A: Place an amount of powdered Tablets, equivalent to 15 to 20 mg of probucol, in the depression of a spot plate, and add two drops of sulfuric acid: an intense yellow color is produced immediately. The color corresponds to that obtained from a similar preparation of USP Probucol RS.

B: Transfer a quantity of finely ground Tablets, equivalent to about 1000 mg of probucol, to a copper centrifuge tube. Add 5 mL of methylene chloride, cap the tube, and mix for about 30 seconds on a vortex mixer. Centrifuge at about 7000 rpm for 10 minutes. Decant the supernatant, and place about 1 mL of this solution on each side of a potassium bromide plate, holding the plate at an angle so that a thin film is formed on the plate: the IR absorption spectrum of the clear film so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Probucol RS.

Mobile phase, System suitability preparation, and Chromatographic system— Proceed as directed in the Assay under Probucol.

Standard preparation— Dissolve an accurately weighed quantity of USP Probucol RS in Mobile phase, and dilute quantitatively and stepwise with Mobile phase to obtain a solution having a known concentration of about 50 µg per mL.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed quantity of the powder, equivalent to about 500 mg of probucol, to a 100-mL volumetric flask, add 80 mL of Mobile phase, and stir the mixture vigorously for about 1 hour. Dilute with Mobile phase to volume, and mix. Transfer 1.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, mix, and filter.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C31H48O2S2 in the portion of Tablets taken by the formula: in which C is the concentration, in µg per mL, of USP Probucol RS in the Standard preparation, and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.