Bruker Daltonics Technical Note # TN-24 Extending the Capacity in Modern Spherical High Capacity Ion Traps (HCT) During the last decade, RF ion traps have become a workhorse of the analytical laboratory [1]. The ion capacity is a key factor affecting ion trap performance. Recently, substantial increases to the ion capacity have been achieved through the introduction of two novel instruments: linear ion traps [2] and high capacity spherical traps (HCT) [3]. The HCT uses phase-coupled excitation at a non-linear resonance [4] resulting from the optimized high-order fields (HOF) introduced by the new HCT geometry. The rationale behind the increased ion capacity of the linear trap seems to be well understood; however, the mechanisms for the increased ion capacity of the HCT still requires a more detailed explanation. This technical note will analyze and describe the mechanisms for increasing the ion capacity in a spherical high capacity trap. Background Before understanding the mechanisms for increasing ion capacity it is important to understand the major factor that influences ion capacity and what is meant by ion capacity. In addition to the trapping and excitation fields in the ion trap influencing ion motion, the electric field produced by the ions themselves has a significant impact on their motion. The interaction of ions with other ions as they are being excited and or ejected is one of the key factors that influence the ion capacity of the ion trap. There are three main different ion capacities to be defined: spectrum limit, isolation limit, and the storage limit [5]. The storage limit is perhaps the simplest to understand; this limit is the maximum number of ions that can be put into the ion trap. Any further ions are either pushed out of the ion trap by Columbic forces or cause already trapped ions to be pushed out. This limit has no qualifications on the spectra quality (e.g. the resolution of peaks). The next isolation and spectrum limits depend on the definition of spectral quality. The isolation limit is the number of ions that can be trapped and still allow a given isolation efficiency. For instance, the isolation limit might be defined as the number of ions that can be trapped and still isolate within a 1 Da window. The isolation limit is affected by ion-ion coupling which prevents resonant selection and thereby achieving the desired isolation width. Typically the isolation limit is roughly ten fold lower than storage limit. Finally the spectrum limit is the number of ions that can be trapped while still achieving the desired resolution, speed and mass accuracy. This limit is the strictest in that as ions are being ejected, the ion-ion interaction can retard the resonant ejection, cause ion coupling, and cause mass shifts. This limit can be fifty to a few hundred times smaller than the isolation limit. In mass spectrometry we are mainly interested in the final mass spectrum and thus the spectrum limit is the most important limit on the experiment. For the remainder of this note ion capacity will be referring to the spectrum limit.
Recent developments in linear ion traps yielded a higher storage limit because of the geometrically larger volume. If the number of ions is kept constant, as shown in Figure 1, their density is decreased, or vice versa, if the density is kept constant, the total number of ions in the trap can be increased. However, from the definitions of spectrum and storage limit it can be seen that if all we did was to increase the storage limit it does not necessarily lead to an increase in the spectrum limit. As discussed above the storage limit is a result of trapping and storage processes where as the spectrum limit occurs during the ejection process. To increase the capacity without principally altering the volume of the system, the ejection process must be changed to limit ion-ion interaction during ion ejection. This is achieved in the HCT through the judicious use of non-linear fields and proper phase correlation of the resonant excitation process. Spherical and linear ion trap designs Spherical Ion Trap Linear Ion Trap Fig. 1: Spherical and linear ion traps have been developed in recent years. Ion trap theory To make a document like this understandable we will cover some ion trap fundaments that may be useful as a refresher; however, so as to keep the document as short as possible these descriptions will not go into details. If these topics seem to be too short or more information is required it may be advantageous to first read a more detailed description of the conventional ion trap operation, some helpful references follow. Ions in the quadrupole ion trap are trapped and stored via the quadrupole field that is formed by the voltage applied to the ring electrode. The Bruker ion traps also have a unique design that gives distortions to this field that are advantageous to the performance of the ion trap. These field distortions are most commonly termed higher order fields and the Bruker ion traps specifically exploit hexapole and octapole fields. The voltage that is applied to the ring electrode is in the radio frequency region and is therefore referred to as the main rf voltage. The ion trap can trap ions of a given m/z range. Theoretically the lowest m/z that can be trapped is defined by the amplitude of the main rf voltage and increasing the rf voltage will result in ions below an increasing m/z to be ejected from the ion trap. In general for this technical note the ions have already been trapped and kinetically cooled by collisions with the background gas (He). Cooled ions are moving near the center