Analytical methods used for the study of stability

Long-term reproducibility is the prime quality of an analytical method used for the study of stability. Stability must be performed on the element or substance to be certified. Extrapolations on the stability of tracers are of little interest. If the uncertainty of the method for an analyte is poor, even for large sample intakes, the analyst must refer to the experience on similar materials with higher contents or to the general chemical properties of the substance or element. In any other situations certification may be impossible. Such difficulties are mainly encountered in organic or organo-metallic analysis. Experience has shown that some substances may be stable in a matrix but unstable in another even similar one [46-47]. All analytical methods suffer from long term reproducibility. In some cases analysts have developed tools and tricks to minimise this effect. 

For organic determinations, the same difficulty exists for the stability studies as for the homogeneity studies, with one major drawback. The instability is much more likely 

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