Internal standard solution 1— Dissolve 100 mg of p-chlorobenzaldehyde in 100 mL of chloroform, and mix.

Internal standard solution 2— Dilute 1.0 mL of Internal standard solution 1 to 10.0 mL with chloroform, and mix.

Standard solution— Prepare a solution of 2,6-dichlorobenzaldehyde in chloroform containing 1.0 mg per mL.

Standard preparation— Transfer 4.0 mL of Standard solution and 1.0 mL of Internal standard solution 1 to a 10-mL volumetric flask, dilute with chloroform to volume, and mix.

Test preparation— Transfer 200 mg of Guanabenz Acetate to a 30-mL glass-stoppered centrifuge tube. Add 10 mL of 0.1 N hydrochloric acid, shake to dissolve, add 1.0 mL of Internal standard solution 2, and shake. Centrifuge, and transfer a portion of the lower layer to a stoppered container. [note—The lower layer must be removed within 10 minutes of adding the acid to the centrifuge tube.]

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and a 1.8-m × 3-mm column packed with 20% phase G1 on 80- to 100-mesh support S1A. The column is maintained at a temperature of about 190, the injection port at about 225, and the detector at about 250. Nitrogen is used as the carrier gas at a flow rate of about 30 mL per minute.

Procedure— Separately inject 2-µL portions of the Standard preparation and the Test preparation, successively, into the gas chromatograph. The resolution between 2,6-dichlorobenzaldehyde and p-chlorobenzaldehyde is not less than 3.0, and the relative retention time for p-chlorobenzaldehyde is 0.5 and for 2,6-dichlorobenzaldehyde is 1.0. The relative peak response ratio obtained from the Test preparation does not exceed that obtained from the Standard preparation (0.2%).

Methanolic formic acid— Prepare a mixture of formic acid and methanol (1 in 2000).

Aminoguanidine bicarbonate solution— Transfer 100 mg of aminoguanidine bicarbonate to a test tube, add 0.05 mL of formic acid, and warm gently to effect solution. Quantitatively transfer the contents of the test tube to a 10-mL volumetric flask, dilute with methanol to volume, and mix.

Standard solution A— Transfer 10 mg of USP Guanabenz Acetate RS to a 100-mL volumetric flask, and dissolve in 50 mL of Methanolic formic acid. Add 1.0 mL of the Aminoguanidine bicarbonate solution, dilute with Methanolic formic acid to volume, and mix.

Standard solution B— Transfer 5.0 mL of Standard solution A to a 10-mL volumetric flask, dilute with Methanolic formic acid to volume, and mix.

Standard solution C— Transfer 2.0 mL of Standard solution A to a 10-mL volumetric flask, dilute with Methanolic formic acid to volume, and mix.

Test solution— Prepare a solution of guanabenz acetate containing 10 mg per mL in Methanolic formic acid.

Procedure— Prepare a chromatographic chamber containing a mixture of chloroform, methanol, and ammonium hydroxide (60:40:1) as the developing solvent, and allow it to equilibrate for at least 30 minutes before use. Prewash a plate coated with a 0.25-mm layer of chromatographic silica gel mixture (see Chromatography 621) by placing it in the chromatographic chamber, allowing the solvent front to rise to the top of the plate, drying it in air and activating it by heating at 105 for 20 minutes. Within 30 minutes after preparation, separately apply 10-µL portions of Standard solutions A, B, and C, the Test solution, and Methanolic formic acid. Allow the spots to dry, and place the plate in the chromatographic chamber. When the solvent has moved about three-fourths of the length of the plate, remove the plate and allow it to air-dry for about 30 minutes. Examine the plate under short-wavelength UV light. Estimate the amount of any secondary spots (other than any secondary spot with the same RF as the Methanolic formic acid) observed in the chromatogram of the Test solution by comparison with the Standard solutions. Place the plate in a chamber saturated with iodine vapors for about 10 minutes. Remove and examine the plate. Estimate the amount of any spot in the chromatogram of the Test solution that has an RF corresponding to the RF of the spot produced by the aminoguanidine bicarbonate by comparison with the Standard solutions. No individual secondary spot is greater in size or intensity than the spot produced by Standard solution B (0.5%), and the total of any such spots observed is not more than 1%.