of the ion trap. When cooled in this manner the ions are referred to as the ion cloud. These ions can interact with each other, because like charges repel (columbic interaction) each other, ions will spread out against the focusing forces by the main rf to avoid each other. Ions of a given m/z value that are trapped in the ion trap have a unique oscillatory motion that is a function of the ion trap dimensions, the voltage that is applied to the ring electrode, and the ion s m/z. Every ion of a different m/z has a different oscillation frequency (secular frequency), thus if an ac voltage with the same frequency is applied across the endcaps the ion with that frequency is excited. This process is quite similar to pushing a child on a swing or increasing the motion of an already moving pendulum and is referred to as resonance excitation. If the amplitude of the applied voltage is large enough ions can be ejected out of the ion trap, termed resonance ejection. Keeping the frequency of the excitation ac voltage constant while increasing the amplitude of the main rf will drive ions of increasing m/z into resonance with this excitation ac voltage, causing the ejection of these ions. A mass spectrum is created by detecting the amount of resonantly ejected ions as a function of the currently applied main rf amplitude. For Bruker ion traps, the frequency of the exciting ac voltage applied to the endcaps is chosen to be exactly 1/3 of the frequency of the main rf applied to the ring electrode. This additional ac voltage is at a frequency which is also in resonance with the hexapole higher order fields. Because ions can interact with each other the ejection process is influenced by the number of the ions in the ion trap. This technical note will focus on how to exploit the ejection process of the ion trap so as to allow more ions to be in the into trap during the ejection process.
Model An improvement of the ion capacity can be achieved during resonance ejection at a specific non-linear resonance. [5] A model for this is illustrated in Figure 2 as a sequence of events during the acquisition of a mass spectrum. For this improvement to occur, the ions of interest are first excited (t 1 ) away from the other ions into a higher orbit by the dipolar resonance excitation. As shown in Figure 2a and Figure 2b, the ions of red m/z are resonantly excited to a higher orbit away from the other ions in the ion trap. Because these ions are at a higher orbit they now have a lower density than before resonant excitation. As a result of this separation both spatially and by their velocities these ions have a significantly reduced interaction with the ions that are not resonantly excited in the center of the ion trap. As the ions orbit slowly continues to increase (t 1 + t d ) they are precisely ejected utilizing the higher order fields that are greater in strength near the end caps, Figure 2c. This process would then begin again for the green m/z ions and so on, Figure 2d. As can be seen the elongated ion cloud that would be created in this process is analogous to the elongation of the ion cloud achieved in the linear ion trap. And perhaps even more important: this larger volume is populated by ions of one m/z only, thus further decreasing the effect of ion-ion coupling. To achieve this process it is necessary to have two distinct resonant activation events. In this case the first excitation process is achieved with a simple dipolar resonance excitation and the second is achieved utilizing excitation at the non-linear resonances that are inherent to the Bruker Daltonik s novel spherical ion trap geometry, the HCT. Experimental These experiments will explore the exact timing of the ejection of ions from the ion trap. To experimentally demonstrate the enhancements achieved with the new geometry of the HCT over our previous generations, experiments were performed with electrode geometries of a Bruker esquire 3000 and Bruker HCTultra. These experiments were conducted in the same instrument with only changes to the electrode configuration between sets of experiments. The supplemental AC voltage applied during activation and ejection of ions is derived from the fundamental RF voltage for all experiments. The scan rate of 26,000 Th/sec was used for both electrode configurations. ESI Tuning Mix (Agilent; Palo Alto, CA), which has ions covering the mass range from m/z 100 3000, was used in these experiments. The exact timing of the ejection process was analyzed by varying the phase between the fundamental RF voltage and the resonance excitation voltage. The relative mass position, mass resolving power, and ion abundance were monitored, at each phase. Simulations were performed using SIMION 7.0 and custom calculation programs were written to understand excitation with dipolar fields coupled in the presence Non-linear resonance ejection improves ion capacity Fig. 2: Non-linear ion ejection is a two-step process: First (see 2a), ions are cooled and located in the trap center. Second (see 2b), during the scan, the RF is ramped and ions of a first mass (red) come into resonance with the AC. They are driven to a higher orbit, while the ions of higher mass (green, blue) remain cooled at the center. Third, the excited ions (red, see 2c) meet the non-linear resonance and are ejected instantly. The cooled ions in the center are unaffected by this. Then, the process repeats itself for higher masses (see 2d, green). The ion capacity is improved because right before the non-linear ejection the ions fill up a large volume (see 2b) which reduced space charge effects for these ions.
