Standard preparation
Prepare a solution of
USP Dexamethasone RS in methanol having a known concentration of about 7.5 mg per mL. Dilute an accurately measured volume of this solution with the
Mobile phase to obtain a
Standard preparation having a known concentration of about 0.3 mg per mL.
Procedure
Introduce equal volumes (between 15 and 30 µL) of the
Assay preparation and the
Standard preparation into a high-pressure liquid chromatograph (see
Chromatography 621) operated at room temperature by means of a suitable microsyringe or sampling valve, adjusting the operating parameters such that the peak obtained with the
Standard preparation is 60% full-scale. Typically, the apparatus is fitted with a 4-mm × 25-cm column containing packing L7, is equipped with an UV detector capable of monitoring absorption at 254 nm and a suitable recorder, and is operated at about 1000 psi. Five replicate injections of the
Standard preparation show a relative standard deviation of not more than 3.0%. Determine the peak responses, at equivalent retention times, obtained with the
Assay preparation and the
Standard preparation, and calculate the quantity, in mg, of C
22H
29FO
5 in the portion of Dexamethasone taken by the formula:
100C(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Dexamethasone RS in the
Standard preparation, and
rU and
rS are the peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.