Methane

General Notices

(Ph. Eur. monograph 2413)
CH4  16.04 74-82-8 

Ph Eur

DEFINITION

Content

Minimum 99.5 per cent V/V of CH4.

This monograph applies to methane obtained from natural gas and intended for medicinal use.

CHARACTERS

Appearance

Colourless gas, flammable in air.

Solubility

At 20 °C and at a pressure of 101 kPa, 1 volume dissolves in about 27 volumes of water.

IDENTIFICATION

Examine the chromatograms obtained in the assay.

Results The principal peak in the chromatogram obtained with the gas to be examined is similar in retention time to the principal peak in the chromatogram obtained with reference gas (b).

TESTS

Nitrogen

Gas chromatography (2.2.28).

Gas to be examined The substance to be examined.

Reference gas Mixture containing 500 ppm V/V of nitrogen R1 in methane R1.

Column:
— material: stainless steel;
— size: l = 2 m, Ø = 2 mm;
— stationary phase: molecular sieve for chromatography R (0.5 nm).

Carrier gashelium for chromatography R.

Flow rate 30 mL/min.

Temperature:
— column: 80 °C;
— detector: 150 °C.

Detection Thermal conductivity.

Injection 1 mL.

Relative retention With reference to methane (retention time = about 3 min): nitrogen = about 0.7.

System suitability Reference gas:
— resolution: minimum 1.5 between the peaks due to nitrogen and methane.
Calculation of percentage content:
— use the concentration of nitrogen in the reference gas.
Limit:
— maximum 500 ppm V/V.

C2-C4 Hydrocarbons

Gas chromatography (2.2.28).

Gas to be examined The substance to be examined.

Reference gas Use the following mixture of gases in methane R: ethane R (impurity C) (100 ppm V/V), propane R (impurity D) (100 ppm V/V), i-butane R (impurity E) (100 ppm V/V) and n-butane R (impurity F) (100 ppm V/V).

Column:
— material: fused silica;
— size: l = 30 m, Ø = 0.53 mm;
— stationary phase: deactivated aluminium oxide for chromatography R.

Carrier gashelium for chromatography R.

Flow rate 113 mL/min.

Split ratio 1:10.

Temperature:

Detection Flame ionisation. Use appropriate fuel gases: hydrocarbon-free air R and hydrogen for chromatography R.

Injection 0.1 mL.

Relative retention With reference to methane (retention time = about 1.2 min): impurity C = about 1.2; impurity D = about 1.8; impurity E = about 2.5; impurity F = about 2.6.

System suitability Reference gas:
— resolution: minimum 1.5 between the peaks due to methane and impurity C; minimum 1.5 between the peaks due to impurity E and impurity F.
Calculation of percentage contents:
— for each of the impurities C, D, E and F, use the corresponding concentration in the reference gas.
Limit:
— total (sum of impurities C, D, E and F): maximum 100 ppm V/V.

Water (2.5.28)

Maximum 10.0 ppm V/V.

ASSAY

Gas chromatography (2.2.28).

Gas to be examined The substance to be examined.

Reference gas (a)Methane R1.

Reference gas (b) Mixture containing 1 per cent V/V of nitrogen R1 in methane R1.

Column:
— material: stainless steel;
— size: l = 2 m, Ø = 2 mm;
— stationary phase: molecular sieve for chromatography R (0.5 nm).

Carrier gashelium for chromatography R.

Flow rate 30 mL/min.

Temperature:
— column: 80 °C;
— detector: 150 °C.

Detection Thermal conductivity.

Injection 10 µL.

Relative retention With reference to methane (retention time = about 3 min): nitrogen = about 0.7.

System suitability: reference gas (b):
— resolution: minimum 1.5 between the peaks due to nitrogen and methane.

Calculate the percentage content of CH4, using the chromatogram obtained with reference gas (a).

STORAGE

As a compressed gas, in appropriate containers complying with the legal regulations.

IMPURITIES

Specified impurities A, B, C, D, E, F

A. N2: nitrogen,
B. H2O: water,
C. ethane,
D. propane,
E. 2-methylpropane (i-butane),
F. butane (n-butane).

Ph Eur