Non-linear resonance ejection at different phases alters ion ejection Ejection by non-linear resonance Ions excited by AC over entire time delay Ion motion (a) (b) (c) RF or m/z Fig.3: The process of Fig. 2 is explained on a time (=RF-amplitude, amu) axis. (a) corresponds to Fig.2 (a), (b) to Fig. 2 (b) and (c) to Fig. 2 (c). Ions are resonantly excited (red line) and ions motion increases. For a given phase correlation between the resonance ac and the main RF ions would be ejected quickly (blue line). At a different more optimal phase correlation between the resonance ac and the main RF ion ejection is delayed allowing ions to spend more time in the RF at higher orbit and a lower space charge conditions. of non-linear excitation fields. These data and simulations/ calculations provide insight into how the ion capacity can be influenced as a result of the ejection process, and also how it is improved with proper experimental design. Results and discussion An improvement of the ion capacity can be achieved during resonance ejection at a specific non-linear resonance. For this to occur, the ions of interest are first excited away from the other ions into a higher orbit by the dipolar resonance excitation. After the ions are excited to this higher orbit, they are later precisely ejected from the ion trap by the non-linear resonance. Importantly the non-linear resonance ejection occurs with high temporal precision. The decoupling of the initial excitation and ejection can be observed by altering the relative phase between the supplemental AC and the main RF. As shown in Figure 3, during the resonance ejection process ions experience a delay from the initial excitation (red line) process to the ejection of the ions. In conventional experiments found in the literature ions are ejected at higher amplitudes to minimize ion-ion interaction. In the experiments described here, as the relative phase is changed, a delay in the ejection process occurs from the normal delay (blue line) to the optimal delay (vertical green line). In the experiments this would be manifested as a shift in the entire mass spectrum; however, because the scan rate is known the relative ejection delay can be calculated. Simulations Simulations of the dipolar excitation process coupled with a non-linear resonance give an insight into what range ejection delay can be expected. Shown in Figure 4 are the results of dipolar excitation coupled with hexapole fields performed at two different phases. It is important to note that higher order fields are strongest near the endcaps of the ion trap and these simulations are predominately impacted by the dipolar fields until the ions are closer to the endcaps and higher order fields take effect. The plots show the arbitrary amplitude of the ions motion, where the amplitude of ~30 would result in ejection of the ions from the ion trap. At the displacement where higher order fields take effect and when phases of the dipolar excitation and hexapole fields are matching at this point in time (plot on the left) ions can be quickly excited by both the dipolar resonance as well as the non-linear resonance, thus ions are rapidly ejected from the ion trap (150μsec). When the relative phases at the same excitation level are appropriate (plot on the right) the ions are first quickly excited to an intermediate level. The amplitude of their motion is then retarded until their phase matches the phase of the higher order fields allowing for rapid excitation and ejection by the higher order fields. In the second simulation because the non-linear resonance and the dipole field are not in phase the two fields deconstructively interact. After the ions are rapidly excited by the dipolar field and when the ions approach a region where
Simulation of AC excitation and non-linear ejection process 30 (a) Ejection 30 (b) 20 20 Ion motion (arb. units) 10 0 Amplitude -10-20 10 0 Amplitude -10-20 -30 0 100 200 300 400 500 Time (μsec) -30 0 100 200 300 400 500 Time (μsec) Fig. 4: Simulations of the AC ion excitation and non-linear ejection process, for two different settings of the relative phase between AC and RF. In these simulations ions are ejected with their amplitude of their motion reaches a value of 30 arb units (noted by the green line). In (a), the AC excitation and the non-linear ejection are in phase, ion are rapidly ejected and no distinction between two excitation processes is possible. In (b), they are out of phase, and the non-linear ejection is delayed by several hundred microseconds (ejection delay). This gives the ions the time needed to fill a large volume before being ejected by the non-linear resonance. Only macromotion is shown. the non-linear resonance is stronger the ions are slowed or de-excited by the non-linear resonance. Then once the ions become in phase with non-linear resonance they are quickly excited out of the ion trap. Even though the ions are de-excited by the non-linear resonance they do not cool back down to the center of the trap because the dipolar excitation is still present during this time. These simulations agree with the model of two distinct and coupled excitation events. Ejection experiments Figure 5 shows the ejection delays as a function of phase observed for