The UV/Vis absorption spectroscopyis often exploited for its application to quantitative measurements. Depending on the nature of the samples, a variety of techniques have been developed. Direct quantitative determinationscan be made when the analyte molecule contains a chromophore. Indirect determinations are used when analyte molecule does not have suitable chromophore but the analyte can be made to quantitatively react with a molecule containing a suitable chromophore or which produces a chromophoric group. It should be noted that it is possible to determine the concentrations of two or more species present in ananalytical sample by suitably modifying the Beer-Lambert law. In biological applications, concentrations of two components in a mixture are easily determined by using this method.