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Introduction To Spectrophotometric Methods Of Analysis For Drugs In Combination2 Simultaneous estimation of drug combination is generally done by separation using chromatographic methods like HPLC, GC and HPTLC etc. These methods are accurate and precise with good reproducibility, but the cost of analysis is quite high owing to expensive instrumentation, reagent and expertise. Hence it is worthwhile to develop simpler and cost effective method for simultaneous estimation of drugs for routine analysis of formulation. Spectrophotometric analysis fulfils such requirement where the simultaneous estimation of the drug combination can be done with similar effectiveness as that of chromatographic methods.

The spectrophotometric assay of drugs rarely involves the measurement of absorbance of samples containing only one absorbing component. The pharmaceutical analyst frequently encounters the situation where the concentration of one or more substances is required in samples known to contain other absorbing substances, which potentially interfere in the assay. If the formula of the samples is known, the identity and concentration of the interfering substance are known and the extent of interference in the assay may be determined.

A number of modifications to the simple spectrophotometric procedure are available to the analyst, which may eliminate certain sources of interference and permit the accurate determination of all of the absorbing components. Each modification of the basic procedure may be applied if certain criteria are satisfied.

The basis of all the spectrophotometric techniques for multicomponent samples is the property that at all wavelengths:

the absorbance of a solution is the sum of absorbance of the individual components or   the measured absorbance is the difference between the total absorbance of the solution in the sample cell and that of the solution in the reference cell. There are various spectrophotometric methods are available which can be used for the analysis of a combination samples. Following methods can be used

Simultaneous equation method Derivative spectrophotometric method Absorbance ratio method ( Q-Absorbance method) Difference spectrophotometry Solvent extraction method