A: Infrared Absorption 197S—

B: Transfer about 100 mg of it, accurately weighed, to a 100-mL volumetric flask, and dissolve in 25 mL of alcohol. Dilute quantitatively and stepwise with sodium hydroxide solution (1 in 250) to obtain a 1 in 50,000 solution: the UV absorption spectrum of this solution exhibits maxima and minima only at the same wavelengths as that of a similar solution of USP Trimethoprim RS, concomitantly measured; and the respective absorptivities, calculated on the dried basis for the test sample only, at the wavelength of maximum absorbance at about 287 nm do not differ by more than 3.0%.

Buffer solution— Prepare a 10 mM sodium perchlorate solution in water, adjust with phosphoric acid to a pH of 3.6, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and methanol (7:3). Make adjustments if necessary (see System Suitability under Chromatography 621).

Resolution solution— Dissolve accurately weighed quantities of USP Trimethoprim RS and diaveridine; and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having known concentrations of about 10 µg per mL and 5 µg per mL, respectively.

Test solution— Transfer about 25.0 mg of Trimethoprim, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains base-deactivated packing L1. The flow rate is 1.3 mL per minute. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the resolution, R, between the peaks for trimethoprim and diaveridine is not less than 2.5; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Inject a volume (about 20 µL) of the Test solution into the chromatograph, record the chromatogram for not less than 11 times the retention time of the trimethoprim peak, and measure all of the peak responses. Calculate the percentage of each impurity in the portion of Trimethoprim taken by the formula: in which F is a relative response factor, and is equal to 0.5 for any peak having a relative retention time of 0.9, 2.3, 2.7, or 10.3, and is equal to 1.0 for all other peaks; ri is the peak response for each impurity; and rT is the peak response for trimethoprim obtained from the Test solution: not more than 0.1% of any individual impurity is found; and not more than 0.2% of total impurities is found.