Identification—
Transfer a volume of Topical Solution to a 100-mL volumetric flask, and dilute quantitatively and stepwise with alcohol to obtain a concentration of about 8 µg per mL: the UV absorption spectrum of this solution exhibits maxima and minima at the same wavelengths as that of a similar solution of
USP Methoxsalen RS, concomitantly measured.
Assay—
Mobile phase
,
Internal standard preparation,
Standard preparation, and
Chromatographic system—Prepare as directed in the
Assay under
Methoxsalen.
Assay preparation—
Transfer an accurately measured volume of Topical Solution, equivalent to about 20 mg of methoxsalen, to a 100-mL volumetric flask. Dilute with alcohol to volume, and mix. Transfer 2.0 mL of this solution and 2.0 mL of Internal standard preparation to a 100-mL volumetric flask. Dilute with Mobile phase to volume, and mix. Pass through a 0.45-µm disk before using.
Procedure—
Proceed as directed for
Procedure in the
Assay under
Methoxsalen. Calculate the quantity, in mg, of methoxsalen (C
12H
8O
4) in each mL of the Topical Solution taken by the formula:
5(C/V)(RU /RS),
in which
V is the volume, in mL, of Topical Solution taken, and the other terms are as defined therein.
467
:
meets the requirements.