the m/z 1522 ion ejected from the HCTultra and the esquire 3000. For both instruments the fastest ejection (smallest delay) occurs at the same phase ~225. The maximum delay for both instruments was roughly 170 μsec. This corresponds to roughly 44 oscillation cycles, so this delay is not a result of slight damping or columbic interaction. This plot illustrates the need for strong phase control during the excitation and ejection process. Without it mass spectra could be acquired at random phases which would result in a variety of ejection delays (wider mass peaks). In these instruments, the excitation RF is electronically derived from the main RF in these instruments, which results in an excellent phase control. The real question that arises is which phase is a good phase to utilize? From a reliability standpoint, operating in a region which has minimal change in the ejection delay at a given change in phase is ideal (horizontal tangent). This is desirable because if the region is flat, then for a given fluctuation in the ion motion there will be minimal impact on the MS peaks shape and position. For the esquire 3000 there is only one option for operation, which is at the earliest ejection point (~225, Figure 5). Whereas for the HCTultra there are two options, one at a delayed ejection (~120, Figure 5) and one at the earliest ejection point (~225, Figure 5). Figure 6 and Figure 7 show how the other figures of merit, abundance and resolving power, change as a function of relative phase for the esquire 3000 and for the HCTultra respectively. From a phase of 50 to 140 the resolving power of the esquire 3000 (Figure 6) is increasing reaching a maximum around 125 ; however, neither the signal intensity nor the ejection delay are reaching steady plateaus in this phase region. For the esquire 3000 the stable and robust operation would be at about 240. It is important to note that the absolute performance of the esquire 3000 in these experiments is poorer than standard instruments because it is operating at the faster scan rate of 26,000 Th/sec (The scan rate for the HCT). The general trends do not change at the standard scan rates for this instrument (13,000 Th/sec) but the absolute resolving power and intensities become greater. In the HCTultra both signal intensity and resolving power reach a maximum at 120. Both of these parameters also reach a plateau for about 15 showing further robustness at this operating point. The resolving power at the operating points for the HCT and the esquire 3000 were
Ejection delay as a function of phase Ejection Delay (µsec) 180 160 140 120 μ sec) 100 80 60 Time Delay ( 40 20 HCT Esquire 3000 0 0 50 100 150 200 250 300 350 Phase (Degrees) between main RF and AC Fig. 5: Ejection delay as function of phase between AC and RF. Instruments are best operated at a point of zero derivative. For the esquire3000, one such point exists (at 225 ). For the high capacity ion trap (HCT), two points exist (at 120, 225 ). Operation at the additional point at 120 results in a large ejection delay and hence improved ion capacity. 3500 and 1600 respectively. This resolving power occurs at an ICC value that is five fold greater for the HCT than the esquire 3000. This result shows that this ejection delay means a higher capacity with improved resolving power. In this delayed ejection process, ions are initially excited to a higher orbit separating them from other ions in the trap. The excited ions still pass through the remaining ion cloud at the lower orbit but at a higher speed. Thus, the excited ions cannot couple with other ions and the final excitation is performed in a lower charge density environment, which is the key for achieving the higher capacity ion trap. Conclusions A model and experimental evidence for achieving a higher capacity ion trap have been presented. This method involves decreasing the ion density of the ions being excited before ejecting them from the ion trap. For this to occur, the ions of interest are first excited away from the other ions into a higher orbit by the dipolar resonance excitation. After the ions are excited to this higher orbit, they are later precisely in-time ejected from the ion trap by the non-linear resonance. Importantly this ejection occurs with great temporal precision, which leads to rapid ejection with high resolving power at a high capacity. Simulations and calculations were in good agreement with this model. Experimentally the ejection timing can be controlled by setting the relative phase of dipolar excitation voltage and the main RF voltage. The ejection delay is advantageous because ions spend longer in the excitation field resulting in better resolving power. For the HCT there is an optimal phase and ejection delay where both the resolving power and ion abundance are at a maximum. This maximum has a plateau, which makes for stable robust operation. In these experiments, under the optimal operating conditions, the HCT showed a greater than ten fold increase in ion capacity over an esquire 3000. These results show that a precise design of the ion trap geometry together with the appropriate ejection methods lead to a robust high capacity ion trap that also has improvements in speed, resolving power, and mass accuracy.
