Mobile phase— Prepare a filtered and degassed mixture of 0.04Maqueous monobasic potassium phosphate and methanol (55:45).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Prepare a solution of methylparaben in Mobile phasehaving a concentration of about 100µg per mL.

Standard preparation— Prepare a solution of USP Thiamine Hydrochloride RSin Mobile phasehaving an accurately known concentration of about 500µg per mL.Pipet 10mLof this solution and 10mLof Internal standard solutioninto a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix to obtain a Standard preparationhaving a known concentration of about 50µg per mL.

Assay preparation— Quantitatively dilute an accurately measured volume of Oral Solution with Mobile phaseto obtain a solution containing about 500µg of thiamine hydrochloride per mL.Pipet 10mLof the resulting solution and 10mLof Internal standard solutioninto a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 1.0mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.35for thiamine and 1.0for methylparaben;the resolution,R,between the thiamine and methylparaben peaks is not less than 6.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 25µL)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 thiamine hydrochloride (C12H17ClN4OS·HCl)in each mLof the Oral Solution taken by the formula: in which Cis the concentration,in mg per mL,of USP Thiamine Hydrochloride RSin the Standard preparation;Lis the labeled quantity,in mg per mL,of thiamine hydrochloride in the Oral Solution;Dis the concentration,in mg per mL,of thiamine hydrochloride in the Assay preparationon the basis of the labeled quantity and the extent of dilution;and RUand RSare the ratios of the peak responses of thiamine to methylparaben obtained from the Assay preparationand the Standard preparation,respectively.