Differences between propane in nitrogen versus air matrix analyzed using gas chromatography with flame-ionization detection

Abstract

New US Federal low-level automobile emission requirements, such as Zero Level Emission Vehicle (ZLEV), for hydrocarbons and other species have resulted in manufacturers need for new certified reference materials. The new emission requirement for hydrocarbons requires the use, by automobile manufacturing testing facilities, of 100 nmol/mol (ppb) propane in air gas standard. Emission measurement instruments are required, by Federal law, to be calibrated with the US National Institute of Standards and Technology (NIST) traceable reference materials. A NIST Standard Reference Material (SRM) containing 100 nmol/mol propane has been developed. During the development of this SRM a critical question arose as to the matrix of the primary propane standards. The automobile companies make their measurements using total hydrocarbon analyzers with flame-ionization detectors which integrate all hydrocarbons in a sample. NIST uses gas chromatography/flame-ionization detection (GC/FID) with a column to separate all components. Since the SRM mixtures were in air, the question as to the effect of oxygen on the detector arose. To investigate this effect, two suites of propane primary standards were developed: one in air and the other in nitrogen. The two suites of primary standards were analyzed using NIST methods, and the concentration of propane in an air mixture was determined. The results show that there was a difference of 0.63% in the propane concentration determined versus air and nitrogen suites.