Gas-chromatographic method for the determination of low relative molecular mass alcohols and methyl tert-butyl ether in gasoline

Abstract

Increasing use is being made of oxygenated compounds such as methanol, 2-methylpropanol and methyl tert-butyl ether (MTBE) as octane improvers in gasoline. As a consequence, reliable methods are required for the determination of these oxygenated compounds when they are present in commercial fuels. A rapid gas-chromatographic method has been developed for the simultaneous determination of MTBE and C₁–C₄ alcohols in gasoline that shows excellent precision over a wide concentration range (1–20%V/V) of any of the component oxygenates. The sample, with an internal standard added, is injected on to a column containing ethylene glycol succinate on Chromosorb P and the lower boiling hydrocarbons (up to heptane) are allowed to elute and are vented. The flow of the carrier gas is then switched and the higher boiling hydrocarbons, as well as the oxygenated compounds, which are retained on this polar phase, are back-flushed on to a second column containing Porapak P. The oxygenates, which are less volatile than the remaining hydrocarbons, are separated individually from the latter and then quantified. The flow is again switched in order to back-flush the heavy hydrocarbons as soon as possible. Both columns can be operated at 150 °C and can be fitted into virtually any standard, single oven gas chromatograph. The analysis takes 25 min to complete.