Internal standard solution— Mix 4 volumes of ethyl acetate and 1 volume of dehydrated alcohol.

Standard preparation— Transfer 2.0 mL of isopropyl alcohol and 5.0 mL of Internal standard solution to a 200-mL volumetric flask, dilute with ethyl acetate to volume, and mix.

Test preparation— Transfer an accurately measured volume of Malathion Lotion, equivalent to about 2.0 mL of isopropyl alcohol, to a 200-mL volumetric flask. Add 5.0 mL of Internal standard solution, dilute with ethyl acetate to volume, and mix.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm × 1.8-m glass column packed with 110- to 120-mesh support S2. Maintain the temperatures of the column, the injector port, and the detector block at 130, 200, and 220, respectively. Use dry nitrogen as the carrier gas at a flow rate of about 7 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation of the ratio of the isopropyl alcohol peak response to the internal standard peak response for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard preparation and the Test preparation into the gas chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the percentage of isopropyl alcohol (C3H8O) in the Lotion by the formula: in which V is the volume, in mL, of Lotion taken; and RU and RS are the ratios of the peak responses of isopropyl alcohol to internal standard obtained from the Test preparation and the Standard preparation, respectively: between 90% and 110% of the labeled amount of C3H8O is found.

Solvent mixture— Mix 4 volumes of methyl ethyl ketone and 1 volume of n-hexane.

Internal standard solution— Prepare a solution of parathion in Solvent mixture containing about 2 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Malathion RS in Solvent mixture to obtain a solution having a known concentration of about 2 mg per mL. Transfer 5.0 mL of this solution to a 25-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with Solvent mixture to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Lotion, equivalent to about 10 mg of malathion, to a 25-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with Solvent mixture to volume, and mix.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm × 1.8-m glass column packed with 5% G6 liquid phase on 110- to 120-mesh support S1A. Maintain temperatures of the column, the injector port, and the detector block at 190, 230, and 250, respectively. Use dry nitrogen as the carrier gas at a flow rate of about 15 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are 1.0 for malathion and about 1.3 for parathion; the resolution, R, between the malathion and parathion peaks is not less than 3.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses of the major peaks. Calculate the quantity, in mg, of malathion (C10H19O6PS2) in each mL of the Lotion taken by the formula: in which C is the concentration, in mg per mL, of USP Malathion RS in the Standard preparation; V is the volume, in mL, of Lotion taken; and RU and RS are the ratios of the peak responses of malathion to parathion obtained from the Assay preparation and the Standard preparation, respectively.