Tretinoin Gel »Tretinoin Gel contains not less than 90.0percent and not more than 130.0percent of the labeled amount of C20H28O2. Packaging and storage— Preserve in tight containers,protected from light. USP Reference standards á11ñ USP Tretinoin RS. Identification— 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. Minimum fill á755ñ: meets the requirements. 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 Gel,equivalent to about 375µg of tretinoin,and dissolve in about 70mLof chloroform,dilute with chloroform to volume,and mix.Dissolve an accurately weighed quantity of USP Tretinoin RSin chloroform,and dilute quantitatively and stepwise with chloroform 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 365nm,with a suitable spectrophotometer,using chloroform as the blank.Calculate the quantity,in µg,of C20H28O2in the portion of Gel 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 Gel and the Standard solution,respectively. Auxiliary Information— Staff Liaison:Lawrence Evans,III,Ph.D.,Scientist Expert Committee:(PA6)Pharmaceutical Analysis 6 USP28–NF23Page 1957 Phone Number:1-301-816-8389