Applications of multiphoton ionization mass spectrometry: small protected nucleosides and nucleotides

Abstract

Pulsed laser desorption with a 90 ns pulse-width IR laser is used to evaporate protected nucleosides and nucleotides as neutrals into an expanding supersonic beam. The addition of the IR-absorbing matrix material Na 3PO 4 increases the desorption yield of neutral deoxynucleotides. The neutral molecules are then ionized by tunable multiphoton absorption and mass analysed in a high resolution reflectron time-of-light mass spectrometer. Since all investigated compounds contain heterocyclic aromatic bases and aromatic protecting groups, resonant multiphoton ionization can occur readily by exciting the S 1 ← S 0 transition of the aromatic π-electron system. As found previously for other systems, the molecular ion is clearly detected for each nucleotide at low photon fluxes. Increased photon densities lead to the formation of extensive fragment ions, yielding information about the nucleotide's primary structure. The general fragmentation path via charging of the aromatic protecting function reflects the fact that photoionization is taking place in these protecting groups as well as in the heterocyclic aromatic nucleobases.