Identification—
Prepare a pH 9.0 buffer solution by dissolving 3.1 g of boric acid, 203 mg of magnesium chloride, and 860 mg of sodium hydroxide in water to make 1000 mL. Dissolve 50 mg of alkaline phosphatase enzyme in 50 mL of the pH 9.0 buffer solution, and transfer 5 mL of the resulting solution to a glass-stoppered, 50-mL tube containing 5 mL of the
Assay preparation, prepared as directed in the
Assay. Incubate at 37
for 45 minutes, then add 25 mL of methylene chloride, and shake for 2 minutes. The methylene chloride extract so obtained responds to the
Identification test under
Dexamethasone Sodium Phosphate Injection, beginning with “Evaporate 15 mL of the methylene chloride extract.”
Assay—
Alcohol-aqueous phosphate buffer—
Dissolve 0.29 g of dibasic sodium phosphate in 450 mL of water, add 550 mL of alcohol, and mix.
0.05 M Phosphate buffer—
In a 1-L volumetric flask, dissolve 6.9 g of monobasic sodium phosphate in 500 mL of water, dilute with water to volume, and mix.
Mobile phase—
Prepare a suitable degassed solution of methanol and 0.05 M Phosphate buffer (52:48) which, at ambient temperature and at a flow rate of 1.5 mL per minute, gives a retention time of about 8.5 minutes for dexamethasone phosphate.
Standard preparation—
Using an accurately weighed quantity of
USP Dexamethasone Phosphate RS, prepare a solution in
Alcohol-aqueous phosphate buffer having a known concentration of about 30 µg per mL. Prepare this solution fresh.
Assay preparation—
Transfer an accurately weighed quantity of Cream, equivalent to about 3 mg of dexamethasone phosphate, to a 150-mL beaker. Add 65 mL of Alcohol-aqueous phosphate buffer, and heat just to boiling. Pour the contents of the beaker into a 125-mL separator containing 45 mL of isooctane. After shaking for 1 minute, decant the lower layer into a 100-mL volumetric flask with the aid of a glass funnel. Rinse the 150-mL beaker with two 15-mL portions of Alcohol-aqueous phosphate buffer, extracting the remaining isooctane in the separator with each portion and decanting the lower layer from each extraction into the 100-mL volumetric flask. Dilute with Alcohol-aqueous phosphate buffer to volume, and mix. Filter through a membrane filter before injecting.
Chromatographic system
(see
Chromatography 
621
)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains packing L1. Chromatograph five replicate injections of the
Standard preparation, and record the peak responses as directed under
Procedure: the relative standard deviation is not more than 1.5%.
Procedure—
By means of a suitable sampling valve, separately inject equal volumes (about 20 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 mg, of C
22H
30FO
8P in the portion of Cream taken by the formula:
0.1C(rU / rS),
in which
C is the concentration, in µg per mL, of
USP Dexamethasone Phosphate RS in the
Standard preparation, and
rU and
rS are the peak responses at equivalent retention times obtained from the
Assay preparation and the
Standard preparation, respectively.
