Assay techniques colorimetry

Several collaborating laboratories (usually five participating laboratories) test the proposed substance using a variety of techniques. The relative reactivity or relative absorbance of the impurities present in a substance must be checked when a nonspecific assay method is employed, e.g. by colorimetry or ultraviolet spectrophotometry. It is particularly important to quantify the impurities when a selective assay is employed. In such a case, it is best to examine the proposed substance by as many methods as practicable, including, where possible, absolute methods. For acidic and basic substances, titration with alkali or acid is simple but other reactions which are known to be stoichiometric may be used. Phase solubility analysis and differential scanning calorimetry may also be employed in certain cases. 

Thin-layer chromatography Historically TEC has had great merits in vitamin E assays as a cleanup step in connection with colorimetry or GC and, for a long time, as the only technique capable of separating jS- and y-tocopherols. This separation was accomplished either by using complex mobile phases in one-dimensional approach or, alternatively, in two-dimensional systems. For quantitative purposes photo-densitometric scanning after HPTEC separation has been conducted both in the transmittance and the... 

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