Application of Carbon Matrices to Screening Pentacylic Triterpenoids in Plant Feedstock by MALDI Mass Spectrometry

An approach based on the use of carbon matrices is proposed to obtain MALDI mass spectra of the most important secondary plant metabolites, pentacyclic triterpenoids (PCTs). The best results were obtained using carboxylated carbon nanotubes as a matrix; they form the most homogeneous solid phase with PCTs on a MALDI target plate surface. The cationization of analytes with lithium ions made it possible to obtain the most intense signals in the mass spectra of betulin, lupeol, and betulinic acid and ensured limits of detection for these analytes at a level of 15–25 ng. Because of the occurrence of side processes of the hydration and dehydration of cationized molecules, as well as of the replacement of labile protons by lithium ions, the resulting mass spectra are characteristic for various classes of PCTs and allow their reliable detection without using tandem mass spectrometry. The developed approach was successfully tested on real samples of birch bark extracts, and also on extracts from cranberry and lingonberry peels.