Isoflurane

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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C₃H₂ClF₅O 184.49 

Ethane, 2-chloro-2-(difluoromethoxy)-1,1,1-triuoro-; 

1-Chloro-2,2,2-triuoroethyl difluoromethyl ether CAS RN®: 26675-46-7; UNII: CYS9AKD70P. 

1 DEFINITION 

Isoflurane contains NLT 99.9% and NMT 100.0% of isoflurane (C₃H₂ClF₅O). 

IDENTIFICATION 

The IR absorption spectrum of isoflurane, obtained using a gas cell, exhibits maxima only at the same wavelengths as those of a similar preparation of USP isoflurane RS. 

2 ASSAY

Procedure

Analysis: Using the results from the test for Organic Impurities, calculate the percentage of isoflurane (C₃H₂ClF₅O) in the sample of isoflurane taken by subtracting the sum of percentages for the impurities found from 100.0%. 

Acceptance criteria: 99.9%–100.0% 

3 IMPURITIES 

Chloride 

Sample solution: Pipet 10 mL of isoflurane into a suitable vessel containing 60 mL of isopropyl alcohol and 4 drops of dilute nitric acid (1:1), and stir to dissolve. 

Analysis: Titrate potentiometrically with 0.002 N silver nitrate in isopropyl alcohol VS. 

Acceptance criteria: NMT 2.11 mL is required (NMT 10 ppm) 

Limit of Fluoride 

Use plasticware throughout this test. 

Buffer: Dissolve 110 g of sodium chloride and 1 g of sodium citrate in 700 mL of water in a 2-L 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. [Note—This solution may be used for 6 weeks if stored at room temperature.] 

Solution A: Transfer 55 mg of USP Sodium Fluoride RS to a 25-mL volumetric  flask. Add 5 mL of water, and mix to dissolve. Add 1.0 mL of 0.0025 N sodium hydroxide, dilute with water to volume, and mix. Each mL of this solution contains 1 mg of fluoride ion. Store in a tightly closed plastic container. [Note—This solution may be used for 2 weeks if stored in a refrigerator.] 

Standard stock solution 1: 2.0 µg/mL of fluoride in water from Solution A 

Standard stock solution 2: 6.0 µg/mL of fluoride in water from Solution A 

Standard stock solution 3: 10.0 µg/mL of fluoride in water from Solution A 

Standard stock solution 4: 20.0 µg/mL of fluoride in water from Solution A 

Standard solution 1: 1.0 µg/mL of fluoride in Buffer from Standard stock solution 1 

Standard solution 2: 3.0 µg/mL of fluoride in Buffer from Standard stock solution 2 

Standard solution 3: 5.0 µg/mL of fluoride in Buffer from Standard stock solution 3 

Standard solution 4: 10.0 µg/mL of fluoride in Buffer from Standard stock solution 4 

Sample solution: Shake 50.0 mL of isoflurane with 50.0 mL of water for 5 min, and allow the liquids to separate completely. Transfer 25.0 mL of the water layer to a 50-mL volumetric  flask, dilute with Buffer to volume, and mix. 

Analysis 

Samples: Standard solutions 1–4 and Sample solution 

(See pH 〈791〉.) 

Concomitantly measure the potentials in mV, of Standard solutions 1–4 and the Sample 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 or a double junction fluoride ion-selective combination electrode. 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 with an insulated top until equilibrium is attained (1–2 min), 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 Standard solution 1 and Standard solution 4, having fluoride concentrations of 1.0 and 10.0 µg/mL, respectively, is in the range of 50–60 mV. Plot the logarithm of the fluoride ion concentrations, in µg/mL, of Standard solutions 1–4 versus potential, in mV. From the measured potential of the Sample solution and the standard response line, determine the concentration, in µg/mL, of fluoride in the Sample solution. 

Acceptance criteria: NMT 5 µg/mL of fluoride in the Sample solution [NMT 0.001% (w/v) fluoride in isoflurane] 

Nonvolatile Residue 

Analysis: Transfer 10.0 mL of isoflurane to a suitable weighed evaporating dish, evaporate with the aid of a current of air or stream of nitrogen to dryness, and dry the residue at 50° for 2 h. 

Acceptance criteria: The weight of the residue does not exceed 2.0 mg. 

Change to read: 

Organic Impurities 

Standard stock solution: To a suitable tared vial, fitted with a septum, add 20 mL (29.8 g) of isoflurane. Seal and reweigh the vial to determine the weight of the isoflurane added. To this vial sequentially add 20 µL (30 mg) of USP isoflurane Related Compound A RS, 21 µL (30 mg) of USP isoflurane Related Compound B RS, and 38 µL (30 mg) of USP Acetone RS. Record the weight after the addition of each impurity and determine the total weight. 

