method is really measuring the substance of interest. Interferences of all types affect the specificity of several methods e.g. chromatographic separations with non-specific detectors such as FID or ECD, etc. 

Specificity is the basic parameter for microbiology measurements and should for this field of metrology be placed first of the basic characteristics. Nearly all the specificity of the final counting relies on the selectivity of the culture medium and growth conditions. 

Sensitivity is mainly a limiting factor in trace analysis. It is evident that the method applied should be sensitive enough so that the concentration of substance to be determined is accessible (limit of detection and determination) or the colonies of microbes countable. It must also deliver a measurable difference for a small change in content. When the sensitivity of the method of final detection is a limiting factor the analyst may have several possibilities which all will influence the selection, optimisation and validation of the other steps of the procedure. He may  

The determination of the property of interest in a simple solution will indicate to the analyst his working range in terms of sensitivity of the signal. The degree of trueness and precision are properties quantified in the validation process. Robustness or ruggedness concerns the ability of the method to remain unaffected by environmental changes (analyst, time, fluctuations in supplies, etc,). All six characteristics have to be established and quantified in the validation of the method (section 2.3). 

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