Assay—
Buffer—
Dissolve 3.5 g of monobasic sodium phosphate monohydrate in 900 mL of water. Adjust with phosphoric acid to a pH of 3.5. Dilute with water to 1 L.
Mobile phase—
Prepare a filtered and degassed mixture of
Buffer and acetonitrile (1:1). Make adjustments if necessary (see
System Suitability under
Chromatography 
621
).
Standard stock preparation—
Transfer an accurately weighed amount of
USP Valproic Acid RS to a suitable volumetric flask, and dissolve in
Mobile phase to obtain a solution having a known concentration of about 5.0 mg per mL.
Impurity stock preparation—
Dissolve an accurately weighed amount of
USP Valproic Acid Related Compound A RS in acetonitrile to obtain a solution having a known concentration of about 0.5 mg per mL of valproic acid related compound A.
System suitability preparation—
Accurately transfer 10 mL of Standard stock preparation and 1 mL of Impurity stock preparation to a 100-mL volumetric flask, and dilute with Mobile phase to obtain a solution having concentrations of 0.5 mg per mL of valproic acid and 0.005 mg per mL of valproic acid related compound A.
Standard preparation—
Quantitatively dilute the Standard stock preparation with Mobile phase to obtain a solution having a known concentration of about 0.5 mg per mL of valproic acid.
Assay preparation—
Transfer about 50 mg of Divalproex Sodium, accurately weighed, to a 100-mL volumetric flask, and dissolve in and dilute with Mobile phase to volume. This solution has a concentration of about 0.5 mg per mL of divalproex sodium.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L7. The flow rate is about 1.0 mL per minute. Chromatograph the
System suitability preparation, and record the peak responses as directed for
Procedure: the relative retention time for valproic acid related compound A is about 0.69, and that for valproic acid is 1.0; the resolution,
R, between valproic acid related compound A and valproic acid is not less than 5.0; for the valproic acid peak, the tailing factor is not more than 1.5; and the relative standard deviation for replicate injections 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 the major peaks. Calculate the percentage of available valproic acid (C
8H
16O
2) in the portion of Divalproex Sodium taken by the formula:
100(CS / CU)(rU / rS)(310.41/144.21)(½)
in which
CS is the concentration, in mg per mL, of valproic acid in the
Standard preparation; CU is the concentration of Divalproex Sodium, in mg per mL, in the
Assay preparation; rU and
rS are the peak areas for valproic acid obtained from the
Assay preparation and the
Standard preparation; 310.41 and 144.21 are the molecular weights for divalproex sodium repeating unit and valproic acid, respectively; and 2 is the number of moles of valproic acid per mole of divalproex sodium repeating unit.
USP31
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