Question

A Solphadine tablet was analysed by HPLC in order to determine the quantity of caffeine. One tablet (0.3478g) was dissolved in water and made up to 200 mL. Caffeine standards were prepared from a stock solution of 100mg/ 100mL of caffeine. 10, 20, 30, 40, and 50 mL aliquots were taken and made up to a final volume of 100 mL in water. The standards and tablet sample were injected into the HPLC using a 20?L injection loop. The separation was achieved using a 15cm 4.6mm id ODS stationary phase with 60:40 H₂O:CH₃OH as the mobile phase. Detection was by uv absorption at 265nm. .

Table 1 gives the peak areas obtained for each solution

Caffeine Conc (mg/100mL)	Std1	Std2	Std3	Std4	Std5	tablet
Caffeine peak area	990	1900	2894	3818	4584	2742

Plot a suitable graph and determine the caffeine content of the tablets on in mg

Calculate the % caffeine in the tablet.

Answer 1

Caffeine standards:

100mg/100mL x aliquot mL / 100mL = conc. in standard (see table)

Caffeine Conc (mg/100mL)	Std1 10	Std2 20	Std3 30	Std4 40	Std5 50	tablet
Caffeine peak area	990	1900	2894	3818	4584	2742

Calibration curve:

10	20	30	40	50
990	1900	2894	3818	4584

slope =	91.06
intercept=	105.4

Area = 91.06c +105.4

Tablet sample concentration:

2742 = 91.06c +105.4

C = (2742-105.4)/91.06 = 28.93 mg/100mL

Mass of caffeine = 28.93 mg/100mL x 200mL = 57.86 mg

Contration in tablet C= 100 x 57.86 mg/ 347.8mg = 16.63 %