Identification—
Dilute a volume of Oral Solution with alcohol to obtain a concentration of about 1 mg of pentobarbital per mL. Apply 50 µL of this solution and 50 µL of a Standard solution of
USP Pentobarbital RS in alcohol containing 1 mg per mL as streaks about 1 cm in length along the spotting line to a suitable thin layer chromatographic plate (see
Chromatography 
621
) coated with a 0.25-mm layer of chromatographic silica gel mixture. Allow the streaks to dry, and develop the chromatogram in a solvent system consisting of a mixture of isopropyl alcohol, ammonium hydroxide, chloroform, and acetone (9:4:2:2) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots by viewing the plate under short-wavelength light: the
RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.
Assay—
Internal standard solution—
Dissolve an accurately weighed quantity of n-tricosane in chloroform, and quantitatively dilute with chloroform to obtain a solution having a known concentration of about 0.6 mg per mL.
Standard preparation—
Dissolve accurately weighed quantities of
USP Pentobarbital RS and
n-tricosane in chloroform, and quantitatively dilute with chloroform to obtain a solution that contains, in each mL, known amounts of about 1 mg of
USP Pentobarbital RS and about 0.4 mg of
n-tricosane.
Assay preparation—
Transfer an accurately measured volume of Oral Solution, equivalent to about 20 mg of pentobarbital, to a separator, add 1 mL of dilute hydrochloric acid (1 in 5), and extract with four 10-mL portions of chloroform. Filter the extracts through about 15 g of anhydrous sodium sulfate that is supported on a funnel by a small pledget of glass wool. Collect the combined filtrate in a 50-mL volumetric flask, wash the sodium sulfate with 5 mL of chloroform, dilute with chloroform to volume, and mix. Combine 4.0 mL of this solution with 1.0 mL of Internal standard solution in a suitable container, and reduce the volume to about 1.5 mL by evaporation, with the aid of a stream of dry nitrogen, at room temperature.
Chromatographic system and System suitability—
Proceed as directed for
Chromatographic System and
System Suitability under
Barbiturate Assay 361, the resolution,
R, between pentobarbital and
n-tricosane being not less than 2.3.
[NOTE—Relative retention times are, approximately, 0.5 for
n-tricosane and 1.0 for pentobarbital.
]
Procedure—
Proceed as directed for
Procedure under
Barbiturate Assay 361. Calculate the quantity, in mg, of pentobarbital (C
11H
18N
2O
3) in each mL of the Oral Solution taken by the formula:
12.5(RU)(QS)(Ci) / V(RS),
in which
V is the volume, in mL, of Oral Solution taken; and the other terms are as defined therein.
Pentobarbital Elixir