Qualitative and quantitative determination of oxygenates in gasolines using 1H nuclear magnetic resonance spectroscopy

Oxygenous compounds are added in motor gasolines for increase of knocking resistance and to reduce vehicular emissions. In this paper the use of nuclear magnetic resonance (NMR) spectroscopy for the determination of oxygenates in untreated commercial gasolines is described. Nine different oxygenates were added individually or as mixtures in different amounts from 1% (v/v) up to 17% (v/v) to a base gasoline. All oxygenates could be successfully determined both qualitatively and quantitatively from the H-1-NMR spectra of the gasolines. The analysis of dilute mixtures of methanol, ethanol, n-propanol, iso-propanol, n-butanol, 2-butanol, tert-butanol (TBA), methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) was also possible. Tn order to increase the accuracy of the quantitative determination, the H-1-NMR spectra were C-13 decoupled during the acquisition time using the GARP pulse-sequence. The high certainty of the qualitative assignment, the speed of the analyses of small untreated sample volumes and the good agreement of the quantitative determination (correlation coefficient R = 0.998) are the most impressive advantages of this method. The assignments of the H-1-NMR signals to the single oxygenates were ensured by two-dimensional C-13-shift correlated NMR spectrum recorded in the inverse mode (HSQC) in only a few minutes. Additionally it is shown that results obtained from NMR spectra were also suitable for the determination of the influences of single oxygenates on the knocking behaviour. By a multiple linear regression (MLR) a partial octane number was calculated for each oxygenate, comparable with its blending octane number. (C) 1999 Elsevier Science B.V. All rights reserved.