Identification—
A:
Thin-Layer Chromatographic Identification Test 
201
—
Test solution—
Disperse a quantity of Cream, equivalent to about 25 mg of neomycin, with 20 mL of chloroform in a 60-mL separator. Add 0.2 mL of 2.5 N hydrochloric acid, and shake. Allow the layers to separate for about 30 minutes. Discard the lower chloroform layer, and centrifuge the upper aqueous layer. Use a portion of the centrifuged aqueous layer.
Standard solution—
Dissolve suitable quantities of
USP Neomycin Sulfate RS and
USP Polymyxin B Sulfate RS in 0.1 N hydrochloric acid to obtain a solution containing the equivalent of about 3.5 mg of neomycin and 10,000 USP Polymyxin B Units per mL.
Developing solvent system—
Dissolve 0.1 g of benzalkonium chloride in a mixture of isopropyl alcohol, water, and ammonium hydroxide (60:40:10).
Procedure—
Proceed as directed in the chapter. Place the plate in a chromatographic chamber saturated with
Developing solvent system, and develop the chromatogram. Dry the plate at 105
for about 10 minutes, spray with a solution of ninhydrin in butyl alcohol (1 in 200), and heat the plate at 105
for about 15 minutes. The
RF values of the two principal spots in the chromatogram obtained from the
Test solution correspond to those of the two principal spots in the chromatogram obtained from the
Standard solution.
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 for pramoxine hydrochloride.
Assay for neomycin—
Proceed as directed for neomycin under
Antibiotics—Microbial Assays 81, using an accurately weighed portion of Cream, equivalent to about 3.5 mg of neomycin, blended for 3 to 5 minutes in a high-speed blender with 249 mL of
Buffer No. 3 and 1 mL of polysorbate 80. Quantitatively dilute an accurately measured volume of this solution with
Buffer No. 3 to obtain a
Test Dilution having a concentration of neomycin assumed to be equal to the median level of the Standard (1 µg of neomycin per mL).
Assay for polymyxin—
Proceed as directed for polymyxin B under
Antibiotics—Microbial Assays 81, using an accurately weighed portion of Cream, equivalent to about 10,000 USP Polymyxin B Units, blended for 3 to 5 minutes in a high-speed blender with 199 mL of
Buffer No. 6 and 1 mL of polysorbate 80. Quantitatively dilute an accurately measured volume of this solution with
Buffer No. 6 to obtain a
Test Dilution having a concentration of polymyxin B assumed to be equal to the median dose level of the Standard (10 USP Polymyxin B Units per mL).
Assay for pramoxine hydrochloride—
Mobile phase—
Dissolve 3.5 g of dibasic potassium phosphate in 1000 mL of water. Prepare a mixture of this solution, acetonitrile, and triethylamine (700:300:2), and adjust with phosphoric acid to a pH of 4.0 ± 0.1. Filter and degas. Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Standard preparation—
Prepare a solution of
USP Pramoxine Hydrochloride RS in methanol to obtain a solution having a known concentration of about 0.2 mg per mL.
Assay preparation—
Transfer an accurately weighed portion of Cream, equivalent to about 10 mg of pramoxine hydrochloride, to a 50-mL volumetric flask, add about 5 mL of chloroform, and sonicate at about 40
to disperse the Cream. Allow to cool to room temperature, dilute with methanol to volume, and mix. Pass a portion of this solution through a glass fiber filter and a PTFE filter having a 0.45-µm porosity, discarding the first few mL of the filtrate.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 280-nm detector, a guard column that contains packing L7, and a 4.6-mm × 25-cm analytical column that contains packing L7. The column is maintained at a constant temperature of about 40
. The flow rate is about 2 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: 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 areas for the major peaks. Calculate the quantity, in mg, of pramoxine hydrochloride (C
17H
27NO
3·HCl) in each g of Cream taken by the formula:
50(C/W)(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Pramoxine Hydrochloride RS in the
Standard preparation; W is the weight, in g, of Cream taken to prepare the
Assay preparation; and
rU and
rS are the peak areas for pramoxine obtained from the
Assay preparation and the
Standard preparation, respectively.
