Identification—
Transfer an amount of powdered Tablets, equivalent to about 8 mg of guanabenz, to a 60-mL separator. Add 10 mL of 0.1 N hydrochloric acid, and shake to disperse the powder. Shake the mixture with three 10-mL portions of chloroform, discarding the chloroform phase each time. Add 5 mL of 1 N sodium hydroxide, and extract with two 25-mL portions of ether, filtering the ether extracts. Evaporate the combined extracts with the aid of a current of air to dryness: the IR absorption spectrum of a potassium bromide dispersion of the residue so obtained exhibits maxima at the same wavelengths as that of a similar preparation of
USP Guanabenz Acetate RS.
Dissolution 
711
—
Medium:
water; 1000 mL.
Apparatus 2:
50 rpm.
Time:
60 minutes.
Procedure—
Determine the amount of C
8H
8N
4Cl
2 dissolved from UV absorbances at the wavelength of maximum absorbance at about 272 nm of filtered portions of the solution under test, suitably diluted with
Medium, in comparison with a Standard solution having a known concentration of
USP Guanabenz Acetate RS in the same medium.
Tolerances—
Not less than 75% (Q) of the labeled amount of C8H8N4Cl2 is dissolved in 60 minutes.
Chromatographic purity—
Extracting solvent, Mobile phase, Standard preparation I, System suitability solution, Chromatographic system, and Assay preparation—
Proceed as directed in the
Assay.
Standard preparation II—
Pipet 2 mL of Standard preparation I into a 100-mL volumetric flask, dilute with Extracting solvent to volume, and mix.
Procedure—
Proceed as directed for
Procedure in the
Assay, except to substitute
Standard preparation II for
Standard preparation I. Calculate the quantity of any impurity observed having a relative retention time corresponding to the component eluting before guanabenz obtained from the
System suitability solution. The amount of any such impurity observed is not more than 2%.
Assay—
Extracting solvent—
Dissolve 8.2 g of sodium acetate in 20 mL of water, add 5.7 mL of glacial acetic acid, dilute to 1 L with methanol, and mix.
Mobile phase—
Prepare a filtered and degassed mixture of water, methanol, and phosphoric acid (57:43:0.3), making adjustments if necessary (see
System Suitability under
Chromatography 621).
Standard preparation I—
Transfer about 25 mg of
USP Guanabenz Acetate RS, accurately weighed, to a 250-mL volumetric flask. Add 25 mL of water, shake to dissolve the solids, dilute with
Extracting solvent to volume, and mix.
Assay preparation—
Transfer 10 Tablets to a 500-mL volumetric flask. Add 50 mL of water, stir by mechanical means until the solids are well dispersed, add 400 mL of Extracting solvent, and stir for 45 minutes. Dilute with Extracting solvent to volume, mix, and centrifuge a portion of the mixture until a clear supernatant is obtained. If necessary, dilute a portion of the supernatant quantitatively with a mixture of Extracting solvent and water (9:1) to obtain a solution containing about 0.08 mg of guanabenz per mL.
System suitability solution—
Transfer about 30 mg of guanabenz acetate to a 100-mL stoppered flask. Add about 50 mL of 0.1 N hydrochloric acid, heat on a steam bath for 60 minutes, and allow the solution to cool.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 245-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph replicate injections of
Standard preparation I and record the peak responses as directed for
Procedure: the relative standard deviation is not more than 2.0%. Inject a volume (about 20 µL) of the
System suitability solution into the chromatograph and record the chromatogram: the resolution between guanabenz and the peak eluting before it is not less than 1.6.
Procedure—
Separately inject equal volumes (about 20 µL) of
Standard preparation I and the
Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C
8H
8N
4Cl
2 in the portion of Tablets taken by the formula:
(231.08 / 291.13)(CD)(rU / rS),
in which 231.08 and 291.13 are the molecular weights of guanabenz and guanabenz acetate, respectively;
C is the concentration, in mg per mL, of
USP Guanabenz Acetate RS in
Standard preparation I; D is the
Assay preparation dilution factor, in mL per Tablet; and
rU and
rS are the peak responses of the
Assay preparation and
Standard preparation I, respectively.
