(Official January 1, 2007)

Standard preparations— Prepare a solution of freshly ignited zinc oxide in dilute hydrochloric acid (1 in 60) to obtain the equivalent of 1.0 mg of zinc per mL. Dilute quantitatively with water to obtain separate solutions containing the equivalent of 15 and 30 µg of zinc per mL.

Assay preparation— Transfer about 1.0 g of Ointment, accurately weighed, to a 100-mL beaker. Add 25 mL of dilute hydrochloric acid (1 in 20), swirl, and heat carefully until the mixture is liquefied. Cool, and transfer the mixture to a 250-mL separator. Complete the transfer of the waxy residue by thoroughly rinsing the beaker with 50 mL of water and two 50-mL portions of chloroform and adding the rinsings to the separator. Equilibrate the mixture, and transfer the chloroform extract to a 500-mL separator. Extract the aqueous phase with another 100-mL portion of chloroform, combine the second chloroform extract with the main extract in the 500-mL separator, and transfer the aqueous phase to a 200-mL volumetric flask. Wash the combined chloroform extracts with three 25-mL portions of water, add the aqueous washings to the 200-mL volumetric flask, dilute with water to volume, and mix to obtain a specimen stock solution. [NOTE—Retain the chloroform extract for the Assay for undecylenic acid. ] Transfer 15.0 mL or other suitable volume (see Procedure) of this specimen stock solution to a 100-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Aspirate each Standard preparation and the Assay preparation into the flame of a suitable atomic absorption spectrophotometer, and determine the absorbances of the solutions at 214 nm. Typically, an acetylene–air mixture is adjusted to obtain a blue flame about 7 mm in height with a suitable burner that is rotated to a position perpendicular to the light path. [NOTE—If the absorbance of the Assay preparation is outside the central 70% of the range between the absorbances of the Standard preparations, discard the Assay preparation and prepare another by diluting the specimen stock solution quantitatively as necessary to obtain a suitable absorbance.] Calculate the percentage of zinc undecylenate in the Ointment taken by the formula: in which 431.94 is the molecular weight of zinc undecylenate; 65.39 is the atomic weight of zinc; W is the weight, in g, of Ointment taken; A U, A H, and A L are the absorbances of the Assay preparation and the high- and low-concentration Standard preparations, respectively; and C H and C L are the concentrations, in µg per mL, of the high- and low-concentration Standard preparations, respectively.

Internal standard solution— Prepare a solution in chloroform containing 10 mg of tridecanoic acid in each mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Undecylenic Acid RS in chloroform to obtain a solution having a known concentration of about 3.8 mg per mL. Transfer 5.0 mL of this solution to a 50-mL volumetric flask, add 3.0 mL of Internal standard solution, dilute with chloroform to volume, and mix.

Assay preparation— Pass the chloroform extract prepared from the Ointment as directed in Assay for zinc undecylenate through phase-separating filter paper into a 250-mL volumetric flask. Rinse the separator with three 15-mL portions of chloroform, passing the rinsings through the filter and combining them with the main chloroform solution, add chloroform to volume, and mix. Transfer 20.0 mL of this solution to a 50-mL volumetric flask, add 3.0 mL of Internal standard solution, dilute with chloroform to volume, and mix.

Chromatographic system— Under typical conditions, the gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm × 1.8-m glass column packed with 3% liquid phase G1 on 100- to 200-mesh support S1A. The column is maintained at a temperature of about 165. Dry helium is used as the carrier gas at a flow rate of about 30 mL per minute.

System suitability— Chromatograph five injections of the silylated Standard preparation, and record peak responses as directed for Procedure: the resolution, R (see Chromatography 621), is not less than 3.0.

Procedure— Transfer 1.0-mL portions of the Standard preparation and the Assay preparation to separate, stoppered test tubes. To each tube add 50 µL of bis(trimethylsilyl)trifluoroacetamide, insert the stopper, mix, and allow to stand for 30 minutes. Inject a suitable portion (2 to 5 µL) of the Standard preparation into a suitable gas chromatograph, and record the chromatogram so as to obtain not less than 50% of maximum recorder response. Similarly inject a suitable portion of the Assay preparation, and record the chromatogram. Measure the peak responses for the first (undecylenic acid) and second (tridecanoic acid) peaks of the chromatograms. [NOTE—Relative retention times are, approximately, 0.43 for undecylenic acid and 1.0 for tridecanoic acid.] Calculate the percentage of total undecylenic acid in the Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Undecylenic Acid RS in the Standard preparation; W is the weight, in g, of Ointment taken; and R U and R S are the ratios of the peak responses of undecylenic acid to those of tridecanoic acid from the Assay preparation and the Standard preparation, respectively. The difference between the total undecylenic acid and the undecylenic acid equivalent to the determined zinc undecylenate (the weight of zinc undecylenate multiplied by 0.8533 gives the equivalent of undecylenic acid), both expressed as a percentage of the Ointment, gives the percentage of free undecylenic acid in the Ointment.