DYNAMIC ION TRAPPING FOR FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY - SIMULTANEOUS POSITIVE-ION AND NEGATIVE-ION DETECTION

The conventional static electric potential used to trap ions for Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry has been replaced by a low-amplitude [as low as V(ac) almost-equal-to 0.5 V for N2+] alternating (17.5 kHz) electric field applied to the end caps of an ICR ion trap. Ion z-motion is then governed by a Mathieu equation, whose solution leads to a z-stability diagram for which optimal results are obtained at z-stability parameter, q(z) = 4q-gamma-V(ac)/(m-OMEGA(2)) almost-equal-to 0.5, in which m/q is the ion mass-to-charge ratio, OMEGA is the RF frequency, and gamma = 2.7737/d2 for a cubic trap of edge length, d. A triangular waveform appears to be more effective than sinusoidal modulation. We demonstrate experimentally three major additional advantages of RF trapping for FTICR mass spectrometry: (a) both positive and negative ions may be trapped and detected simultaneously; (b) magnetron motion is eliminated, along with the electrostatic radial field-induced ICR frequency shift and sidebands; and (c) mass calibration follows a simpler law (m = a/upsilon, in which a is a constant and upsilon is the measured ICR orbital frequency) than for electrostatic trapping (m = a/upsilon + b/upsilon(2), in which a and b are constants). All prior FTICR mass spectrometric capabilities are preserved, except that (as in an RF-only quadrupole ion trap) optimal sensitivity is observed only for 0.4 less-than-or-equal-to q(z) less-than-or-equal-to 0.7, so that the mass-to-charge ratio range for a single FTICR mass spectrum is limited accordingly.