Probing mass discriminations and mass shifts in the ITMS mass spectra of externally generated MALDI ions with synthetic polymers

Synthetic polymers are demonstrated to be very useful probes for the characterization of a newly constructed external ion source matrix-assisted laser desorption and ionization quadrupole ion trap mass spectrometer (MALDI-ITMS). Mass discrimination effects and space-charge-induced mass shifts can readily be identified and quantified with these synthetic polymer probes. Mass dependencies in the trapping efficiency were evaluated by comparison of measurements of poly(methyl methacrylate) (PMMA) standards in the mass range m/z 400^_3400 with MALDI-TOF-MS measurements of the same samples. This established the optimal experimental conditions for the analysis of mass ranges smaller than 1000 u with negligible mass discrimination. This is demonstrated on the basis of measurements on a tri-block copolymer of poly(ethylene oxide) poly(propylene oxide). As expected, mass measurements were highly influenced by the magnitude of the trapped ion population. Using poly(ethylene glycol) standards we were able to quantify these mass shifts as a function of the total ion load. Consequently, reproducible results can only be obtained by reproducing the total space-charge in the trap. In MALDI experiments, this can be achieved by tuning the laser power just above threshold. This approach allowed the mass determination of three different end groups in a complex Jeffamine D2000 sample with an accuracy of better than 0.1 u.