Assay—
[NOTE—Use low-actinic glassware throughout the
Assay.]
Mobile phase—
Dissolve 1.38 g of monobasic potassium phosphate monohydrate in about 900 mL of water, adjust with phosphoric acid to a pH of 3.0, and dilute with water to 1000 mL. Prepare a filtered and degassed mixture of this buffer solution, methanol, and acetonitrile (65:28:7). Make adjustments if necessary (see
System Suitability under
Chromatography 
621
).
Standard preparation—
Dissolve an accurately weighed quantity of
USP Metolazone RS in methanol to obtain a solution having a known concentration of about 5 µg per mL.
Assay preparation—
Transfer 10 Tablets to a 200-mL volumetric flask, and add 3 mL of water and about 100 mL of methanol. Sonicate for 30 minutes. If disintegration is not complete, sonicate for an additional 30 minutes. Shake by mechanical means for about 30 minutes. Dilute with methanol to volume, and mix. Transfer an accurately measured volume of this stock solution, equivalent to about 0.25 mg of metolazone, to a 50-mL volumetric flask, dilute with methanol to volume, and mix.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 235-nm detector and a 3.9-mm × 15-cm column that contains packing L1. The flow rate is about 1.1 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the relative standard deviation for replicate injections is not less than 2.0%.
Procedure—
Separately inject equal volumes (about 100 µL) of the
Standard preparation and the
Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the metolazone peak. Calculate the quantity, in mg, of metolazone (C
16H
16ClN
3O
3S) in the portion of Tablets taken by the formula:
50C(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Metolazone RS in the
Standard preparation; and
rU and
rS are the peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
.