Accurate determination of double bond position in mono-unsaturated straight-chain fatty acid ethyl esters from conventional electron impact mass spectra by quantitative spectrum—structure modelling

Abstract

Brakstad, F., 1993. Accurate determination of double bond position in mono-unsaturated straight-chain fatty acid ethyl esters from conventional electron impact mass spectra by quantitative spectrum—structure modelling. Chemometrics and Intelligent Laboratory Systems, 19: 87–100. An objective approach is given for selecting the most significant fragments for mass spectrum—structure correlation. The method is based on partial least-squares regression (PLSR) on normalised spectral intensities as independent variables, and the carbon—carbon double bond position as response. Such a correlation between a structural descriptor and the mass spectra of pure compounds has not hitherto been reported in the literature. The procedure is demonstrated using conventional electron impact (70 eV)mass spectra of twelve straight-chain mono-unsaturated fatty acid ethyl esters, ranging in carbon—carbon double bond positions from 5 to 11 (except 7), incorporating both cis- and trans-isomers and chain lengths from C 16 to C 20. On the basis of the relative intensity of thirteen fragments, a quantitative model relating these to the response was developed. At a 95% confidence interval, all the residuals from the cross-validation fall within the confidence interval given by [true value ± 0.46] carbon position units. Thus, the model practically always gives an unambiguous estimate of the carbon—carbon double bond position. Possible further developments of this method are discussed for the use of gas chromatography—mass spectrometry and gas chromatography—Fourier transform infrared spectroscopy in combination.