PRECURSOR IONS FOR GAS-PHASE CATION-ATTACHMENT REACTIONS IN LASER DESORPTION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

The mechanism by which cation-attached organic compounds are formed and and trapped for detection in the infrared laser desorption ionization (LDI)/Fourier transform ion cyclotron resonance (FTICR) mass spectrometry experiment is evaluated. A combination of time-of-flight (TOF), variable trap potential, and double-resonance experiments offers evidence that these ions result from gas-phase reactions in the trapped-ion cell. LDI spectra of KBr-doped organic samples show that as the desorption site is displaced from the trapped-ion cell, the (M + K)+ signal decreases and the K+ signal increases. Optimum LDI/FTICR trapping potentials for (M + K)+ and K+ are less than 3 V and greater than 17 V, respectively, which indicates that substantial differences exist in kinetic energy distributions for these ions. In contrast, salt adduct ions formed by LDI, K2Br+ for example, exhibit trapping profiles that are similar to (M + K)+. Double-resonance experiments to eject suspected precursor ions indicate that it is these adduct ions rather than the bare cation which react with the neutral to form the cation-attached organic species. For example, in LDI/FTICR experiments on a mixture of KCl and dilaurylthiodipropionate (DLTDP), ejection of K+ yields an abundant (M + K)+ ion, while continuous ejection of K2Cl+ precludes formation of any product ion species.