A: It meets the requirements under Thin-Layer Chromatographic Identification Test 201BNP.

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

Mobile phase— Dissolve 4.44 g of docusate sodium in 1000 mL of a mixture of methanol and water (4:1), add 1 mL of 0.1 N sulfuric acid, and mix. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve a suitable quantity of USP Lidocaine RS, accurately weighed, in Mobile phase to obtain a solution having a known concentration of about 0.4 mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ointment, equivalent to about 40 mg of lidocaine, to a separator, add 50 mL of n-hexane, and shake until the specimen is in solution. Add 30 mL of Mobile phase, shake for 1 minute, and allow the layers to separate. Drain the lower layer into a 100-mL volumetric flask, and extract the n-hexane layer remaining in the separator with two 30-mL portions of Mobile phase, combining the lower layers in the volumetric flask. Dilute the combined extracts in the 100-mL volumetric flask with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 230-nm detector and a 4-mm × 25-cm column that contains packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak response as directed for Procedure: the column efficiency determined from the analyte peak is not less than 500 theoretical plates, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 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 lidocaine (C14H22N2O) in the portion of Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Lidocaine RS in the Standard preparation, and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.