Identification—
A:
The retention time of the major peak in the chromatogram of Oral Solution corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.
B:
Chloramine T–trichloroacetic acid reagent—Mix 10 mL of a freshly prepared solution of chloramine T (3 in 100) and 40 mL of a 1 in 4 solution of trichloroacetic acid in dehydrated alcohol.
Spotting solvent—
Prepare a mixture of chloroform and methanol (2:1).
Standard solution—
Dissolve an accurately weighed quantity of
USP Digoxin RS in
Spotting solvent to obtain a solution containing 0.25 mg per mL.
Test solution—
Pipet a volume of Oral Solution, equivalent to 0.5 mg of digoxin, into a separator. Add sufficient water to obtain a final volume of approximately 50 mL. Extract the aqueous layer with three 30-mL portions of chloroform, combining the extracts in a conical flask. Evaporate the combined chloroform extracts on a steam bath with the aid of a current of air to dryness. Add 2 mL of Spotting solvent to the residue, and shake for 2 minutes.
Procedure—
Proceed as directed for
Procedure in the test for
Related glycosides under
Digoxin, except to omit the use of the
Gitoxin standard solution. Examine the plate under long-wavelength UV light: the
RF value of the principal spot in the chromatogram of the
Test solution corresponds to that in the chromatogram of the
Standard solution.
Assay—
Mobile phase—
Prepare a filtered and degassed mixture of water, acetonitrile, and isopropyl alcohol (70:27.5:2.5). Make adjustments if necessary (see
System Suitability under
Chromatography 
621
).
Standard preparation—
Dissolve an accurately weighed quantity of
USP Digoxin RS in diluted alcohol, and dilute quantitatively and stepwise with diluted alcohol to obtain a solution having a known concentration of about 20 µg per mL.
Assay preparation—
Transfer an accurately measured volume of Oral Solution, equivalent to about 500 µg of digoxin, to a 25-mL volumetric flask, dilute with diluted alcohol to volume, and mix.
System suitability preparation—
Prepare as directed in the
Assay under
Digoxin.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 218-nm detector and a 4.6-mm × 15-cm column that contains packing L1. The flow rate is about 0.5 mL per minute. Chromatograph the
System suitability preparation, and record the peak responses as directed for
Procedure: the resolution,
R, between the digoxin and digoxigenin bisdigitoxoside peaks is not less than 2.0; the tailing factor for the analyte peak 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 10 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 µg, of digoxin (C
41H
64O
14) in each mL of the Oral Solution taken by the formula:
(100C / V)(rU / rS),
in which
C is the concentration, in µg per mL, of
USP Digoxin RS in the
Standard preparation; V is the volume, in mL, of Oral Solution taken; and
rU and
rS are the digoxin peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
Digoxin Elixir