Mobile phase— Prepare a suitable degassed solution of methanol and water (50:30). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Isomalathion RS in methanol to obtain a solution having a known concentration of about 0.1 mg per mL.

Test preparation— Dissolve an accurately weighed quantity of Malathion in methanol to obtain a solution having a known concentration of about 20 mg per mL.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 4-mm × 30-cm column that contains 10-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.5 for isomalathion and 1.0 for malathion; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Test preparation into the chromatograph, record the chromatograms, and measure the responses of the major peaks. Calculate the percentage of isomalathion in the Malathion taken by the formula: in which CS is the concentration, in mg per mL, of USP Isomalathion RS in the Standard preparation; CU is the concentration, in mg per mL, of specimen in the Test preparation; P is the stated purity, in percentage, of the USP Isomalathion RS; and rU and rS are the isomalathion peak responses obtained from the Test preparation and the Standard preparation, respectively: not more than 0.3% of isomalathion is found.

(Official January 1, 2007)

Mobile phase— Prepare a suitable degassed solution of methanol and water (50:30). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Dissolve propylparaben in Mobile phase to obtain a solution containing about 0.6 mg per mL.

Standard preparation— Transfer about 100 mg of USP Malathion RS, accurately weighed, to a 10-mL volumetric flask, add 1.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Assay preparation— Transfer about 500 mg of Malathion, accurately weighed, to a 50-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains 10-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.6 for propylparaben and 1.0 for malathion; and the resolution, R, of the propylparaben and malathion peaks is not less than 4, and the relative standard deviation for replicate injections is not more than 2.0%.

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 mg, of C10H19O6PS2 in the portion of Malathion taken by the formula: in which C is the concentration, in mg per mL, of USP Malathion RS in the Standard preparation; and RU and RS are the ratios of the peak responses of malathion to propylparaben obtained from the Assay preparation and the Standard preparation, respectively.