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Keywords:

  • Epicuticular wax;
  • anteiso- and iso-branched alkyl esters;
  • swede;
  • Brassica napus;
  • gas chromatography-mass spectrometry;
  • structural determination

Abstract

A method for the separation and identification of complex mixtures of more than 50 different intact esters in epicuticular wax of swede, Brassica napus var. rapifera cv. Doon Major, using combined gas chromatography-mass spectrometry (GC-MS) has been developed. The esters were separated at temperatures of up to 350°C by automatic pressure programming (for capillary GC) or by manually increasing helium carrier gas pressure (for GC-MS) in order to maintain flow at high temperatures. Esters in the range C41-C50 were detected, with regular repeating acid/alcohol combinations. Individual acid and alcohol moieties were identified from [RCO2H2]+ ions in the mass spectra of intact esters and, following trans-esterification and silylation, from the mass spectral and chromatographic characteristics of the methyl esters of the acid components (C14-C23) and trimethylsilyl derivatives of the alcohol components (C23-C31). The major compounds were doubly branched C44 and C46 esters, composed of anteiso-C17/anteiso-C27 (32.7%), anteiso-C17/anteiso-C29 (11.9%) and anteiso-C19/anteiso-C27 (13.1%) acid/alcohol combinations, respectively, singly branched C43 and C45 esters, composed of n-C16/anteiso-C27 (8.5%), anteiso-C17/n-C26 (3.9%), n-C18/anteiso-C27 (6.8%), anteiso-C17/n-C28 (2.3%) and anteiso-C19/n-C26 (1.6%), were minor components.