Fig. 7

a GC–MS total ion chromatogram (TIC) and mass spectra (insets) of the trimethylsilyl (TMS) and trimethylsilyl-oxime (TMS-oxime) derivatives for compounds of pectin hydrolysis at 45 °C and pH 5.4 using peh28. b Predicted fragmentation pattern of oxime-TMS derivative of galacturonic acid showing prominent ions of m/z = 218, (1), and m/z = 333, (2), relative to what was reported by Peterson [104]. As shown in the elution profile, galacturonic acid (GA) exists in two different configuration forms of open-chain and cyclic-pyranose which correspond to TMS (GA1-2) and TMS-oxime (GA3-4) derivatives. Galacturonic acid is the main product displayed relative to the other less dominant compound galactose (Gal). Those components were not identified in the blank profile, figure not shown, and were expected as a result of pectin hydrolysis using peh28. Two peaks with different retention times and identical mass fragments were detected in case of TMS and TMS-oxime derivatives of galacturonic acid in their alpha- and beta-stereoisomers and syn- and anti-oxime stereoisomers, respectively