Comparison of ionization behaviors of ring and linear carbohydrates in MALDI-TOFMS

Ionization efficiencies of cyclodextrins and their linear compounds in matrix-assisted laser desorption and ionisation (MALDI) analysis were compared, and differences in the ionization efficiencies of α- and β-cyclodextrins were also studied. The mass spectra showed a series of the [M+cation]+ ions but not the [M+H]+ ions. Alkali metal salts of Li+, Na+, K+, and Cs+ were used as the cationizing agents to enhance the ionization efficiency. Relative ion intensities of the ring compounds (α- and β-cyclodextrins) were much larger than those of the linear ones (maltohexaose and maltoheptaose), and the difference showed an increasing trend with the size of the alkali metal cation. β-Cyclodextrin had higher ionization efficiency than α-cyclodextrin and the difference increased by increasing the size of the alkali metal cation. It was also found that the ionization efficiency was affected by the counter anion of the salt. The higher ionization efficiencies of cyclodextrins were explained with the number of coordination sites and the binding energies.