Identification—
A:
Mix 5mLof Topical Solution with 5mLof water:a voluminous yellow precipitate is formed.
B:
The absorption spectrum,obtained between wavelengths of 300nm and 450nm,of the solution employed for measurement of absorbance in the
Assayexhibits maxima and minima at the same wavelengths as that of a similar solution of
USP Tretinoin RS,concomitantly measured.
Assay—
[NOTE—Avoid exposure to strong light,and use low-actinic glassware in the performance of the following procedure.
]Transfer to a 100-mLvolumetric flask an accurately weighed quantity of Topical Solution,equivalent to about 375µg of tretinoin,dilute with acidified isopropyl alcohol (prepared by mixing 1mLof 0.01Nhydrochloric acid with isopropyl alcohol to make 1000mLof solution)to volume,and mix.Dissolve an accurately weighed quantity of
USP Tretinoin RSin the acidified isopropyl alcohol,and dilute quantitatively and stepwise with the same solvent to obtain a Standard solution having a known concentration of about 3.75µg per mL.Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 352nm,with a suitable spectrophotometer,using the acidified isopropyl alcohol as the blank.Calculate the quantity,in µg,of C
20H
28O
2in the portion of Topical Solution taken by the formula:
100C(AU/AS),
in which
Cis the concentration,in µg per mL,of
USP Tretinoin RSin the Standard solution,and
AUand
ASare the absorbances of the solution from the Topical Solution and the Standard solution,respectively.