MoP-09
INFLUENCE OF SAMPLING CONE POTENTIAL ON ELECTROSPRAY IONISATION MASS SPECTRA: IMPLICATIONS FOR SYNTHETIC POLYMERS
Susan M. Hunt and Margaret M. Sheil
Department of Chemistry, University of Wollongong, NSW 2522
Peter J. Derrick and Mike Belov
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
Electrospray ionisation is increasingly being applied to the characterisation of synthetic polymers. Molecular weight distributions based on ESI mass spectra, however, rely on the observed spectrum being truly representative of all the components of the oligomeric mixture. It is known that the distribution of multiply-charged ions observed in ESI spectra of a single species such as a protein, is dependent on a number of factors including the voltage applied to the sampling cone in the ESI source. This has previously been attributed to focussing, collisional activation in the intermediate pressure region and charge stripping. We demonstrate here the influence of sampling cone potential on the observed distributions of ions with different mass-to-charge ratios (including singly-charged polymer ions of different mass and multiply-charged peptide and polymer ions of the same mass). The dependence on sampling cone potential is found to correlate primarily with the mass-to-charge ratio of the ions. These effects have been modelled mathematically and are shown to arise from a combination of two influences. Firstly, the ions undergo a supersonic jet expansion as it passes through the sampling cone into the lower pressure region. This is accompanied by the formation of a shock wave, known as the Mach disk, which cause considerable scattering of the ions. Ions with different masses will scattered to different regions of the Mach disk. Secondly, the ion mobilities after leaving the Mach disk are dependent on the charge of the ion. The implications of these findings for the molecular weight determinations of synthetic polymers will be discussed.