» Erythromycin Ointment is Erythromycin in a suitable ointment base. It contains not less than 90.0 percent and not more than 125.0 percent of the labeled amount of erythromycin(C37H67NO13).
Packaging and storage Preserve in collapsible tubes or in other tight containers, preferably at controlled room temperature.
USP Reference standards 11
USP Erythromycin B RS
USP Erythromycin C RS
A: Transfer a quantity of Ointment, equivalent to about 5 mg of erythromycin, to a separator containing 50 mL of solvent hexane. Shake until dissolved. Extract with three separate 20-mL portions of methanol. Combine the methanol extracts in a beaker, and evaporate to dryness. Dissolve the residue in 2 mL of methanol (test solution). Proceed as directed in the Identification test under Erythromycin Delayed-Release Capsules, beginning with Prepare a Standard solution of USP Erythromycin RS.
B: The retention times of the peaks for erythromycin A, erythromycin B, and erythromycin C in the chromatogram of the Assay preparation correspond to those in the chromatograms of the Standard preparation and Erythromycins B and C standard preparation, as obtained in the Assay.
Minimum fill 755: meets the requirements.
Water, Method I 921: not more than 1.0%, 20 mL of a mixture of toluene and methanol (7:3) being used in place of methanol in the titration vessel.
Solution A Prepare a degassed mixture of acetonitrile and water (90:10). Store in a reservoir protected from air by sparging with helium.
Solution B To 1000 mL of degassed water add 0.5 mL of sodium hydroxide solution (1 in 2), using a suitable syringe and needle to minimize exposure to air. Degas, and store in a reservoir protected from air by sparging with helium.
Solution C Use degassed water, and store in a reservoir protected from air by sparging with helium.
Mobile phase Using a suitable pumping system, pump Solution A, Solution C, and Solution B from the respective reservoirs in the ratio of 56:37:7. Make any necessary adjustments (see System Suitability under Chromatography 621).
Diluent Prepare a mixture of methanol and water (50:50).
Standard preparation Quantitatively prepare a solution of USP Erythromycin RS in Diluent having a known concentration of about 0.66 mg per mL.
Erythromycins B and C standard preparation Quantitatively prepare a solution in Diluent having known concentrations of about 34 µg each of USP Erythromycin B RS and USP Erythromycin C RS per mL.
System suitability solution Transfer about 2 mg of USP Erythromycin Related Compound N RS to a 10-mL volumetric flask, add 0.4 mL of the Standard preparation and 6 mL of Erythromycins B and C standard preparation, and mix. Dilute with Erythromycins B and C standard preparation to volume, and mix.
Assay preparation Transfer an accurately weighed portion of Ointment, equivalent to about 60 mg of erythromycin, to a 125-mL separator. Add 50 mL of solvent hexane, and shake until dissolved. Extract with four separate 20-mL portions of Diluent, collecting the extracts in a 100-mL volumetric flask. Dilute the combined extracts with Diluent to volume, mix, and pass a portion of the solution through a filter having a 0.45-µm porosity. Use the clear filtrate as the Assay preparation.
Chromatographic system (see Chromatography 621) The liquid chromatograph is equipped with an electrochemical detector, a glassy carbon electrode fitted with three gaskets, a 4-mm × 5-cm guard column that contains 8-µm packing L50, and a 4-mm × 25-cm analytical column that contains 8-µm packing L50. The electrochemical detector is used in the pulsed integrated amperometric mode with a range of 10 nC, an output of 1 V full scale, a rise time of 0.6 second, positive polarity, E = 0.9 V; t1 = 400 ms; E2 = 0.9 V; t2 = 100 ms; E3 = 0.9 V; t3 = 100 ms. The flow rate is about 1 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.4 for erythromycin related compound N, 0.5 for erythromycin C, 1.0 for erythromycin A, and 1.6 for erythromycin B; the resolution, R, between erythromycin related compound N and erythromycin C is not less than 0.6, between erythromycin C and erythromycin A not less than 2.5, and between erythromycin A and erythromycin B not less than 2.5. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 2; and the relative standard deviation for replicate injections is not more than 3%. [noteTurn off the electrochemical detector before stopping the flow of the Mobile phase. ]
Procedure Separately inject equal volumes (about 10 µL) of the Standard preparation, Erythromycins B and C standard preparation, and the Assay preparation into the chromatograph, record the chromatograms, and measure the peak areas. Calculate the percentage of erythromycin A in the portion of Ointment taken by the formula:
0.1(CP/W)(rU / rS)in which C is the concentration, in mg per mL, of USP Erythromycin RS in the Standard preparation; P is the designated percentage of erythromycin A in USP Erythromycin RS; W is the quantity, in g, of Ointment taken to prepare the Assay preparation; and rU and rS are the erythromycin A peak areas obtained from the Assay preparation and the Standard preparation, respectively. Calculate the percentages of erythromycin B and erythromycin C in the portion of Ointment taken by the formula:
0.0001(CP/W)(rU / rS)in which C is the concentration, in µg per mL, of the relevant USP Reference Standard in the Erythromycins B and C standard preparation; P is the designated percentage of erythromycin B or erythromycin C in the relevant USP Reference Standard; W is the quantity, in g, of Ointment taken to prepare the Assay preparation; and rU and rS are the peak areas for the relevant analyte obtained from the Assay preparation and the Erythromycins B and C standard preparation, respectively. Calculate the percentage content of erythromycin in the Ointment by adding the percentages of erythromycin A, erythromycin B, and erythromycin C found.
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USP35NF30 Page 3081Pharmacopeial Forum: Volume No. 28(2) Page 297