100(1/F)CS / CT)(ri / rS)

Compound Name	Relative Retention Time1 (approximate)	Relative Response Factor2	Limit (%)
Gabapentin related compound E	2.9	1.0	0.10
Gabapentin related compound A	3.5	5.3	0.1
Gabapentin related compound B	3.8	0.35	0.06
Individual unknown impurity	—	0.41	0.10
1  The relative retention times are calculated based on the retention time of gabapentin. [note—This information is for identification purposes only.]
2  The relative response factors are calculated based on the response of gabapentin related compound E due to the low absorptivity of gabapentin at the monitoring wavelength (215 nm).

Buffer solution— Proceed as directed in the Assay.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution, acetonitrile, and methanol (35:35:30). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Dissolve an accurately weighed quantity of USP Gabapentin Related Compound D RS in a small amount of methanol, and dilute quantitatively, and stepwise if necessary, with Diluent to obtain a solution having a known concentration of about 0.0028 mg per mL.

Test solution— Use the Assay preparation.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1 mL per minute. The column temperature is maintained at 40. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the column efficiency is not less than 13,600 theoretical plates for the gabapentin related compound D peak; and the relative standard deviation for replicate injections is not more than 7.0%.

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 responses for the major peaks. [note—Disregard all the peaks having relative retention times of 0.35 or less relative to gabapentin related compound D, as these are quantified in the test for Limit of Early Eluting Impurities.] Calculate the percentage of any impurity in the portion of Gabapentin taken by the formula: in which F is the relative response factor of the impurity (relative to gabapentin related compound D) which is 1.0 for gabapentin related compound D and 0.025 for all other impurities, respectively; CS is the concentration, in mg per mL, of USP Gabapentin Related Compound D RS in the Standard solution; CT is the concentration of Gabapentin, in mg per mL, in the Test solution; ri is the peak area for any impurity in the Test solution; and rS is the peak area for gabapentin related compound D in the Standard solution: not more than 0.10% of any impurity is found, and not more than 0.5% of total impurities (including the impurities quantified in Limit of early eluting impurities) is found.