Calculate the percentage of each impurity in the Standard stock solution: 

Result = (W_I/W_T) × P_J 

W_I = weight of each impurity added (g) 

W_T = total weight of the Standard stock solution (g) 

P_J = purity of each impurity added (%) 

Standard solution: To a suitable tared vial, fitted with a septum, add 10 mL (15 g) of isoflurane. Seal and reweigh the vial to determine the weight of the isoflurane added. To this vial add 300 µL of the Standard stock solution, and record the weight to determine the weight of the Standard stock solution added and the nal weight of the Standard solution. 

Calculate the spiked concentration (C ) of each impurity in the Standard solution: 

Result = (W_I /W_T ) × C_J   

W_I = weight of the Standard stock solution added (g)  

W_T = total weight of the Standard solution (g) 

C_J = concentration of each impurity in the Standard stock solution (%) 

System suitability solution: To a suitable vial, fitted with a septum, add 10 mL (15 g) of isoflurane. Seal the vial. To this vial add 100 µL of the Standard stock solution. 

Sample solution: isoflurane 

Chromatographic system 

(See Chromatography 〈621〉, System Suitability.) 

Mode: GC 

Detector: Flame ionization 

Column: 0.32-mm × 60-m; fused-silica capillary coated with a 1.0-µm lm of G16 

Carrier gas: Helium 

Flow rate: 1.7 mL/min (constant ow mode) 

Temperatures 

Injection port: 175° 

Detector: 200° 

Column: See Table 1. 

Table 1 

Injection volume: 2 µL 

Injection type: Split; split ratio, 23:1 

System suitability 

Samples: Standard solution and System suitability solution 

Suitability requirements 

Tailing factor: NMT 1.5 for acetone, Standard solution 

Relative standard deviation: NMT 5% each for isoflurane related compound A, isoflurane related compound B, and acetone, Standard solution 

Signal-to-noise ratio: NLT 15 for isoflurane related compound B, System suitability solution 

Analysis: Injections of isoflurane used to prepare the Standard solution must be made to estimate the amount of known impurities that may be present in the solvent. The nal concentration of each impurity is equal to the concentration of the impurity added plus the concentration inherent in the matrix. 

Samples: Standard solution and Sample solution 

Calculate the nal concentration (C ) of each impurity in the Standard solution: 

Result = [r_U/(r_S − r_U) × C_S ] + C_S 

r_U = peak response of each impurity from the isoflurane used as the solvent

r_S = peak response of each impurity from the Standard solution

C_S = spiked concentration of each impurity in the Standard solution (%)

Calculate the percentage of isoflurane related compound A, isoflurane related compound B, and acetone observed in the Sample solution: 

Result = (r_U/r_S) × C_F 

r_U = peak response of each impurity from the Sample solution 

r_S = peak response of each impurity from the Standard solution

C_F   = final concentration of each impurity in the Standard solution

Calculate the percentage of all other impurities: 

Result = (r_U/r_S) × C_F × (1/F) 

r_U = peak response of the impurity from the Sample solution 

r_S = peak response of isoflurane related compound B from the Standard solution 

C_F   = final concentration of USP isoflurane Related Compound B RS in the Standard solution (%) F = relative response factor relative to isoflurane related compound B (see Table 2) 

Acceptance criteria: See Table 2. 

Table 2 

^aRelative to isoflurane.

^b Relative to isoflurane related compound B. 

^c 1,1-Dichloro-1-(chlorodifluoromethoxy)-2,2,2-triuoroethane. 

^d 2-(chlorodifluoromethoxy)-1,1,1-triuoroethane. 

^e 1,1-Dichloro-1-(difluoromethoxy)-2,2,2-triuoroethane. 

4 SPECIFIC TESTS 

Refractive Index 〈831〉: 1.2990–1.3005 at 20° 

Water Determination 〈921〉, Method I: NMT 0.10% 

Acidity or Alkalinity 

Sample: Transfer 5 mL of isoflurane and 2 mL of carbon dioxide-free water to a 10-mL glass-stoppered graduated cylinder, shake for 3 min, and allow the layers to separate. 

Analysis: Test the aqueous layer (upper) with both red litmus paper and blue litmus paper. 

Acceptance criteria: The aqueous layer is neutral to litmus paper. The aqueous phase should not turn the red litmus paper blue, nor should it turn the blue litmus paper red. 

5 ADDITIONAL REQUIREMENTS 

Packaging and Storage: Preserve in tight containers. Store at controlled room temperature. Replace the cap securely after each use. USP Reference Standards 〈11〉 

USP Acetone RS 

USP isoflurane RS 

USP isoflurane Related Compound A RS 

1-Chloro-2,2,2-triuoroethyl chlorodifluoromethyl ether. 

C₃HCl₂F₅O 218.94 

USP isoflurane Related Compound B RS 

2,2,2-Triuoroethyl difluoromethyl ether. 

C₃H₃F₅O 150.05 

USP Sodium Fluoride RS