External ion accumulation of low molecular weight poly(ethylene glycol) by electrospray ionization fourier transform mass spectrometry

The combination of electrospray ionization (ESI) with Fourier transform mass spectrometry (FTMS) is a powerful tool in characterizing synthetic polymers. ESI permits the generation of intact, multiply charged high mass ions, whereas FTMS provides high mass resolution and correspondingly improved mass accuracy. However, under “nonideal” empirically determined operating conditions, a mass discrimination effect occurs as a function of accumulation time that can result in significant differences for calculated average mass values (M_n, M_w) and polydispersities. A multidimensional tuning process to eliminate the deleterious effects of mass bias is demonstrated for several sodiated poly(ethylene glycol) samples containing oligomers with masses covering a 600–3350 Da mass regime. In addition, experiments are performed in order to elucidate the possible mechanism(s) that cause the mass discrimination effect. It is proposed that extended collisions (reactive and nonreactive) occur in the hexapole to alter the energy (and velocity) distributions of the ions before injection into the trap. By choosing higher skimmer potentials, ions over a narrower energy “window” are preselected in the hexapole and exhibit lower overall mass bias effects.