Liquid chromatography coupled with ultraviolet absorbance detection, electrospray ionization, collision-induced dissociation and tandem mass spectrometry on a triple quadrupole for the on-line characterization of polyphenols and methylxanthines in green coffee beans

Liquid chromatography coupled with a photodiode array detector, electrospray ionization, collision-induced dissociation and tandem mass spectrometry (LC-DAD/ESI-CID-MS/MS) on a triple quadrupole (QqQ) has been used to detect and characterize polyphenols and methylxanthines in green coffee beans: three phenolic acids (caffeic acid, ferulic acid and dimethoxycinnamic acid), three isomeric caffeoylquinic acids (M-r 354), three feruloylquinic acids (M-r 368), one p-coumaroylquinic acid (M-r 338), three dicaffeoylquinic acids (M-r 516), three feruloyl-caffeoylquinic acids (M-r 530), four p-coumaroyl-caffeoylquinic acids (M-r 500), three diferuloylquinic acids (M-r 544), six dimethoxycinnamoyl-caffeoylquinic acids (M-r 544), three dimethoxycinnamoyl-feruloylquinic acids (M-r 558), six cinnamoyl-amino acid conjugates, three cinnamoyl glycosides, and three methylxanthines (caffeine, theobromine and theophylline). Dimethoxycinnamic acid, three isomers of dimethoxycinnamoyl-caffeoylquinic acids and another three of dimethoxycinnamoyl-feruloylquinic acids, as well as the three cinnamoyl glycosides, had not previously been reported in coffee beans. Structures have been assigned on the basis of the complementary information obtained from UV-visible spectra, relative hydrophobicity, scan mode MS spectra, and fragmentation patterns in MS2 spectra (both in the positive and negative ion modes) obtained using a QqQ at different collision energies. A structure diagnosis scheme is provided for the identification of different isomers of polyphenols and methylxanthines. Copyright (C) 2009 John Wiley & Sons, Ltd.