pH5.25Buffer— Dissolve 110g of sodium chloride and 1g of sodium citrate in 700mLof water in a 2000-mLvolumetric flask.Cautiously add 150g of sodium hydroxide,and dissolve with shaking.Cool to room temperature,and,while stirring,cautiously add 450mLof glacial acetic acid to the cooled solution.Cool,add 600mLof isopropyl alcohol,dilute with water to volume,and mix:the pHof this solution is between 5.0and 5.5.This solution may be used for 6weeks if stored at room temperature.

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

Standard solutions— Quantitatively dilute portions of the Standard stock solutionwith water to obtain 100-mLvolumes of stock solutions having concentrations of 2.0,6.0,10.0,and 20.0µg of fluoride per mL.Transfer 25.0mLof each of these stock solutions to separate 50-mLvolumetric flasks,dilute with pH5.25Bufferto volume,and mix.

Test solution— Shake 50.0mLof Isoflurane with 50.0mLof water for 5minutes,and allow the liquids to separate completely.Transfer 25.0mLof the water layer to a 50-mLvolumetric flask,dilute with pH5.25Bufferto volume,and mix.

Procedure— Concomitantly measure the potentials (see pHá791ñ),in mV,of the Standard solutionsand the Test solutionwith a pHmeter capable of a minimum reproducibility of ±0.2mVand 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-mLbeaker containing a polytef-coated stirring bar.Allow to stir on a magnetic stirrer having an insulated top until equilibrium is attained (1to 2minutes),and record the potential.Rinse and dry the electrodes between measurements,taking care to avoid damaging the crystal of the fluoride ion electrode.]Asatisfactory response is achieved if the difference in potential between the potentials obtained with the Standard solutionshaving fluoride concentrations of 1.0µg per mLand 10.0µg per mLis in the range of 50to 60mV.Plot the logarithm of the fluoride ion concentrations,in µg per mL,of the Standard solutionsversus potential,in mV.From the measured potential of the Test solutionand the standard response line,determine the concentration,in µg per mL,of fluoride in the Test solution:not more than 5µg per mLis found [0.001%(w/v)].

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

Standard solution— To 95mLof Isoflurane in a 100-mLvolumetric flask,add 10.0µLof USP Isoflurane Related Compound A RS,7.0µLof USP Isoflurane Related Compound B RS,10.0µLof acetone,and 250µLof 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 20.0mLof Isoflurane add 50.0µLof Internal standard solution,and mix.It contains about 0.0025%(w/v)of normal butyl acetate.

Chromatographic system (see Chromatography á621ñ)— 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 G31and 15%phase G18on 60-to 80-mesh sodium hydroxide-washed support S1C.Helium is used as the carrier gas at a flow rate of about 25mLper minute.The column temperature is programmed for 7minutes at 65,then increases to 110at a rate of 4per minute.The injection port temperature is maintained at about 150and 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 3µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms for 40minutes,and measure the responses for all the peaks.Separately calculate the percentages of acetone,isoflurane related compound A,and isoflurane related compound Bin the portion of Isoflurane taken by the formula: in which Pis the percentage of the relevant analyte in the Standard solution;and RUand RSare the peak response ratios obtained from the Test solutionand 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 Bare found.Calculate the percentage of any other individual impurity by the formula: in which Pis the percentage of isoflurane related compound Bin the Standard solution;Riis the peak response ratio of any individual impurity to the internal standard obtained from the Test solution;and RSis the peak response ratio of isoflurane related compound Bto the internal standard obtained from the Standard solution:not more than 0.003%of any other individual impurity is found.