Mobile phase and Diluent— Prepare as directed in the Assay under Doxycycline Hyclate.

System suitability solution— Prepare as directed for the Resolution solution in the Assay under Doxycycline Hyclate.

Methacycline standard stock solution, Standard solution 1, Standard solution 2, and Chromatographic system— Prepare as directed for the Related compounds test under Doxycycline Hyclate.

Test solution— Use the Assay preparation, prepared as directed in the Assay.

Procedure— Proceed as directed for the Related compounds test under Doxycycline Hyclate. Calculate the percentage of methacycline in the portion of Doxycycline taken by the formula: in which CM is the concentration, in mg per mL, of USP Methacycline Hydrochloride RS in Standard solution 2; W is the weight, in mg, of Doxycycline taken to prepare the Test solution; and rU and rM are the methacycline peak responses obtained from the Test solution and Standard solution 2, respectively. Not more than 2% of methacycline is found. Calculate the percentage of each related compound, other than methacycline, in the portion of Doxycycline taken by the formula: in which CS is the concentration, in mg per mL, of USP Doxycycline Hyclate RS in Standard solution 2; W is the weight, in mg, of Doxycycline taken to prepare the Test solution; ri is the peak response for each impurity obtained from the Test solution; and rS is the doxycycline peak response obtained from Standard solution 2. Not more than 0.5% of any impurity eluting before methacycline is found; not more than 2% of 6-epidoxycycline is found; and not more than 0.5% of any impurity eluting after the main doxycycline peak is found.

Mobile phase, Diluent, Resolution solution, Standard preparation, and Chromatographic system— Prepare as directed in the Assay under Doxycycline Hyclate.

note—Throughout the following sections, protect the Standard preparation and the Assay preparation from light.

Assay preparation— Transfer about 55 mg of Doxycycline, accurately weighed, to a 50-mL volumetric flask, add 12 mL of 0.1 N hydrochloric acid, swirl to dissolve, dilute with Diluent to volume, and mix. Filter through a filter of 0.5 µm or finer porosity.

Procedure— 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 µg, of doxycycline (C22H24N2O8) in each mg of the Doxycycline taken by the formula: in which C is the concentration, in mg per mL, of USP Doxycycline Hyclate RS in the Standard preparation, P is the potency, in µg of doxycycline per mg, of USP Doxycycline Hyclate RS, W is the weight, in mg, of Doxycycline taken to prepare the Assay preparation, and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.