A tuning method for electrically compensated ion cyclotron resonance mass spectrometer traps

Abstract

We describe a method for tuning electrically compensated ion cyclotron resonance (ICR) traps by tracking the observed cyclotron frequency of an ion cloud at different oscillation mode amplitudes. Although we have used this method to tune the compensation voltages of a custom-built electrically compensated trap, the approach is applicable to other designs that incorporate electrical compensation. To evaluate the effectiveness of tuning, we examined the frequency shift as a function of cyclotron orbit size at different z-mode oscillation amplitudes. The cyclotron frequencies varied initially by ϳ12 ppm for ions with low z-mode oscillation amplitudes compared with those with high z-mode amplitudes. This frequency difference decreased to ϳ1 ppm by one iteration of trap tuning. (J Am Soc Mass Spectrom 2010, 21, 451-454) © 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry F ourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers ultra high mass resolving powers and accurate mass measurements that surpass those of other methods. The trap serves as the heart of the instrument, allowing ions to be stored, fragmented, and analyzed. Ion motion in traps is confined in the radial direction by a strong homogeneous magnetic field, and in the axial direction by a three-dimensional quadrupolar electrostatic trapping well. As a consequence of the finite dimensions of the trapping-plate electrodes, the electrostatic trapping potential within the trap is only approximately quadrupolar. The resulting outwardly directed radial electric field (E r ) opposes the effect of the magnetic field and causes a shift in the observed cyclotron frequency of the ions and a procession of the ion clouds' guiding center (magnetron motion).A problem arises because the effect of the E r field is not constant, causing the observed cyclotron frequency to depend on ion-mode amplitudes within the trap. Among the efforts to design a trap in which the ion cyclotron frequency is independent of its mode amplitudes are those that attempt to eliminate the radial electric field by creating an electrostatic well shape that approximates a particle-in-a-box [1-9] (E r ϭ 0). Other strategies seek a design that more closely approximates the desired three-dimensional quadrupolar trapping potential [10 -15] (E r /r ϭ constant). The outcomes are traps with complex electrode shapes that follow the equipotential lines of a perfect, three-dimensional quadrupolar potential (i.e., a hyperbolic trap) and others with electrodes segmented into many parts to which individual compensation voltages are applied to shape the trapping potential.Our solution to this problem is an electrically compensated trap [10] whose implementation requires that correct voltages be applied to the electrodes to reduce the sensitivity of cyclotron frequency as a function of mode amplitudes (i.e., d /dA z,r ¡ 0). Despite our best efforts, the compensation voltages calculated by theory do not afford optimum performance, although we did find an imp...

Trapping Radial Electric Field Optimization in Compensated FTICR Cells

“…The approach also minimizes frequency shifts due to ion-ion interactions, by enabling the use of increased excitation radii [7]. The goal can be achieved by creating a potential distribution close to the quadrupolar potential [3,4,20]:…”

“…The radial electric field distribution of the OC-7S cell was also compared with that of the ring [3,4] and leaf [5] cells, as described below.…”

“…Recent compensated cell publications initiated the discussion whether theoretically obtained potentials should be applied directly or optimum values must be experimentally determined [1][2][3][4]. Figure 1 demonstrates that using the theoretically calculated compensation voltages does in fact deliver the desired outcome in terms of the ion frequency profiles, suggesting that the use of the theoretical compensated potentials is at least a valid starting point.…”

“…These designs have been based on the previous cell work [1,2], prior compensated cell designs [3,4], or relatively new designs in electrode geometry [5,6]. The compensated FTICR cell provided improved homogeneity of the radial electric field and allowed operation at excitation radii increased up to 70% of the cell radius [7,8].…”

Towards analytically useful two-dimensional Fourier transform ion cyclotron resonance mass spectrometry

“…This ICR cell was used, among others, for the analysis of bovine serum albumin mentioned earlier. The compensated ICR cell introduced by Gabrielse et al [19] for high-accuracy isotope mass measurement has been extended to analytical FT-ICR by Tolmachev et al [20] and Brustkern et al [21], who showed that it eliminates harmonics in the signal [16]. An asymmetric variation on this geometry in which the excitation and detection plates do not have the same geometry was developed by Kaiser et al [22] for complex mixtures.…”