The furosine, CML and CEL residues were derivatised
by esterification with isobutanol and acylation with heptafluorobutyric
anhydride as described for amino acids
derivatization, with the exception that the BHT solution
was not added after the esterification with isobutanol [24].
The furosine, CML and CEL residues were analysed by
gas chromatography–quadrupole mass spectrometry (GC–
MS, Agilent). The column and the temperature programme
used were the same as for amino acids analysis. Selected
ion monitoring (SIM) was used for the detection of the
compounds. The ions at m/z 110, 621 and 578 were used,
respectively, for furosine, CML and CEL detection. For
qualification, additional ions at m/z 280 and 240, m/z 565
and 519 and m/z 533 and 511, respectively, were used.
Quantification was performed using the lysine residues as
internal standard. The MS was operated in the electron
impact mode with an electron impact energy of 70 eV and
data collected at a rate of 3 scan s-1. The ion source and
the transfer line were kept at 230 C [20].