Packaging and storage—
Preserve in single-dose containers, or in multiple-dose containers not exceeding 60 mL in volume, preferably of Type I glass, at a temperature between 2
and 30
. Avoid freezing and exposure to light.
Assay—
Mobile phase—
Prepare a suitable filtered and degassed mixture of 0.1
M monobasic sodium phosphate and methanol (65:35). Make adjustments if necessary (see
System Suitability under
Chromatography 
621
).
Standard preparation—
Dissolve an accurately weighed quantity of
USP Labetalol Hydrochloride RS in
Mobile phase to obtain a solution having a known concentration of about 0.5 mg per mL.
Resolution solution—
Dissolve a quantity of methylparaben in the
Standard preparation to obtain a solution containing about 0.08 mg per mL.
Assay preparation—
Transfer an accurately measured volume of Injection, equivalent to about 50 mg of labetalol hydrochloride, to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 20-cm column that contains packing L1 and is maintained at 60 ± 1
. The flow rate is about 1.5 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the column efficiency determined from the analyte peak is not less than 700 theoretical plates; 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 1.5%. Chromatograph the
Resolution solution, and record the peak responses as directed for
Procedure: the relative retention times are about 0.6 for methylparaben and 1.0 for labetalol; and the resolution,
R, between the methylparaben and labetalol is not less 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 area responses for the major peaks. Calculate the quantity, in mg, of labetalol hydrochloride (C
19H
24N
2O
3·HCl) in each mL of the Injection taken by the formula:
100(C / V)(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Labetalol Hydrochloride RS in the
Standard preparation; V is the volume, in mL, of Injection taken; and
rU and
rS are the peak area responses obtained from the
Assay preparation and the
Standard preparation, respectively.