Large ejection delay results in higher capacity, improved resolving power, and higher ion abundance esquire 3000 HCT 1.2 Operating point 1.2 Operating point 1.0 1.0 0.8 0.8 Normalized Value 0.6 0.4 Normalized Value 0.2 0.0 0 50 100 150 200 250 300 350 Ph as e (Deg rees) 0.6 0.4 0.2 0.0 0 50 100 150 200 250 300 350 Ph ase (Deg rees) Resolution (NL 3.5x10 3 ) Ejection Delay (NL170 µs) Signal Intensity (NL 4.5x10 7 ) Resolution (NL 3.5x10 3 ) Ejection Delay (NL170 µs) Signal Intensity (NL 4.5x10 7 ) Fig. 6: Ejection delay, mass resolution and ion abundance as function of the phase between AC and RF, for esquire3000 (operated at 26,000 Th/s) and high capacity trap (HCT). For HCT, resolution, delay (correlated to the ion capacity) and ion abundance all maximize at the same operating point (120 ), resulting in highest scan speeds, highest ion capacity and highest sensitivty all at the same time (NL: Normalization Level). References [1] March, R. E. and Todd, J. F. J., A historical Review of the Devlopment of the Quadrupole Ion Trap, in Quadrupole Ion Trap Mass Spectrometry, 2 nd ed., John Wiley & Sons, Inc., Hoboken, NJ, 2005, Chapter 1 [2] March, R. E. and Todd, J. F. J., Linear Quadrupole Ion Trap Mass Spectrometer, in Quadrupole Ion Trap Mass Spectrometry, 2 nd ed., John Wiley & Sons, Inc., Hoboken, NJ, 2005, Chapter 5 [3] C. Baessmann, A. Brekenfeld, G. Zurek, U. Schweiger-Hufnagel, M. Lubek, T. Ledertheil, R. Hartmer, M. Schubert, Proceedings of the 51 st American Society of Mass Spectrometry and Allied Topics, Montreal, PQ, June 8-12 2003 [4] March, R. E. and Todd, J. F. J., Recent Applications of ESI combined with QIT, in Quadrupole Ion Trap Mass Spectrometry, 2 nd ed., John Wiley & Sons, Inc., Hoboken, NJ, 2005, Chapter 8 [5] March, R. E. and Todd, J. F. J., Dynamics of Ion Trapping, in Quadrupole Ion Trap Mass Spectrometry, 2 nd ed., John Wiley & Sons, Inc., Hoboken, NJ, 2005, Chapter 3 Authors Desmond Kaplan, Andreas Brekenfeld, Ralf Hartmer, Thorsten Ledertheil, Christoph Gebhardt, Michael Schubert Bruker Daltonics Keywords Ion Capacity Ion Density Higher Order Fields Quadrupole Ion Trap Spherical Ion Trap Instrumentation & Software HCTultra esquire 3000
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