Initial Experimental Characterization of a New Ultra-High Resolution FTICR Cell with Dynamic Harmonization

B American Society for Mass Spectrometry, 2011 J. Am. Soc. Mass Spectrom. (2011) 22:1125Y1133 DOI: 10.1007/s13361-011-0125-9 RESEARCH ARTICLE Initial Experimental Characterization of a New Ultra-High Resolution FTICR Cell with Dynamic Harmonization Eugene N. Nikolaev,1,2,3 Ivan A. Boldin,1,2 Roland Jertz,4 Gökhan Baykut4 1 The Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences, Leninskij pr. 38, k.2, Moscow, Russia119334 2 Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia 3 The Institute of Biomedical Chemistry, Russian Academy of Medical Sciences, Moscow, Russia 4 Bruker Daltonik GmbH, Bremen, Germany Abstract A new Fourier transform ion cyclotron resonance (FTICR) cell based on completely new principles of formation of the effective electric potential distribution in Penning type traps, Boldin and Nikolaev (Proceedings of the 58th ASMS Conference, 2010), Boldin and Nikolaev (Rapid Commun Mass Spectrom 25:122–126, 2011) is constructed and tested experimentally. Its operation is based on the concept of electric potential space-averaging via charged particle cyclotron motion. Such an averaging process permits an effective electric force distribution in the entire volume of a cylindrical Penning trap to be equal to its distribution in the field created by hyperbolic electrodes in an ideal Penning trap. The excitation and detection electrodes of this new cell are shaped for generating a quadratic dependence on axial coordinates of an averaged (along cyclotron motion orbit) electric potential at any radius of the cyclotron motion. These electrodes together with the trapping segments form a cylindrical surface like in a conventional cylindrical cell. In excitation mode this cell being elongated behaves almost like an open cylindrical cell of the same length. It is more effective in ion motion harmonization at larger cyclotron radii than a Gabrielse et al.-type (Int J Mass Spectrom Ion Processes 88:319–332, 1989) cylindrical cell with four compensation sections. A mass resolving power of more than twenty millions of reserpine (m/z 609) and more than one million of highly charged BSA molecular ions (m/z 1357) has been obtained in a 7T magnetic field. Key words: FT ICR MS, Penning trap, Dynamic harmonization, Ultra-high mass resolution Introduction I n order to let an ion cloud generate a long time domain signal in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer and, hence, to increase its resolving power it is necessary not only to have an ultra high vacuum but also to keep the cyclotron frequency and phase the same for all ions of the same mass-to-charge ratio during the signal detection time. In general, even in a perfect homogenous magnetic field, this is not the case due to the Correspondence to: Eugene N. Nikolaev; e-mail: necessity of an electric field, which is required for trapping the ions in axial direction. In conventional ICR cells, such as cubic, cylindrical, or “open cells” ions of different zoscillation (trapping-oscillation) amplitude have slightly different so-called “effective” cyclotron frequencies. Generally speaking, the effective cyclotron frequency is equal to the cyclotron frequency (the frequency of rotation in magnetic field in absence of any electric field) minus the drift motion frequency (the frequency of motion perpendicular to both magnetic and electric fields), which converges into magnetron motion for hyperbolic geometry electric field distribution. The effective cyclotron frequency is lower for Received: 2 December 2010 Revised: 3 March 2011 Accepted: 8 March 2011 Published online: 19 April 2011 1126 E. N. Nikolaev, et al.: Ultra-High Resolution Harmonized FTICR Cell higher z-oscillation amplitude because the drift frequency is linearly proportional to the electric potential gradient perpendicular to magnetic field. This gradient increases with increasing z-amplitude in cubic and cylindrical cells. It results in gradual dephasing of an ion packet (due to high z-amplitude ions rotating with smaller circular velocities) and, consequently, in signal decay. This effect limits the signal duration, causes a frequency drift, and, hence, reduces the resolving power of an FTICR mass spectrometer. Signal duration increases in higher magnetic fields because the drift motion frequency is inversely proportional to magnetic field. Signal duration does also significantly increase when the number of ions in a cell is sufficient for phase locking [4]. The effect of phase locking may prolong the signal detection time but, in turn, it decreases the resolving power because ions of close mass-to-charge ratios get phase locked, i.e., not resolved in FTICR spectrum. There are two approaches to reduce electric field influences. The first one is to reduce a radial electric field in the region of ion detection [5–8]. The second one is to create an electric field configuration that leaves the cyclotron frequency of ions independent of their axial motion—that is a hyperbolic field:  1
2 g 2z  r2 ; ð1Þ 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi where Φ is the electric potential, r ¼ x2 þ y2 , γ is a coefficient proportional to the trapping voltage. The z-axis is directed along the magnetic field vector. For this potential distribution, assuming the magnetic field to be uniform, the equations of motion can be solved exactly [9]. In this field, the variables get separated and the motion of an ion consists of three independent modes:
ðr; zÞ ¼ 1) Oscillation along z-axis at ωz frequency that is defined by wz ¼ rffiffiffiffiffiffiffiffi 2qg ; m ð2Þ 2) Rotation in xy-plane at ω+ frequency, i.e., cyclotron motion, wc þ wþ ¼ 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  w 2 w 2 c z ;  2 2 ð3Þ 3) Rotation in xy-plane at ω− frequency, i.e., magnetron motion, wc w ¼  2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  w 2 w 2 c z ;  2 2 ð4Þ Where ωc = qB/m, q and m are particle’s charge and mass, B is magnetic field strength [9]. One can see that the ω+ frequency (effective cyclotron frequency for this case as we called it earlier), which is detected during a FTICR experiment, stays the same for all ions of the same mass-to-charge ratio that are present in an ICR cell, being independent on ions’ initial positions and velocities. A nearly ideal hyperbolic field is provided by a 3D hyperbolic trap (see for instance [10, 11]), but it has the major disadvantage of not effective usage of room temperature bore space of high magnetic field homogeneity, i.e., the region where ions can circle is much smaller than the magnet bore. Because of space charge effects such cells will have limited charge capacity, hence limited dynamic range. Another approach to create an ICR cell with trapping field that is close to hyperbolic (also called “harmonization” of an ICR cell [1]) is the segmentation of electrodes. Gabrielse et al. implemented this approach by inserting compensation rings into an open cylindrical cell [3] between the center section and the trapp (...truncated)