Prepared and presented by:Pinak R. Patel

Assistant ProfessorDepartment of pharmaceutical Chemistry

Dharmaj, degree pharmacy college

Multi-component analysis in pharmaceutical dosage forms

by UV Spectrophotometry

• 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.

Some of the commonly used Spectrophotometric methods are as follows,

1. Simultaneous equation method (Vierdott’s method)

2. Derivative Spectrophotometric method

3. Absorbance ratio method ( Q-Absorbance method)

4. Solvent extraction method

5. Dual wavelength method

6. Geometric correction method

7. Orthogonal poly nominal method

8. H-point standard addition method

9. Least square approximation method

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.

1. Simultaneous equation method• If a sample contain two absorbing drugs (X & Y) each of this

absorbs at the λmax of each other i.e. λ1 and λ2 (figure 2), it may be possible to determine both the drugs by the technique of simultaneous equation method provided that certain criteria apply.

• The information required is:• The absorptivity of X at λ1 and λ2 and ax1 and ax2 respectively.

• The absorptivity of Y at λ1 and ay1 and ay2 respectively.

• The absorbance of the diluted sample at λ1 and λ2, A1 and A2 respectively.

• Let Cx & Cy be the concentration of X & Y respectively in the diluted sample. Two equations are constructed based upon the fact that at λ1 and λ2 the absorbance of the mixture is the sum of the individual absorbance of X& Y.

• At λ1 A1 = ax1 bcx + ay1 bcy ………(1)

• At λ2 A2 = ax2 bcx + ay2 bcy ………(2)

• Rearrange eq. (2). cy = A2 - ax2cx / ay2

• Substituting for cy in eq. (1). And rearranging gives

cx = A2ay1 – A1ay2 / ax2ay1 – ax1ay2

And cy = A1ax2 – A2ax1 / ax2ay1 – ax1ay2

• 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,

• A2/A1/ ax2/ ax1 and ay2/ay1/ A2/A1

• Should lie outside the range of 0.1-20, for the precise determination of Y and X respectively

• These criteria are satisfied only when the λmax 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.