Determination of oxygen-containing additives in gasoline by gas chromatography–microwave-induced plasma atomic emission spectrometry

Abstract

The development and characterization of a method for the determination of oxygen-containing (oxygenated) species in a complex mixture of hydrocarbons is described. A microwave-induced plasma (MIP) atomic emission detector is coupled to a gas chromatograph, and atomic oxygen emission is monitored as an indication of oxygenated compounds in the gas chromatograph effluent. On-line helium purification and the use of a tangential-flow torch provide conditions ideal for oxygen-selective analysis without make-up gases or extensive background correction. Furthermore, large amounts of solvent or sample do not extinguish the plasma so samples can be analysed directly without dilution, addition of reagent gases or solvent venting procedures. Both axial and lateral viewing positions were investigated for analytical use. Detection limits for oxygen were 1 ng s^–1 for both axial and lateral positions but sensitivity was higher in the axial position. However, selectivity over carbon in the lateral position was 4850 : 1 compared with 155 : 1 in the axial position. The gas chromatography MIP method was applied, using lateral viewing, to the analysis of alcohols in National Institute of Standards and Technology (NIST) reference fuel standards and to commercial gasoline samples collected from Columbia, SC, USA and Greensboro, NC, USA. Experimental results of oxygen-selective analysis of NIST reference fuel are in good agreement with the certified values. Results obtained for the determination of oxygenated compounds in gasoline samples further demonstrate the usefulness of the method for the analysis of a complex mixture of hydrocarbons. Most spike recoveries were within 10% of the expected values. Examples of oxygen-selective chromatograms are presented to demonstrate the usefulness and selectivity of the method.