Identification—
To a quantity of the Oral Suspension add a volume of methanol sufficient to yield a solution having a concentration equivalent to about 2.5 mg of erythromycin per mL. Shake this mixture by mechanical means for about 30 minutes. Centrifuge a portion of this mixture, and use the clear supernatant as the test solution. Prepare a solution of
USP Erythromycin Estolate RS in methanol containing about 3 mg per mL (
Standard solution A). Prepare a solution of
USP Sulfisoxazole Acetyl RS in methanol containing about 8.7 mg per mL (
Standard solution B). Apply separately 10 µL each of the test solution and the two Standard solutions to a suitable thin-layer chromatographic plate (see
Chromatography 
621
) coated with a 0.25-mm layer of chromatographic silica gel mixture, and allow to dry. Place the plate in an unlined chromatographic chamber, and develop the chromatograms in a solvent system consisting of a mixture of methanol and chloroform (85:15) until the solvent front has moved about 9 cm. Remove the plate from the chamber, mark the solvent front, and allow the solvent to evaporate. Spray the plate with a mixture of dehydrated alcohol,
p-methoxybenzaldehyde, and sulfuric acid (90:5:5). Heat the plate at 100
for 10 minutes, and examine the chromatograms, in which the erythromycin estolate appears as a black-to-purple spot and the sulfisoxazole acetyl appears as a yellow spot: the
RF value of the principal black-to-purple spot obtained from the test solution corresponds to that obtained from
Standard solution A, and the
RF value of the principal yellow spot obtained from the test solution corresponds to that obtained from
Standard solution B.
Assay for erythromycin—
Dilute an accurately measured volume of Oral Suspension, freshly mixed and free from air bubbles, quantitatively with methanol to obtain a solution containing the equivalent of 2.5 mg of erythromycin per mL. Dilute with 1.5 volumes of
Buffer No. 3, and allow to stand at room temperature for 18 hours. Proceed as directed for erythromycin under
Antibiotics—Microbial Assays 81, using an accurately measured volume of this stock test solution diluted quantitatively with
Buffer No. 3 to yield a
Test Dilution having a concentration assumed to be equal to the median dose level of the Standard (1.0 µg of erythromycin per mL).
Assay for sulfisoxazole—
Mobile solvent—
Mix 40 volumes of acetonitrile and 60 volumes of water. The acetonitrile concentration may be varied to meet system suitability requirements and to provide a suitable elution time for sulfisoxazole acetyl. Filter the solution through a membrane filter (1-µm or finer porosity).
Internal standard solution—
Prepare a solution of benzanilide in acetonitrile having a concentration of about 0.33 mg per mL. Filter the solution through a membrane filter (1-µm or finer porosity).
Standard preparation—
Prepare a solution of
USP Sulfisoxazole Acetyl RS in
Internal standard solution having a known concentration of about 1 mg per mL.
Assay preparation—
Transfer an accurately measured volume of Oral Suspension, freshly mixed and free from air bubbles, equivalent to about 600 mg of sulfisoxazole, to a 125-mL separator, and extract with three 75-mL portions of chloroform. Collect the chloroform extracts in a 250-mL volumetric flask, dilute with chloroform to volume, and mix. Filter a portion of this solution through a membrane filter (1-µm or finer porosity). Pipet 4 mL of the filtrate into a glass-stoppered, 25-mL conical flask, and evaporate with the aid of a current of dry air to dryness. Add 10.0 mL of Internal standard solution, and mix.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 1.2 mL per minute. Chromatograph replicate injections of the
Standard preparation, and record the peak responses as directed under
Procedure: the resolution factor between sulfisoxazole acetyl and benzanilide is not less than 3.0.
Procedure—
Separately inject equal volumes (about 5 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 sulfisoxazole (C
11H
13N
3O
3S) in each mL of the Oral Suspension taken by the formula:
(267.31 / 309.35)(625C / V)(RU / RS),
in which 267.31 and 309.35 are the molecular weights of sulfisoxazole and sulfisoxazole acetyl, respectively;
C is the concentration, in mg, of
USP Sulfisoxazole Acetyl RS in each mL of the
Standard preparation; V is the volume, in mL, of Oral Suspension taken; and
RU and
RS are the ratios of peak responses of sulfisoxazole acetyl peak to benzanilide peak obtained from the
Assay preparation and the
Standard preparation, respectively.
