Direct Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Imaging of Cellulose and Hemicellulose in Populus Tissue

Imaging applied toward lignocellulosic materials requires high molecular specificity to map specific compounds within intact tissue. Although secondary ionization mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) with a single stage of MS have been used to image lignocellulosic biomass, the complexity of the plant tissue requires tandem MS, which limits the interpretation of simple MS. MALDI linear ion trap (LIT) tandem MS offers the high molecular specificity needed for lignocellulosic analyses. MALDI-LIT MS analyses of cellulose and xylan (hemicellulose) standards were performed to determine mass-to-charge ratios and fragmentation pathways for identification of these compounds in intact tissue. The MALDI-LIT-MS images of young Populus wood stem showed even distribution of both cellulose and hemicellulose ions; in contrast, the tandem MS images of cellulose and hemicellulose generated by plotting characteristic fragment ions resulted in drastically different images. This demonstrates that isobaric ions are present during MALDI-LIT-MS analyses of wood tissue and tandem MS is necessary to distinguish between isobaric species for selective imaging of carbohydrates in biomass.