A: Transfer a quantity of Capsule contents, equivalent to about 250 mg of mexiletine hydrochloride, to a suitable test tube, add 10 mL of methanol, and mix on a vortex mixer for 1 minute. Filter the mixture, evaporate the filtrate under a stream of nitrogen to dryness, and dry the residue in vacuum at 60 for 1 hour: the IR absorption spectrum of a mineral oil dispersion of the dried residue so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Mexiletine Hydrochloride RS.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C11H17NO·HCl dissolved from the difference between first derivative values at the wavelengths of maximum and minimum first derivative absorbance in the wavelength range from 230 to 290 nm on filtered portions of the solution under test, suitably diluted with Dissolution Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Mexiletine Hydrochloride RS in the same Medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C11H17NO·HCl is dissolved in 30 minutes.

Mobile phase, Standard preparation, and Resolution solution— Prepare as directed in the Assay under Mexiletine Hydrochloride.

Standard solution— Transfer 10.0 mL of the Standard preparation prepared as directed in the Assay under Mexiletine Hydrochloride to a 1000-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 20 µg of USP Mexiletine Hydrochloride RS per mL.

Test solution— Use the Assay preparation prepared as directed in the Assay.

Chromatographic system (see Chromatography 621)— Prepare as directed in the Assay under Mexiletine Hydrochloride, except that the relative standard deviation of replicate injections of the Standard solution is not more than 3.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into the chromatograph; record the chromatograms using a high sensitivity setting for the recorder; and measure the areas for the peaks. Calculate the percentage of each impurity observed by the formula: in which C is the concentration, in mg per mL, of USP Mexiletine Hydrochloride RS in the Standard solution; L is the quantity, in mg, of mexiletine hydrochloride in each mL of the Test solution, based on the labeled amount in the portion of Capsule contents used to prepare the Assay preparation and the extent of dilution; rU is the peak area obtained from an individual impurity observed in the chromatogram of the Test solution; and rS is the mexiletine peak area obtained from the Standard solution: not more than 1% of any individual impurity is found; and the total of all observed impurities is not more than 1.5%.

Mobile phase, Standard preparation, Resolution solution, and Chromatographic system— Prepare as directed in the Assay under Mexiletine Hydrochloride.

Assay preparation— Weigh the contents of not fewer than 20 Capsules, and calculate the average weight per Capsule. Mix the combined contents of the Capsules, and transfer an accurately weighed portion, equivalent to about 50 mg of mexiletine hydrochloride, to a stoppered, 50-mL centrifuge tube. Add 25.0 mL of Mobile phase, insert the stopper, and shake by mechanical means for 15 minutes. Centrifuge, and use the clear supernatant as the Assay preparation. [note—Reserve a portion of this solution for use as the Test solution in the test for Chromatographic purity.]

Procedure— Proceed as directed for Procedure in the Assay under Mexiletine Hydrochloride. Calculate the quantity, in mg, of mexiletine hydrochloride (C11H17NO·HCl) in the portion of Capsule contents taken by the formula: in which C is the concentration, in mg per mL, of USP Mexiletine Hydrochloride RS in the Standard preparation; and rU and rS are the mexiletine peak responses obtained from the Assay preparation and the Standard preparation, respectively.