Gas Analysis: Hydrocarbons and Fixed Gases

This Technical Bulletin explains the gas chromatographic separation and detection of these gases.

The composition of gasses in a system can indicate whether the system has developed a leak, oxidation or some other chemical reaction has occurred, or if the system has been correctly handled. Bubbles can develop in products through electrolysis, oxidation, or microbiological action. Many processes require accurate control of the atmospheres. The identity and purity of gasses in hospitals, manufacturing, medical devices, and products is very important.

Fixed gasses may be analyzed using mass spectrometry (MS), infrared spectroscopy (IR), or gas chromatography (GC). This Technical Bulletin explains the gas chromatographic separation and detection of these gasses. By injecting a sample onto two different columns with two different detectors both the Fixed gasses (N₂, O₂, CO, CO₂, CH₄) and hydrocarbons (C₁-C₉) can be determined.

The fixed gasses are chromatographically separated based on molecular size. A molecular sieve packing is used in a conventional 1/8" stainless steel packed column. Samples are introduced using either a gas tight syringe or a sampling valve with a fixed volume loop. Upon injection the sample is swept through the GC with carrier gas. After separating in the column, the carrier gas carries each component into the detector where components are detected based on the difference in thermal conductivity between the carrier gas and the component. For the analysis of hydrogen and helium, argon is used as the carrier. Helium is used for all the other gasses: nitrogen, oxygen, carbon monoxide, carbon dioxide, and methane. Detection limits using a thermal conductivity detector are typically 0.01% by volume.

In addition to the fixed gasses, hydrocarbons such as methane, ethane, ethene, and others up through C₉are separated on a different column. Normal-, iso-, saturated and unsaturated hydrocarbons can be separated up through C₅. Each component is detected using a flame ionization detector with detection limits for hydrocarbons of approximately 1 ppm. The chromatograms (Primary Gas Standard and Mixed Hydrocarbon Standard) illustrate the separation of both fixed gasses and hydrocarbons conducted simultaneously.

The units in gas composition are all based on volume of the component per volume of sample (volume/volume). Since most non-polar gasses approximately follow the ideal gas law, this is the same as mole percent, i.e.: 25% CO by volume equals 25 mole%. The units of ppm actually represent mL/L.

The sample needs to be contained so that it cannot leak or permeate through the container. Hydrogen and helium should be contained in glass or steel because these gasses will permeate most plastic films. Other gasses will also permeate plastic films but at slower rate. The container should also have fittings which allow for the transfer of a gas sample without leakage or contamination. Some gas sampling containers have Swagelok or Cajon fittings, or glass stopcocks which all work well. Plastic fittings should be avoided because they leak. Rubber and most plastic parts should be avoided when trace organic components are being measured because they will both adsorb and desorb contaminants.

If the sample will be pressurized, stainless steel cylinders or canisters with steel valves should be used. If the sample is at ambient or reduced pressure, glass bottles with glass stopcocks may also be used. WCAS carries both glass sampling bulbs (250 mL) and SUMMA polished steel canisters (6 L).

This Technical Bulletin covers only the major gasses listed above. There are a variety of forms of gas analysis including environmental (stack, ambient, indoor), industrial hygiene, petroleum gasses and others in which the determination of both trace organic and inorganic components are important.

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