Inclusion of a MALDI ion source in the ion chromatography technique: conformational information on polymer and biomolecular ions

Abstract

A matrix-assisted laser desorption ionization (MALDI) source has been coupled to the ion chromatography instrument developed at UCSB. The source produces a strong, consistent signal for several hours on a single sample. In this paper we report the application of this method to a series of poly(ethylene glycol) (PEG) polymers cationized by sodium. Data have been taken for Na +PEG5 to Na +PEG19. The temperature dependence of the ion mobility (collision cross-section) in He gas for Na +PEG9, Na +PEG13 and Na +PEG17 has been measured from 80 to 580 K. A detailed analysis of these three systems has been accomplished in order to extract the conformations of the ion and how they vary with temperature. This analysis included several significant changes from methods used previously. Molecular mechanics methods were used both to obtain the lowest energy 0 K structures and to predict how these structures would change as temperature increases. In order to account for the observed low temperature results, a 12-6-4 potential was incorporated in place of the hard-sphere potential used previously. For all three systems studied in detail, the oxygen atoms on the PEG units solvated the Na + ion, forming a crown ether type ring of five oxygens surrounding Na + and several others above and below this ring. The molecular mechanics model was also applied to neutral PEG13. In this instance a quite compact structure is obtained for T ≤ 200 K but a sudden melting type transition occurs between 200 and 300 K and chaotic motion dominates at and above 300 K. Data are also reported on the temperature dependence of the ion mobility of C 60 +. This ion is expected to change shape only slightly over the temperature range reported here. Consequently it provided an excellent set of calibration data for evaluating the intramolecular interaction potentials used to describe the collision process.