pH 5.25 Buffer— Dissolve 110 g of sodium chloride and 1 g of sodium citrate in 700 mL of water in a 2000-mL volumetric flask. Cautiously add 150 g of sodium hydroxide, and dissolve with shaking. Cool to room temperature, and, while stirring, cautiously add 450 mL of glacial acetic acid to the cooled solution. Cool, add 600 mL of isopropyl alcohol, dilute with water to volume, and mix: the pH of this solution is between 5.0 and 5.5. This solution may be used for 6 weeks if stored at room temperature.

Standard stock solution— Transfer 55 mg of USP Sodium Fluoride RS, previously dried at 150 for 4 hours, to a 25-mL volumetric flask, add about 5 mL of water, and mix to dissolve. Add 1.0 mL of sodium hydroxide solution (1 in 10,000), dilute with water to volume, and mix. Each mL of this solution contains 1 mg of fluoride ions. Store in a tightly closed plastic container. This solution may be used for 2 weeks if stored in a refrigerator.

Standard solutions— Quantitatively dilute portions of the Standard stock solution with water to obtain 100-mL volumes of stock solutions having concentrations of 2.0, 6.0, 10.0, and 20.0 µg of fluoride per mL. Transfer 25.0 mL of each of these stock solutions to separate 50-mL volumetric flasks, dilute with pH 5.25 Buffer to volume, and mix.

Test solution— Shake 50.0 mL of Isoflurane with 50.0 mL of water for 5 minutes, and allow the liquids to separate completely. Transfer 25.0 mL of the water layer to a 50-mL volumetric flask, dilute with pH 5.25 Buffer to volume, and mix.

Procedure— Concomitantly measure the potentials (see pH 791), in mV, of the Standard solutions and the Test solution with a pH meter capable of a minimum reproducibility of ±0.2 mV and equipped with a fluoride ion electrode and a glass-sleeved calomel reference electrode. [note—When taking measurements, immerse the electrodes in the solution under test, which has been transferred to a 150-mL beaker containing a polytef-coated stirring bar. Allow to stir on a magnetic stirrer having an insulated top until equilibrium is attained (1 to 2 minutes), and record the potential. Rinse and dry the electrodes between measurements, taking care to avoid damaging the crystal of the fluoride ion electrode.] A satisfactory response is achieved if the difference in potential between the potentials obtained with the Standard solutions having fluoride concentrations of 1.0 µg per mL and 10.0 µg per mL is in the range of 50 to 60 mV. Plot the logarithm of the fluoride ion concentrations, in µg per mL, of the Standard solutions versus potential, in mV. From the measured potential of the Test solution and the standard response line, determine the concentration, in µg per mL, of fluoride in the Test solution: not more than 5 µg per mL is found [0.001% (w/v)].

Internal standard solution— Transfer about 1 g of normal butyl acetate, accurately weighed, to a 100-mL volumetric flask, dilute with Isoflurane to volume, and mix.

Standard solution— To 95 mL of Isoflurane in a 100-mL volumetric flask, add 10.0 µL of USP Isoflurane Related Compound A RS, 7.0 µL of USP Isoflurane Related Compound B RS, 10.0 µL of acetone, and 250 µL of Internal standard solution, dilute with Isoflurane to volume, and mix. It contains 0.01% of isoflurane related compound A, 0.007% of isoflurane related compound B, and 0.01% of acetone.

Test solution— To 20.0 mL of Isoflurane add 50.0 µL of Internal standard solution, and mix. It contains about 0.0025% (w/v) of normal butyl acetate.

Chromatographic system (see Chromatography 621)— The gas chromatograph is equipped with a flame-ionization detector and a 2.4-mm × 3.7-m nickel or stainless steel column packed with 10% phase G31 and 15% phase G18 on 60- to 80-mesh sodium hydroxide-washed support S1C. Helium is used as the carrier gas at a flow rate of about 25 mL per minute. The column temperature is programmed for 7 minutes at 65, then increases to 110 at a rate of 4 per minute. The injection port temperature is maintained at about 150 and the detector temperature is maintained at about 200. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the tailing factor for the normal butyl acetate peak is not more than 1.5; and the relative standard deviation of the ratio of the response of the acetone peak to the response of the normal butyl acetate peak for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 3 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms for 40 minutes, and measure the responses for all the peaks. Separately calculate the percentages of acetone, isoflurane related compound A, and isoflurane related compound B in the portion of Isoflurane taken by the formula: in which P is the percentage of the relevant analyte in the Standard solution; and RU and RS are the peak response ratios obtained from the Test solution and the Standard solution, respectively: not more than 0.01% of acetone, not more than 0.01% of isoflurane related compound A, and not more than 0.007% of isoflurane related compound B are found. Calculate the percentage of any other individual impurity by the formula: in which P is the percentage of isoflurane related compound B in the Standard solution; Ri is the peak response ratio of any individual impurity to the internal standard obtained from the Test solution; and RS is the peak response ratio of isoflurane related compound B to the internal standard obtained from the Standard solution: not more than 0.003% of any other individual impurity is found.