Gravimetric preparation of reference gas mixtures for natural gas
Gas chromatography is used to analyse the key components of natural gas. Natural gas has 11 main components: nitrogen (N2), carbon dioxide (CO2), methane (CH4), ethane (C2H6), propane (C3H8), iso-butane (i.C4H10), n-butane (n.C4H10), neo-pentane (neo.C5H12), iso-pentane (i.C5H12), n-pentane (n.C5H12) and n-hexane (n.C6H14).
The aim of this study is to develop reference standards for natural gas, so that on-site concentration measurements for its main components can be traced to these reference standards.
The reference standards will be 11-component gas mixtures, prepared using the gravimetric method and then validated by gas chromatography.
A quality assurance system will be applied to the preparation and validation procedures so that the gas mixtures can subsequently be accredited by French accreditation committee COFRAC.
Once the gas mixtures are validated, they can be used in two ways:
LNE uses a special mixing rack to produce "natural gas" mixtures. Unlike standard mixing racks, it has electropolished piping, with stainless steel joints and orbital welding.
The gravimetric reference gas mixtures prepared by the laboratory are validated with a Varian CP 2003 Quad micro gas chromatograph. This model has three independent channels, each consisting of a specific separation column and a thermal conductivity detector (TCD), allowing all components to be analysed simultaneously.
In 2003, gas mixtures with 7 components (C3H8, CO2, C2H6, N2, n.C4H10 and i.C4H10 in CH4) were prepared by the gravimetric method and then validated by gas chromatography.
In 2004 the study was extended as follows: