A: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

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

Phosphate buffer solution and Mobile phase— Proceed as directed in the Assay.

Standard solution— Transfer accurately weighed quantities of USP Galactose RS, USP Anhydrous Lactose RS, USP Epilactose RS, and USP Fructose RS to a 10-mL volumetric 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 6.4 mg per mL, 4.8 mg per mL, 3.2 mg per mL and 0.4 mg per mL, respectively.

Test solution— Prepare as directed for the Assay preparation in the Assay.

Chromatographic system— Proceed as directed in the Assay. To evaluate the system suitability requirements, use the Standard preparation prepared as directed in the Assay.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak responses. Identify the peaks based on their relative retention times given in Table 1. Calculate the percentages of galactose, lactose, epilactose, and fructose, if found, in the portion of Concentrate taken by the formula: in which C is the concentration, in mg per mL, of the relevant USP Reference Standard in the Standard solution; V is the volume, in mL, of the Test solution; W is the weight, in mg, of lactulose in the Test solution; and rU and rS are the peak responses for the relevant related compounds obtained from the Test solution and the Standard 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 1.15 g of monobasic sodium phosphate in 1000 mL of water.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile and Phosphate buffer solution (82:18). Make adjustments if necessary (see System Suitability under Chromatography 621). [note—Ensure that the concentration of acetonitrile in the Mobile 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 RS to a 10-mL volumetric 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 40 mg per mL, 4.8 mg per mL, and 3.2 mg per mL, respectively.

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

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a refractive index detector maintained at a temperature of 40 ± 1 and 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 1.3 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure. Identify the components based on their relative retention times given in Table 1: 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 20 µL) of the Standard preparation and the Assay preparation into 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 C is the concentration, in mg per mL, of USP Lactulose RS in the Standard preparation; V is the volume, in mL, of the Assay preparation; and rU and rS are the peak responses for lactulose obtained from the Assay preparation and the Standard preparation, respectively.