Abstract
Four simple, precise and accurate spectrophotometric methods have been developed for simultaneous determination of Allopurinol (ALP) and Benzbromarone (BENZ) in their bulk powder and pharmaceutical dosage form. Method (I) is dual wavelength analysis, method (II) is Q-analysis (graphical absorbance ratio) method, method (III) is the mean centering of ratio spectrophotometric (MCR) method while method (IV) is the extended ratio subtraction method (EXRSM). In method (I) two wavelengths were selected for each drug in such a way that the difference in absorbance was zero for the second drug. At wavelengths 238.2 and 261.2 nm ALP had equal absorbance values; therefore, these two wavelengths have been used to determine BENZ; on a similar basis 253 and 274.4 nm were selected to determine ALP. Method (II) involves the formation of Q-absorbance equation using the respective absorptivity values at 245.8 nm (isoabsorptive point) and 250 nm (λmax of ALP). In method (III) the absorption spectra of both ALP and BENZ with different concentrations were recorded over 210-280 and 210-275 nm, respectively, divided by the spectrum of suitable divisor of both ALP and BENZ and then they obtained ratio spectra were mean centered. Method (IV) starts with the normal ratio subtraction method (RSM) for determination of ALP at its λmax (250 nm), while an extension of the already developed method has been established as a new approach for BENZ determination at its λmax (238 nm). Accuracy, precision and recovery studies of the developed methods have been carried out in order to confirm their accuracy. Specificity of the methods was also tested by their application for determination of different synthetic mixtures and they have been successfully used for drugs determination in their combined dosage form. Statistical comparison of the developed methods with the reported HPLC one showed no significant difference.
Highlights
In method (I) two wavelengths were selected for each drug in such a way that the difference in absorbance was zero for the second drug
The absorption spectra of the prepared solutions were recorded in the range of 200-400 nm and the absorbance values at 245.8nm and 250nm were measured from which the absorptivity values for both drugs at the selected wavelengths were calculated
Linearity and range: The calibration range for ALP and BENZ was established through considerations of the practical range necessary according to adherence to Beer-Lambert's law and the concentration of ALP and BENZ present in the pharmaceutical dosage form to give accurate, precise and linear results
Summary
Spectral characteristics and wavelength selection: The absorption spectra of 20 μgml-1 each of ALP, BENZ and their 1:1 mixture (containing 10 μgml-1 each) in methanol were recorded over the range of 200-400 nm using methanol as a blank. The absorption spectra of the prepared solutions were recorded in the range of 200-400 nm and the absorbance values at 245.8nm (λiso) and 250nm (λmax of ALP) were measured from which the absorptivity values for both drugs at the selected wavelengths were calculated. The absorption spectra of the prepared solutions were recorded in the range of 210-280 nm, divided by the standard spectrum of 20 μgmL-1of BENZ and they obtained ratio spectra were mean centered. Method IV(Extended ratio subtraction method): Standard solutions containing 5-50 μgmL-1 of ALP and 5-20 μgmL-1 of BENZ were prepared separately by appropriate dilutions of their respective working standard solutions in methanol and scanned in the range of 200-400 nm. Recovery studies: To study the accuracy of the proposed methods, recovery studies were carried out by applying the standard addition technique at different levels (80%, 100% and 120%) where known amounts of the studied drugs were separately added to the preanalyzed Alloben® tablets powder and the percentage recoveries were calculated
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