MASS DISCRIMINATION IN LASER DESORPTION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY CATION-ATTACHMENT SPECTRA OF POLYMERS

A mass discrimination effect is demonstrated in laser desorption/ionization (LDI)/Fourier transform ion cyclotron resonance mass spectrometry (FTICR) data for low molecular weight poly(ethylene glycol) (PEG) samples mixed with potassium halide salts. This effect is attributed to factors that influence the relative overlap of gas-phase neutral and ion populations that react to form the cation-attached products detected by FTICR. Specifically, these factors include laser power density and reactant masses which determine velocity distributions, trap potential which serves as an energy filter for detected product ions, and distance between trapped ion cell and desorption site. For example, the number averaged molecular weights, M(n), for PEG-600, PEG-1000, and PEG-1500 samples vary by 7, 10, and 12%, respectively, as the desorption site is displaced over a 10-cm distance from the cell. The polydispersity value for a standard PEG-1000 sample, as determined by vapor-phase osmometry, is achieved at probe distances of 5.4, 7.2, and 8.2 cm, respectively, when different salts, KF, KBr, and KI, are mixed with the polymer. All data are consistent with an ionization mechanism in which a distribution of desorbed low-energy polymer neutrals reacts in the cell with the low-energy tail of a salt adduct population. The well-defined polymer distribution is demonstrated to provide effective time resolution in defining the gas-phase reactions that occur in the FTICR experiment but are not otherwise discernible on the time scale of the detection process.