pH 9.2 Buffer— Transfer 3.80 g of sodium borate decahydrate to a 1000-mL volumetric flask, dissolve in and dilute with water to volume, and mix.

Bromothymol blue solution— Transfer 150 mg of bromothymol blue and 405 mg of sodium carbonate to a 100-mL volumetric flask, dilute with water to volume, and mix.

Standard solution— Dilute 1 mL of 60% benzyltrimethylammonium chloride solution, accurately pipeted, quantitatively and stepwise with water to obtain a stock solution having a concentration of 0.2 ± 0.01 mg per mL [NOTE—Prepare this solution fresh]. Cut a 20- to 25-cm piece of cellulose dialysis tubing* having a molecular weight cut-off of 6,000 to 14,000 and a dry flat width of 5 to 9 cm, and place it in water to hydrate until pliable, appropriately sealing one end. Pipet 5 mL of the stock solution into the tubing, add 5 mL of water, appropriately seal the open end, place the tube in a suitable vessel containing 100 mL of water so that it is completely immersed in the water, and stir the fluid for 16 hours to effect dialysis.

Test solution— Cut a 20- to 25-cm piece of cellulose dialysis tubing* having a molecular weight cut-off of 6,000 to 14,000 and a dry flat width of 5 to 9 cm, and place it in water to hydrate until pliable, appropriately sealing one end. Weigh 2 ± 0.01 g of Cholestyramine Resin, and carefully transfer the specimen into the tubing, taking care to ensure that none adheres to the upper walls of the tubing. Add 10 mL of water to the contents of the tube, appropriately seal the open end, and place the tube in a suitable vessel containing 100 mL of water so that it is completely immersed in the water. Stir the fluid for 16 hours to effect dialysis.

Procedure— Pipet the following into each of three separators: separator 1: 5 mL of Standard solution, 5 mL of pH 9.2 Buffer, 1 mL of Bromothymol blue solution, and 10 mL of chloroform; separator 2: 5 mL of Test solution, 5 mL of pH 9.2 Buffer, 1 mL of Bromothymol blue solution, and 10 mL of chloroform; separator 3: 5 mL of water, 5 mL of pH 9.2 Buffer, 1 mL of Bromothymol blue solution, and 10 mL of chloroform. Shake each separator, vigorously, for 1 minute, allow the phases to separate until the chloroform phase is clear, and collect the chloroform extracts in separate 25-mL volumetric flasks. Repeat the extraction process with a second 10-mL portion of chloroform, and combine with the previous extracts. Dilute each solution with chloroform to volume, if necessary, and mix. Concomitantly determine the absorbances of the Test solution and the Standard solution at the wavelength of maximum absorbance at about 420 nm, with a suitable spectrophotometer, using the solution from separator 3 as the blank: the absorbance of the Test solution does not exceed that of the Standard solution (0.05% as benzyltrimethylammonium chloride).

Mobile phase— Prepare a filtered and degassed mixture of 0.08 M monobasic potassium phosphate and acetonitrile (65:35). Adjust with phosphoric acid to a pH of 3.0. Make adjustments if necessary (see System Suitability under Chromatography 621).

Potassium phosphate buffer— Transfer about 4 g of monobasic potassium phosphate and about 12 g of dibasic potassium phosphate to a 1-liter volumetric flask. Dissolve in and dilute with water to volume, and mix.

Sodium glycocholate solution— Transfer about 15 g of sodium glycocholate to a 500-mL volumetric flask, and dissolve in and dilute with Potassium phosphate buffer to volume.

Reference solution— Pipet 4.0 mL of Sodium glycocholate solution into a 100-mL volumetric flask, and dilute with water to volume.

Standard solution— Transfer about 100 mg of USP Cholestyramine Resin RS, accurately weighed, to a 25-mL conical flask. Pipet 15.0 mL of Sodium glycocholate solution into the flask, and stir by mechanical means for 2 hours. Transfer the contents to a centrifuge tube, and centrifuge for 15 minutes. Transfer 5.0 mL of the supernatant to a 50-mL volumetric flask, and dilute with water to volume.

System suitability solution— Prepare a solution in water containing, in each mL, about 0.6 mg of sodium glycocholate and about 0.3 mg of taurodeoxycholic acid.

Test solution— Transfer about 100 mg of anhydrous Cholestyramine Resin, accurately weighed, to a 25-mL conical flask. Pipet 15.0 mL of Sodium glycocholate solution into the flask, and stir by mechanical means for 2 hours. Transfer the contents to a centrifuge tube, and centrifuge for 15 minutes. Transfer 5.0 mL of the supernatant to a 50-mL volumetric flask, and dilute with water to volume.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 214-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between sodium glycocholate and taurodeoxycholic acid is not less than 1.5. Chromatograph the Reference solution, and record the peak responses as directed for Procedure: the tailing factor is not more than 2.5; and the relative standard deviation for replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 50 µL) of the Reference solution, the Standard solution, and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of sodium glycocholate absorbed on each g of the Resin taken by the formula: in which M is the stated value, in mg, of sodium glycocholate absorbed per g of USP Cholestyramine Resin RS; rR, rU, and rS are the peak responses obtained from the Reference solution, the Test solution, and the Standard solution, respectively; WU is the weight, in mg, of Cholestyramine Resin, calculated on the dried basis, taken to prepare the Test solution; and WS is the weight, in mg, of USP Cholestyramine Resin RS taken to prepare the Standard solution.

(Official January 1, 2007)