Multicomponant analysis

Simultaneous qeuation method

1. Prepared and presented by: Pinak R. Patel Assistant Professor Department of pharmaceutical Chemistry Dharmaj, degree pharmacy college Multi-component analysis in pharmaceutical dosage forms by UV Spectrophotometry

2. 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 interferents are known and the extent of interference in the assay may be determined.

3. Some of the commonly used Spectrophotometric methods are as follows, Simultaneous equation method ( Vierdott’s method) Derivative Spectrophotometric method Absorbance ratio method ( Q-Absorbance method ) Solvent extraction method Dual wavelength method Geometric correction method Orthogonal poly nominal method H-point standard addition method Least s quare approximation method

4. 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 . And most importantly the excipients present in the formulation are not absorbing at the wavelength of experiment. If all of these conditions are satisfied than we can apply these methods satisfactorily.

5. 1. Simultaneous equation method If a sample contain two absorbing drugs (X & Y) each of this absorbs at the λ max of each other i.e. λ 1 and λ 2 (figure 2), it may be possible to determine both the drugs by the technique of simultaneous equation method provided that certain criteria apply .

6. The information required is: The absorptivity of X at λ 1 and λ 2 and a x1 and a x2 respectively. The absorptivity of Y at λ 1 and a y1 and a y2 respectively. The absorbance of the diluted sample at λ 1 and λ 2, A 1 and A 2 respectively. Let C x & C y be the concentration of X & Y respectively in the diluted sample. Two equations are constructed based upon the fact that at λ 1 and λ 2 the absorbance of the mixture is the sum of the individual absorbance of X& Y . At λ 1 A 1 = a x1 bc x + a y1 bc y ………( 1 ) At λ 2 A 2 = a x2 bc x + a y2 bc y ………( 2)

7. Rearrange eq. (2 ). c y = A 2 - a x2 c x / a y2 Substituting for c y in eq. (1). And rearranging gives c x = A 2 a y1 – A 1 a y2 / a x2 a y1 – a x1 a y2 And c y = A 1 a x2 – A 2 a x1 / a x2 a y1 – a x1 a y2 Criteria for obtaining maximum precision, based upon absorbance ratios, have been suggested that place limit on the relative concentrations of the components of the mixture. The criteria are that the ratios, A 2 /A 1 / a x2 / a x1 and a y2 /a y1 / A 2 /A 1 Should lie outside the range of 0.1-20, for the precise determination of Y and X respectively

8. These criteria are satisfied only when the λ max of the two components are reasonably dissimilar. An additional criterion is that the two components must not interact chemically, thereby negating the initial assumption that the total absorbance is the sum of individual absorbance.