A: Transfer a quantity of Capsule contents,equivalent to about 250mg of mexiletine hydrochloride,to a suitable test tube,add 10mLof methanol,and mix on a vortex mixer for 1minute.Filter the mixture,evaporate the filtrate under a stream of nitrogen to dryness,and dry the residue in vacuum at 60for 1hour:the IRabsorption 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 preparationcorresponds to that in the chromatogram of the Standard preparation,as obtained in the Assay.

Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 30minutes.

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 230to 290nm 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 RSin the same Medium.

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

Mobile phase,Standard preparation,andResolution solution— Prepare as directed in the Assayunder Mexiletine Hydrochloride.

Standard solution— Transfer 10.0mLof the Standard preparationprepared as directed in the Assayunder Mexiletine Hydrochlorideto a 1000-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.This solution contains about 20µg of USP Mexiletine Hydrochloride RSper mL.

Test solution— Use the Assay preparationprepared as directed in the Assay.

Chromatographic system (see Chromatography á621ñ)— Prepare as directed in the Assayunder Mexiletine Hydrochloride,except that the relative standard deviation of replicate injections of the Standard solutionis not more than 3.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto 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 Cis the concentration,in mg per mL,of USP Mexiletine Hydrochloride RSin the Standard solution;Lis the quantity,in mg,of mexiletine hydrochloride in each mLof the Test solution,based on the labeled amount in the portion of Capsule contents used to prepare the Assay preparationand the extent of dilution;rUis the peak area obtained from an individual impurity observed in the chromatogram of the Test solution;and rSis 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 Assayunder Mexiletine Hydrochloride.

Assay preparation— Weigh the contents of not fewer than 20Capsules,and calculate the average weight per Capsule.Mix the combined contents of the Capsules,and transfer an accurately weighed portion,equivalent to about 50mg of mexiletine hydrochloride,to a stoppered,50-mLcentrifuge tube.Add 25.0mLof Mobile phase,insert the stopper,and shake by mechanical means for 15minutes.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 Procedurein the Assayunder Mexiletine Hydrochloride.Calculate the quantity,in mg,of mexiletine hydrochloride (C11H17NO·HCl)in the portion of Capsule contents taken by the formula: in which Cis the concentration,in mg per mL,of USP Mexiletine Hydrochloride RSin the Standard preparation;and rUand rSare the mexiletine peak responses obtained from the Assay preparationand the Standard preparation,respectively.