Evolution study from a spectra set

The simplest method to study a spectra set evolution is to display the spectra on Fig. 10. Several examples can be further shown for degradation tests. More than the general evolution, leading in this case to the decrease of spectra shape and thus of absorbance values, isosbestic points can appears with time. 

If all spectra, or at least several, cross together at one point, this particular point is called isosbestic point (IP). Several isosbestic points may exist from a set of spectra. One classic example of such isosbestic points is, for instance, a set of spectra of one component in solution, at various pH values, which shows an equilibrium between acidic and basic forms, the proportion of these depending on pH value. Most of the studies on isosbestic points were made on reacting systems involving pure components artificially mixed in the laboratory (for example, see Reference 7) this phenomenon can be observed for the discharge of wastewater into a river (Fig. 11). 

In practice, because of some instrumental or environmental errors, an isosbestic point is rather a small surface than a real point, and a procedure has been proposed for its detection [8,9]. An isosbestic point is defined by the wavelength XjP (orX/p for hidden IP) as a point where the apparent coefficient of variation (CV ) is lower than a limit value (fixed to 2.5%, value obtained from a statistical study on repeatability)  

The search for points called outliers, responsible for a coefficient of variation greater than the fixed value is based on a statistical test (Dixon test). The UV spectra eliminated following this test are considered as not representative of the studied flux. Then, a final statistical test is carried out (Rank test, for example) in order to check if the revealed point is a true isosbestic point. This final test is carried out at X/p 10 nm. 

The presence of at least one isosbestic point shows that there is a mass conservation between all samples, which can thus be considered as a mixture of compounds. More often, this indicates the presence of only two major mixtures (considered like pure compounds), the concentrations of which are linked in a way that the mass balance is conserved [10], More precisely, there is a fixed linear relation between the concentrations of the two components (or mixtures of fixed mass composition) of the form  

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