A: The chromatogram obtained from the Assay preparation in the Assay exhibits a major peak for doxapram, the retention time of which corresponds to that of the doxapram peak in the chromatogram of the Standard preparation.

B: Transfer a volume of Injection, equivalent to about 50 mg of doxapram hydrochloride hydrate, to a separator containing 5 mL of water. Add 1 mL of a saturated solution of sodium chloride to the separator, insert the stopper, and mix. Add 5 mL of 2.5 N sodium hydroxide, and extract with three 15-mL portions of chloroform. Pass each extract through a pledget of glass wool, combine the filtrates in a 50-mL volumetric flask, dilute with chloroform to volume, and mix. Evaporate to dryness about 5 mL of this solution. Dissolve the residue in 0.01 N sulfuric acid, dilute with the same solvent to 100 mL, and mix: the UV absorption spectrum of the solution so obtained exhibits maxima and minima at the same wavelengths as a solution similarly prepared, about 50 mg of USP Doxapram Hydrochloride RS, instead of Doxapram Hydrochloride Injection, being used.

(Official January 1, 2007)

Mobile phase— Dissolve 2.8 g of monobasic potassium phosphate in 1 L of water, adjust with 50% phosphoric acid or 1 N potassium hydroxide to a pH of 3.0 ± 0.1, and filter through a 0.5-µm or finer porosity filter. Prepare a suitable mixture of this solution and acetonitrile (65:35). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of diphenhydramine hydrochloride in water containing about 1.5 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Doxapram Hydrochloride RS in water to obtain a solution having a known concentration of about 2 mg per mL. Transfer 5.0 mL of this solution and 5.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with water to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 100 mg of doxapram hydrochloride monohydrate, to a 50-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of this solution and 5.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 225-nm detector and a 4.6-mm × 15-cm column containing 5-µm packing L10, and is maintained at 40. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the responses as directed under Procedure: the relative retention times are about 1.0 for doxapram and 1.2 for diphenhydramine, the resolution, R, between the doxapram and diphenhydramine peaks is not less than 3.0; the tailing factor for the peaks is not more than 2.0, and the relative standard deviation for replicate injections is not more than 2.0%.

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 doxapram hydrochloride hydrate (C24H30N2O2·HCl·H2O) in each mL of the Injection taken by the formula: in which 432.98 and 414.98 are the molecular weights of doxapram hydrochloride monohydrate and anhydrous doxapram hydrochloride, respectively, C is the concentration, in mg per mL, of USP Doxapram Hydrochloride RS in the Standard preparation, V is the volume, in mL, of Injection taken to prepare the Assay preparation, and RU and RS are the ratios of the peak responses of doxapram and diphenhydramine obtained from the Assay preparation and the Standard preparation, respectively.