asymmetry were observed in the ratio of 50:50% v/v of acetonitrile and methanol. It was found to be the optimum mobile phase concentration.3.2. Choice of internal standardSeveral substances were tested as internal standards. Among these, lamotrigine has beenchosen as the most appropriate in the present analysis because it is stable. In the present study, itdid not interfere with the matrix of pharmaceutical samples and it was well separated from TDF.More over, a significant advantage of this IS was its elution time that was shorter than that of TDFres u l t i ng in s ho r t r un t i me , l e s s t han 5 m in . A typ i c a l ch rom a tog ra m o f TD F and IS u s i ng the  proposed method is shown in Fig.2. Sharp and symmetrical peak was obtained with good baselinefor each compound, thus facilitating the accurate measurements of peak area. The average retentiontimes for TDF and IS were found to be 4.11 ± 0.03 and 2.27 ± 0.02 min, respectively. Under thedescribed parameters, the respective compounds were clearly separated and their corresponding  peaks were sharply developed at reasonable retention times.3.3. Validation of methods3.3.1. LinearityFive points calibration graphs were constructed covering a concentration range 1-5 µg/ ml(see section 2.3). Three independent determinations were performed at each concentration. Linear relationships between the ratio of peak area signal of TDF to that of IS versus the correspondingdrug concentration were observed, as shown by the results presented in Table 1. The standarddeviations of the slope and intercept were low. The determination coefficient (r 2) exceeded 0.999.T o d e t e r m i n e w h e t h e r t h e e x p e r i m e n t a l i n t e r c e p t ( a ) o f t h e r e g r e s s i o n e q u a t i o n w a s n o t significantly different from the theoretical zero value, confidence interval (99%) and student’s t-testwere performed. It concerns the comparison of t = a/ sawhere a is the intercept of the regressionequation and sai s t he s t a nda rd dev ia t i on o f a , w i th t a bu la t e d da t a o f t he t - d i s t r i bu t i on . As the calculated t value (t = 1.4350) does not exceed to (0.001, 14) = 4.140, the intercept of regressionequation is not significantly different from 0 (point estimation). By using 99% confidence interval,the value lies between 0.0273 - 0.0452.This shows that the intercept will fall on this range and thedistance from zero is very short (Interval estimation).3.3.2. PrecisionThe repeatability study (n=6) carried out showed a R.S.D. of 1.340 % for the peak a r ea r a t i o o f T DF o f IS ob t a ined , t hus s howi ng tha t t he e qu ipmen t u se d fo r t he s tudy w orked  correctly for the developed analytical method and being highly repetitive. For the intermediate precision a study carried out by the same analyst working on 3 consecutive days (n = 3) indicated aR.S.D. of 0.744 and 1.126 %. Both values were far below 2%, the limit percentage set for the precision and indicated a good method precision.3.3.3. AccuracyThe data for accuracy were expressed in terms of percentage recoveries of TDF inthe real samples. These results are summarized in Table 2. The mean recovery data of TDF in realsample were within the range of 99.18 and 100.84 % for Forzest and 98.70 and 100.85

for Tazzle.Mean % R.S.D. was 1.348 % and 0.880 %, satisfying the acceptance criteria for the study.3.3.4. SpecificityThe HPLC chromatogram recorded for the mixture of the drug excipients revealedno peak within a retention time range of 5 min. The results showed that the developed method wasspecific as none of the excipients interfered with the analytes of interest (Fig.2).3.3.5. StabilityThe stability of TDF in standard and sample solutions containing IS determined by storingthe solutions at ambient temperature (20 ± 1°C) protected from light. The solutions were checkedin triplicate after 3 successive days of storage and the data were compared with freshly preparedsamples. In each case, it could be noticed that solutions were stable for 48 hrs, as during this timethe results did not decrease below 97%. This denotes that TDF is stable in standard and samplesolutions for at least 2 days at ambient temperature, protected from light and is compatible with IS.3.3.6. System suitabilityThe resolution factor between IS and TDF, in the developed method, was above 2.The % R.S.D. of peak area ratios of TDF to that of IS and retention times for both drug and IS werewithin 2% indicating the suitability of the system (Table 3). These results indicate the applicabilityof this method to routine with no problems, its suitability being proved. The system suitability parameter like capacity factor, asymmetric factor, tailing factor, HETP and number of theoretical   p l a t e s a l s o ca l cu l a t ed . I t w as obse rved tha t a l l t he va lues a r e w i t h in t he l im i t s . The s t a t i s t i c a l evaluation of the proposed method revealed its good linearity, reproducibility and its validation for different parameters and let us to the conclusion that it could be used for the rapid and reliabledetermination of TDF in tablet formulation.  3.4. Assay of tablets T h e v a l i d a t e d m e t h o d w a s a p p l i e d f o r t h e a s s a y o f t w o c o m m e r c i a l t a b l e t s con ta in ing 20m g o f TD F: Taz z l e a nd fo r ze s t . Ea ch sa mp le wa s ana lyze d in t r i p l i c a t e a f t e r   e x t r a c t i n g t h e d r u g a s m e n t i o n e d i n a s s a y s a m p l e p r e p a r a t i o n o f t h e e x p e r i m e n t a l s e c t i o n (section 2.4) and injections were carried out in triplicate Fig.2. shows a HPLC chromatogram of   TDF in pharmaceutical tablets. None of the tablet ingredients interfered with the analyte peak. There s u l t s p r e s en t ed in Ta b le 4 a r e i n good ag ree men t w i t h t he l abe l ed con te n t . A ss ay r e s u l t s , e x p r e s s e d a s t h e p e r c e n t a g e o f l a b e l c l a i m , w e r e f o u n d t o b e 9 9 . 2 1 ± 1 . 3 4 0 f o r F o r z e s t ; 99.27 ± 1.253 for Tazzle showing that the content of TDF in tablet formulations confirmed to thecontent requirements (95-105 %) of the label claim. Low values of standard deviation denoted verygood r ep roduc i b i l i t y o f t he meas u rem en t . T he above r e s u l t s dem ons t r a t ed t ha t t he deve l oped method achieved rapid and accurate determination of TDF and could be used for the determinationof TDF drug substance and pharmaceutical formulations.4. ConclusionA validated isocratic HPLC-UV method has been developed for the determination of TDF in dosage forms Forzest and Tazzle. The proposed method is simple, rapid, accurate, precise,and s pec i f i c . I t s ch rom a tog ra ph ic run t i me o f 5 m in a l l ow s t he ana l y s i s o f a l a rge num ber o f   s a mp le s i n a s ho r t pe r iod o f t ime . The re fo re , i t i s su i t ab l e fo r t he rou t ine ana ly s i s o f TD F in   pharmaceutical dosage forms. The simplicity of the method allows for

application in laboratoriesthat lack sophisticated analytical instruments such as LC-MS that is complicated, costly and time consuming rather than a simple HPLC-UV method. Hence the proposed method could be useful for the national quality control laboratories in developing countries