Mobile phase— Prepare a suitable degassed solution of methanol and water (50:30).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

Chromatographic system (see Chromatography á621ñ)—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 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.5for isomalathion and 1.0for malathion;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Test preparationinto 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 CSis the concentration,in mg per mL,of USP Isomalathion RSin the Standard preparation;CUis the concentration,in mg per mL,of specimen in the Test preparation;Pis the stated purity,in percentage,of the USP Isomalathion RS;and rUand rSare the isomalathion peak responses obtained from the Test preparationand the Standard preparation,respectively:not more than 0.3%of isomalathion is found.

Mobile phase— Prepare a suitable degassed solution of methanol and water (50:30).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

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

Chromatographic system (see Chromatography á621ñ)—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 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.6for propylparaben and 1.0for 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 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C10H19O6PS2in the portion of Malathion taken by the formula: in which Cis the concentration,in mg per mL,of USP Malathion RSin the Standard preparation;and RUand RSare the ratios of the peak responses of malathion to propylparaben obtained from the Assay preparationand the Standard preparation,respectively.