Nonvolatile residue
Transfer 10.0 mL to a suitable weighed evaporating dish, evaporate with the aid of a current of air to dryness, and dry the residue at 50
for 2 hours: the weight of the residue does not exceed 2.0 mg.
Limit of fluoride
[NOTEUse plasticware throughout this test.
]
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.
[NOTEWhen 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)].
Related compounds
[NOTEThe
Internal standard solution and the
Standard solution are prepared using the same Isoflurane that is under test. If multiple lots or samples of Isoflurane are under test, one sample may be selected for the
Internal standard solution and the
Standard solution. An appropriate blank correction should be made when determining the percentages of impurities in the other lots or samples.
]
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:
P[
RU /(
RS RU)],
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:
P[
Ri /(
RS Ri)],
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.