Phosphate buffer 0.05M— Dissolve 11.5mLof phosphoric acid in 1900mLof water,adjust with about 100mLof 1Nsodium hydroxide to a pHof 2.4,and mix.

Mobile phase— Dissolve 555mg of sodium 1-heptanesulfonate in 980mLof 0.05M Phosphate bufferwith stirring,add 20mLof methanol,mix,degas,and filter.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Transfer about 10mg of p-aminobenzoic acid to a 100-mLvolumetric flask,dissolve in water,dilute with water to volume,and mix.Transfer 5.0mLof the resulting solution to a 25-mLvolumetric flask,dilute with water to volume,and mix to obtain the Internal standard solution.

Standard preparation— Transfer an accurately weighed quantity of about 10mg of USP Levocarnitine RSto a 5-mLvolumetric flask,add 1.0mLof Internal standard solution,dilute with water to volume,and mix.

Assay preparation— Transfer an accurately measured volume of Oral Solution,equivalent to about 500mg of levocarnitine,to a 50-mLvolumetric flask,dilute with water to volume,and mix.Wash a 10-mm ×4-cm disposable column containing 500mg of packing L1,in the order,with two column volumes of methylene chloride,two column volumes of methanol,and three column volumes of water.Pipet 5.0mLof the solution prepared above into the washed disposable column,and rinse the column twice with 6.0-mLportions of water.Collect the filtrate and washings in a 25-mLvolumetric flask,add 5.0mLof Internal standard solution,dilute with water to volume,and mix to obtain the Assay preparation.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 225-nm detector and a 3.9-mm ×30-cm column that contains 10-µm packing L1.The flow rate is about 1.5mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the resolution,R,between the levocarnitine and internal standard peaks is not less than 1.5;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 40µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.56for levocarnitine and 1.0for p-aminobenzoic acid.Calculate the quantity,in mg,of levocarnitine (C7H15NO3)in each mLof the Oral Solution taken by the formula: in which Cis the concentration,in mg per mL,of USP Levocarnitine RSin the Standard preparation;Vis the volume,in mL,of Oral Solution taken;and RUand RSare the peak response ratios obtained from the Assay preparationand the Standard preparation,respectively.