Circularly polarized quadrature excitation for Fourier-transform ion cyclotron resonance mass spectrometry

A quadrature method for producing circularly polarized excitation for selective detection of positive or negative ions has been proposed for Fourier-transform ion cyclotron resonance mass spectrometry. The new technique achieves the same post-excitation ion cyclotron orbital radius at half the excitation rf voltage of conventional linearly polarized (dipolar) excitation, and is especially useful for exciting and detecting dynamically trapped ions of one charge state in a mixture of simultaneously trapped positive and negative ions. Circularly polarized single-frequency resonant excitation is accomplished experimentally by simultaneous application ofa sinusoidal rf voltage to one pair of electrodes of a cubic ICR ion trap and a 90° or 270° phase-shifted voltage of the same frequency to a second orthogonal pair of electrodes, followed by conventional dipolar detection. Selective excitation of 35C1+ or 35CI− ions from a dynamically trapped mixture of 35C1+ and 35C1− ions is demonstrated. Circularly polarized broadb excitation by use of frequency-sweep or stored-waveform (SWIFT) excitation is proposed and discussed.