A: Dissolve about 500 mg in 10 mL of water in a separator, add 10 mL of 3 N hydrochloric acid, and extract the liberated methohexital with two 25-mL portions of chloroform. Evaporate the combined chloroform extracts to dryness, add 10 mL of ether, evaporate again, and dry the residue in vacuum at 80 for 4 hours. Dissolve 50 mg of the residue so obtained in 5 mL of chloroform: the solution exhibits IR absorption maxima at the same wavelengths as that of a similar preparation of USP Methohexital RS.

B: The methohexital obtained and dried as directed for Identification test A melts between 92 and 96.

Procedure for content uniformity—

Internal standard solution— Dissolve aprobarbital in chloroform to obtain a solution having a concentration of about 1.35 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Methohexital RS in chloroform to obtain a solution having a known concentration of about 0.46 mg per mL. Transfer 5.0 mL of the resulting solution to a 10-mL volumetric flask, add 2.0 mL of Internal standard solution, dilute with chloroform to volume, and mix to obtain a Standard preparation having a known concentration of about 230 µg per mL.

Assay preparation— Combine and mix the constituted solutions prepared from the contents of 5 vials of Methohexital Sodium for Injection. Transfer an accurately measured volume of the resulting solution, equivalent to about 50 mg of methohexital sodium, to a 125-mL separator containing 25 mL of water, and mix. Add 0.2 mL of dilute hydrochloric acid (1 in 2), and mix. Extract with three 25-mL portions of chloroform, shaking each extraction for 2 minutes and filtering the extracts through about 15 g of anhydrous sodium sulfate, that previously has been washed with about 5 mL of chloroform, into a 100-mL volumetric flask. Wash the sodium sulfate with several small portions of chloroform, collecting the washings in the 100-mL volumetric flask. Dilute with chloroform to volume, and mix. Transfer 5.0 mL of this solution to a 10-mL volumetric flask, add 2.0 mL of Internal standard solution, dilute with chloroform to volume, and mix.

Chromatographic system (see Chromatography 621)— The gas chromatograph is equipped with a flame-ionization detector and contains a 1.2-m × 4-mm column packed with 3% phase G10 on support S1AB. The column is maintained at about 230, the injection port at about 265, and the detector block at about 265. Dry helium is used as the carrier gas at a flow rate of about 60 mL per minute. Chromatograph replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between methohexital and aprobarbital is not less than 4.0, and the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 2 µL) of the Assay preparation and the Standard preparation into the gas chromatograph, and measure the peak responses for the major peak. The relative retention times are about 0.6 for methohexital and 1.0 for aprobarbital. Calculate the quantity, in mg, of methohexital sodium (C14H17N2NaO3) in the portion of Methohexital Sodium for Injection taken by the formula: in which 284.29 and 262.30 are the molecular weights of methohexital sodium and methohexital, respectively; C is the concentration, in µg per mL, of USP Methohexital RS in the Standard preparation; and RU and RS are the peak response ratios obtained from the Assay preparation and the Standard preparation, respectively.