Loss on drying 
731
—
Dry about 100 mg, accurately weighed, in a capillary-stoppered bottle in vacuum at 60
for 3 hours: it loses not more than 1.5% of its weight.
Assay—
Mobile phase—
Prepare a mixture of 0.01 M monobasic potassium phosphate and methanol (60:40). Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Standard preparation—
Transfer about 5 mg of
USP Penicillin G Potassium RS, accurately weighed, to a 50-mL volumetric flask, add about 45 mL of water, and shake to dissolve. Dilute with water to volume, and mix. This solution contains the equivalent of about 160 Penicillin G Units per mL.
Assay preparation—
Transfer about 5 mg of Penicillin G Potassium, accurately weighed, to a 50-mL volumetric flask, add about 45 mL of water, and shake to dissolve. Dilute with water to volume, and mix.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm × 10-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph the
Resolution solution, and record the peak responses as directed for
Procedure: the relative retention times are about 0.8 for 2-phenylacetamide and 1.0 for penicillin G; and the resolution,
R, between 2-phenylacetamide and penicillin G is not less than 2.0. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the column efficiency is not less than 1000 theoretical plates; the tailing factor 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 potency, in Penicillin G Units per mg, of the Penicillin G Potassium taken by the formula:
(PWS / WU)(rU / rS),
in which
P is the specified potency, in Penicillin G Units per mg, of
USP Penicillin G Potassium RS;
WS and
WU are the weights, in mg, of
USP Penicillin G Potassium RS and the Penicillin G Potassium taken to prepare the
Standard preparation and the
Assay preparation, respectively; and
rU and
rS are the penicillin G peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
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