The mixed standard solutions of these drugs containing 4�C20 ��g/mL of DTR and 4.5�C22.5 ��g/mL of ETR were prepared by serial dilutions of standard stock solutions in distilled water. Mixed standard solutions were scanned using 20 ��g/mL of DRT solution as blank. Instrument response at 274 nm animal study and 351 nm was measured for ETR and DRT, respectively, and used to prepare the calibration curve. Six replicates of five mixed standard solutions were used to prepare the calibration curve. Correlation coefficient is not true indicator of linearity therefore the Fischer variance ratio (test of linearity) was used. Test of linearity was performed by using MIP Pharmasoft 1.0, software developed and validated at MAEER’S Maharashtra Institute of Pharmacy, Pune.
Analysis of tablet formulation Twenty tablets were weighed accurately and a quantity of tablet powder equivalent to about 80 mg of DRT (90 mg of ETR) was dissolved in the 80 mL of methanol with sonication for 15 min, solution was filtered through Whatmann filter paper No. 41 into a 100 mL volumetric flask. The filter paper was washed by using solvent, filtrate, and washing transferred to a volumetric flask and the volume was made up to the mark. The solution was suitably diluted with distilled water to get of 12 ��g/mL of DRT and 13.5 ��g/mL of ETR. Instrument response for the analytes was measured by following the procedure described in preparation of standard stock solutions and calibration curve section. Accuracy Recovery studies were carried out by applying the method to drug content present in tablet dosage forms to which known amount of mixed standard of ETR and DRT was added at 50%, 100%, and 150% levels.
At each of the levels, three determinations were performed. Precision The precision of repeatability was studied by six replicate analysis of tablet solutions containing 12 ��g/ mL of DRT. The precision was also studied in terms of intra-day changes in absorbance of drug solution on the same day and on three different days. The intra-day precision of the developed method was determined by preparing the tablet samples of the same batch in nine determinations with three concentrations and three replicate each on the same day. The inter-day precision was also determined by assaying the tablets in triplicate per day for consecutive 3 days. The intra-day and inter-day variations were calculated in terms of percentage relative standard deviation.
Precision of the analyst was also determined by repeating the method by another analyst working in the lab. Three concentration used for the study were 6, 12, and 18 ��g/mL of DRT and 7.25, 13.5, and 20.75 ��g/mL of ETR. Precision data were also analysed by using experimental design based on two-way ANOVA. Method sensitivity (LOD and LOQ) The values GSK-3 of LOD and LOQ were calculated by using �� (standard deviation of response) and b (slope of the calibration curve) and by using equations, LOD = (3.3 �� ��)/b and LOQ = (10 �� ��)/b.
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