Matrix-assisted laser desorption/ionisation collisions of bio-molecules

Abstract

During this study the effect of electrostatic analysers on the time-of-flight of ions was studied theoretically, and it was shown that small energy spreads in the ion packet do not affect seriously the time-of-flight of an ion. The initial angle of an ion upon entering the electrostatic analyser does affect the time-of- flight of the ion. A C-shape configuration with two electrostatic analysers has been proposed as a solution eliminating time broadening due to spread in initial angle. An energy resolved time-of-flight mass (ER-TOF) spectrometer was constructed. Metastable fragmentation and collision-induced dissociation experiments were carried out with different molecular-mass proteins and 3- nitrobenzyl alcohol as matrix. It has been shown that in matrix-assisted laser desorption/ionisation (MALDI) there is a substantial number of ions that decay after acceleration and prior to detection. The energy resolved time-of- flight spectra show that there is a peak broadening resulting from the lack of stability of the high-mass ions during their flight through a time-of-flight instrument. The effect of the electric field above the target on the kinetic energy of the ions when 3-nitrobenzyl alcohol was used as matrix has been studied, and the energy spreads explained as being predominately energy deficits arising from the combination of high electrostatic fields, the liquid nature of the matrix and the inherent characteristics of the desorption / ionisation process. Collision experiments with different molecular-mass proteins ranging from 34% Da to 18300 Da and different collision gases over a wide range of kinetic energies showed that there is a decrease in the analyte signal with respect to the matrix signal for specific collision gases and collision energies. The ions were collected with an off-axis post acceleration detector. The decrease in the analyte signal has been explained as loss of the charge possessed by the analyte ion through charge exchange with the collision gas