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

Standard preparation— Transfer 2.0mLof isopropyl alcohol and 5.0mLof Internal standard solutionto a 200-mLvolumetric flask,dilute with ethyl acetate to volume,and mix.

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

Chromatographic system (see Chromatography á621ñ)—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 7mLper 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 1µL)of the Standard preparationand the Test preparationinto 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 Vis the volume,in mL,of Lotion taken;and RUand RSare the ratios of the peak responses of isopropyl alcohol to internal standard obtained from the Test preparationand the Standard preparation,respectively:between 90%and 110%of the labeled amount of C3H8Ois found.

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

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

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

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

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm ×1.8-m glass column packed with 5%G6liquid 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 15mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are 1.0for malathion and about 1.3for 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 1µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses of the major peaks.Calculate the quantity,in mg,of malathion (C10H19O6PS2)in each mLof the Lotion taken by the formula: in which Cis the concentration,in mg per mL,of USP Malathion RSin the Standard preparation;Vis the volume,in mL,of Lotion taken;and RUand RSare the ratios of the peak responses of malathion to parathion obtained from the Assay preparationand the Standard preparation,respectively.