Optical Detection and Charge-State Analysis of MALDI-Generated Particles with Molecular Masses Larger Than 5 MDa

Charged polystyrene nanoparticles are generated by matrix-assisted laser desorption/ionization (MALDI) and detected by laser-induced fluorescence (LIF) in a quadrupole ion trap. Employing the LIF technique, observations of individual fluorescent nanospheres (27 nm in diameter and containing 180 fluorescein dye equivalents) have been achieved with an average signal-to-noise ratio of ∼10. With the trap operating at a frequency around 5 kHz, charge state analysis of the particles reveals that the number of charges carried by the spheres is between 1 and 10. It suggests a mass-to-charge ratio (m/z) in the range of 105−106 for the MALDI-generated particles. To effectively trap such large particles (m > 5 MDa), damping of the particles' motions by using ∼50 mTorr He buffer gas is absolutely required. Similar findings are obtained for particles with a nominal size of 1 μm in diameter, demonstrating that production of charged particles with a molecular mass as high as 1012 Da is possible using the MALDI technique.