A: The retention time of the major peak in the chromatogram of theAssay preparationcorresponds to that in the chromatogram of theStandard preparation,as obtained in theAssay.

B: Add a few drops of a solution (1in 20)to 5mLof hot alkaline cupric tartrate TS:a red precipitate of cuprous oxide is formed.

Phosphate buffer solutionand Mobile phase— Proceed as directed in theAssay.

Standard solution— Transfer accurately weighed quantities of USP Galactose RS,USP Anydrous Lactose RS,USP Epilactose RS,and USP Fructose RSto a 10-mLvolumetric flask,and dissolve in and dilute with a mixture of water and acetonitrile (1:1)to volume to obtain a solution having known concentrations of about 6.4mg per mL,4.8mg per mL,3.2mg per mLand 0.4mg per mL,respectively.

Test solution— Prepare as directed for theAssay preparationin theAssay.

Chromatographic system— Proceed as directed in theAssay.To evaluate the system suitability requirements,use theStandard preparationprepared as directed in theAssay.

Procedure— Separately inject equal volumes (about 20µL)of theStandard solutionand theTest solutioninto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the percentages of galactose,lactose,epilactose,and fructose,if found,in the portion of Concentrate taken by the formula: in whichCis the concentration,in mg per mL,of the relevant USP Reference Standard in theStandard solution;Wis the weight,in mg,of lactulose in theTest solution;andrUand rSare the peak responses for the relevant related compounds obtained from theTest solution and theStandard solution,respectively:relative to lactulose,not more than 16%of galactose is found,not more than 12%of lactose is found,not more than 8%of epilactose is found,and not more than 1%of fructose is found.

Phosphate buffer solution— Dissolve 1.15g of monobasic sodium phosphate in 1000mLof water.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile andPhosphate buffer solution(82:18).Make adjustments if necessary (seeSystem Suitability underChromatography á621ñ).[NOTE—Ensure that the concentration of acetonitrile in theMobile phase is between 78%and 85%to obtain appropriate retention times.]

Standard preparation— Transfer accurately weighed quantities of USP Lactulose RS,USP Anhydrous Lactose RS,and USP Epilactose RSto a 10-mLvolumetric flask,and dissolve in and dilute with a mixture of water and acetonitrile (1:1)to volume,to obtain a solution having known concentrations of 40mg per mL,4.8mg per mL,and 3.2mg per mL,respectively.

Assay preparation— Transfer an accurately weighed quantity of Concentrate containing about 2.0g of lactulose to a 50-mLvolumetric flask,and dissolve in 20mLof water.Add 25.0mLof acetonitrile,mix,allow the solution to reach ambient temperature,dilute with water to volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a refractive index detector maintained at a temperature of 40±1and a 4.6-mm ×15-cm column that contains 3-µm packing L8.The column temperature is maintained at 40±1.The flow rate is about 1.3mLper minute.Chromatograph theStandard preparation,and record the peak responses as directed forProcedure:the relative retention times are about 0.30for fructose,0.42for galactose,0.85for epilactose,1.0for lactulose,and 1.1for lactose;the resolution,R,between lactulose and lactose is not less than 1.5,and that between lactulose and epilactose is not less than 0.9;and the relative standard deviation for replicate injections determined from the main peak is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of theStandard preparationand theAssay preparationinto the chromatograph,record the chromatograms,and measure the responses for all the peaks.Calculate the quantity,in mg,of lactulose (C12H22O11)in the portion of Concentrate taken by the formula: in which Cis the concentration,in mg per mL,of USP Lactulose RSin theStandard preparation;and rUand rSare the peak responses for lactulose obtained from theAssay preparationand theStandard preparation,respectively.