Chemical mass shifts in resonance ejection experiments in quadrupole ion traps and ESI-FT-orbitrap mass spectrometry

Hongyan Li, Purdue University


Chemical mass shifts are measured in quadrupole ion traps operated in the mass-selective instability scan with resonance ejection. Chemical mass shift is the result of compound-dependent collisional modification of the ejection delay produced by field faults near the endcap electrode apertures. Both dissociative and non-dissociative collisions can occur but the dissociative collisions make the predominant contribution to the chemical mass shifts. Non-linear resonances due to higher order fields lead to dramatic decrease in chemical mass shifts at certain βz values. The large chemical mass shifts associated with the nitro-aromatic compounds could serve as a diagnostic for identifying this class of compounds in mixtures and a simple method of implementing this test is described based on the effect of non-linear resonance on the chemical mass shifts. Uses of chemical mass shift for high-resolution distinction of isobars and isomers are demonstrated by using the nonlinear RF scan functions in chemical mass shift experiments. By careful choice of resonance conditions, ion ejection during the prolonged ejection delay is effected exclusively by dissociative collisions while ions undergo non-dissociative collisions remain in the ion trap which can be ejected by the second RF ramp. The orbitrap mass analyzer, a new version of the Kingdon trap, employs an electrostatic quadro-logarithmic field for ion trapping. A stable ion trajectory in the orbitrap field is a combination of rotation around the central electrode and harmonic oscillation along its axis. The frequency of the axial harmonic oscillation provides the basis for mass analysis that is measured by image current detection followed by the Fourier transformation. An RF front-end quadrupole system successfully couples the orbitrap mass analyzer with a continuous electrospray ionization source. This instrument provides mass resolution up to 200,000 with mass accuracy of several ppm. Applications in the study of serine clustering are reported.




Cooks, Purdue University.

Subject Area

Analytical chemistry

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