DEVELOPMENT OF HYDROCARBON-GAS STANDARDS

Methodology is described for the gravimetric preparation and analytical evaluation of accurate, stable, multicomponent gas standards in compressed gas cylinders containing C-2-C-10 alkane, alkene and aromatic hydrocarbons in pure nitrogen or air. Standards have been prepared containing up to fourteen hydrocarbons in a single mixture at concentrations ranging from 5-2000 nmol/mol (ppb). Analysis of hydrocarbons at the low ppb level requires cyrogenic preconcentration. Depending on the combination of hydrocarbons in any one gas mixture, several analytical gas chromatographic columns may be required to achieve baseline separations of all the compounds. The sum of preparative and analytical error components of the uncertainty associated with the concentrations of the hydrocarbons at the 95% confidence level typically ranges from 0.5-5.0%. This total uncertainty depends on the concentration level and the hydrocarbon. Intercomparative analyses of new and previously prepared standards have verified that such mixtures are stable for at least nine months, with the exception of ethyne (acetylene) which has a stability of less than nine months.