1. Simulation of Ion Isolation in RF Ion Traps Using Notched Broad Band Excitation
MoP-067
Michael Sudakov 1, R.Giles1 and Dimitris Papanastasiou2
1Shimadzu Research Laboratory (Europe) Ltd, Manchester, UK; 2KRATOS Analytical, Manchester, UK
Overview.
In this paper we report simulation of precursor ion isolation using waveforms in ion traps of different kind. Simulation was performed with the use of AXSIM software recently developed in SRL. The software uses potential arrays obtained from SIMION and allows simulations of ion traps in full complexity with account of multiple time dependant signals and collisions with buffer gas particles.
Simulation allows investigation of effect of all parameters that influence resolution and accuracy of precursor ion isolation such as amplitude of excitation waveform, small nonlinear distortions of the ion trap, magnitude of trapping potential and spectral purity of the excitation waveform. Visualising of particle motion both in real space and in a phase space helps to understanding the process of isolation and to optimise it.
Introduction
Resonance excitation of ion vibrations is widely used in ion traps to promote collisional induced dissociation (CID), eject ions to a detector during a mass scan, or remove unwanted ions from a trapping volume. In the latter case a broadband excitation waveform is usually applied to the end-caps in order to simultaneously excite and eject unwanted ions while selecting and storing the mass range of interest. Several types of waveforms were developed for this purpose. The most commonly used are: filtered noise excitation (FNF) [1] and Stored Waveforms produced with the use of Inverse Fourier Transform (SWIFT) [2]. Although precursor ion isolation using such waveforms is commonly used in ion trap MS/MS experiments for over 20 years, simulation studies have never been presented. Here we present a study of precursor ion isolation in a 3D ion trap and in a Linear Ion Trap (LIT) with the use of different kinds of notched waveforms.
Methods
Simulation of precursor ion isolation was performed with using of AXSIM software recently developed in SRL. Fields for simulations were obtained from SIMION using geometry of ion traps under investigation with accuracy of 50mm. FNF waveforms were obtained from AXIMA-IT-TOF instrument manufactured by KRATOS. SWIFT and new waveforms were generated by home-made application and loaded into simulation from external file. AXSIM features advanced Monte-Carlo modeling of ion collisions with neutral particles based on a hard-sphere approximation thus allowing simulation of ion motion accurately and in full complexity. For simulation of precursor ion isolation 3 types of waveforms were used : 1) Filtered Noise waveform ( FNF fig.1A) which is constructed from sinusoidal components and optimized to have as small overall dynamic range as possible [1]. 2) Stored Waveform from Inverse Fourier Transform (SWIFT, fig.1B), which has uniform power spectra in a frequency domain [2]; and an isolation waveform of new design (fig.1.C). All of them have almost uniform intensity of power spectrum from 1kHz to 250kHz with a notch from 70kHz to 71 kHz (fig.2). SWIFT and new waveform are 4ms long, while FNF lasts for 2ms.
Comparison of isolation using different waveforms
Isolation in a LIT with the use of a new waveform
Evaluation and comparison of waveforms was performed on a 3D ion trap of standard size ro=10mm (non stretched) operating with sinusoidal RF at frequency 500kHz. Helium at pressure 0.5mTorr was used as a collision gas. Isolation near mass 1000Da was investigated by repeating simulation for ions of different mass (100 ions at a time) and counting ejected ions. It was found that a single play of FNF does not provide complete ejection in a wide mass range. Advantage of FNF is that it excites all frequencies at the same time and as a result can be applied several times.
The dependence of isolation window from several parameters was investigated. With increasing excitation voltage the window shrinks and distort (fig.6A). Optimum excitation voltage is near 4V. It was found that with higher axial DC trapping voltages the window shifts towards smaller masses (fig.6B) indicating that radial secular frequencies of all ions are reduced proportionally to Z trapping voltage. Isolation at higher q values (fig.6C) produces sharpest boundaries of the window.
Figure 6. Dependence of isolation window from trapping and excitation parameters: A. B. C.
Figure 3. Model of a 3D ion trap used in present investigation.
RF supply ~
Waveform
With 4 times play FNF provides good isolation at amplitudes above 10V (Fig 4.A). SWIFT (fig4.B) and new waveform (fig.4C) provide sufficient isolation in a single play at amplitudes 6V due to a scanning character of these waveforms which excites ions sequentially. New waveform provides smallest excitation voltage as well as cleaner isolation window.
Fig.7 Phase space of ion population during isolation
A. 6.6ms
B. 6.7ms
C. 6.8ms
Fig.8 Result of isolation in two steps
A. Original population
B. After First Isolation at q=0.652
C. Second isolation (RF shifted 150Hz)
0.4Da
R=4.000
Figure 4. Isolation profile of different waveforms at several excitation voltages
A. FNF #4 times (8ms) B. SWIFT 4ms C. New waveform 4ms
Investigation of the phase space of ion population during isolation process reveals physics of isolation process. Just before ejection the ion cloud occupies a loop-shaped region with ions of selected mass range closer to the centre (fig.7A). Unwanted ions are displaced substantially further away from the centre and will be ejected to the rods (fig.7.B and C).
Figure 1. Waveforms used in present investigation (arb. units).
A. FNF.
B. SWIFT.
C. New waveform.
Figure 2. Frequency domain of waveforms used in present investigation: uniform spectral density from 1kHz to 250kHz
Simulations with new waveforms was further performed on a model of a linear ion trap (fig.5). This LIT is based on a segmented quadrupole of inscribed radius 5mm and length of the central segment 30mm. Ions are trapped along the axis of a trap by a negative DC potential (for positively charged particles) at the central section of the trap. Radial trapping is achieved with a Square Wave trapping waveform [3] of 500Vo-p. Frequency of the trapping waveform was adjusted according with desired q value for a mass range near 1560Da.
High resolution isolation can be accomplished in two steps. During the first step a comparatively wide mass range (2Da) is isolated using first waveform. Fig.8. shows initial ion population used for this simulation and result of first waveform application at q=0.652 (RF frequency 438kHz). Waveform of type fig.1C was sampled 4 times every RF cycle an lasted over 9ms in total. Remaining ions (fig 8B) are collisionally cooled with gas for 10ms. After this the RF frequency was increased by 150Hz in order to bring remaining ions at lower q value and isolation waveform was repeated. Resulting ion population (fig.8C) shows isolation resolution of nearly 4000.
Conclusion
Simulation of ion isolation in ion traps using notched waveforms is presented for the first time. 3 waveforms of different type were compared. SWIFT and FNF waveforms show comparable performance in terms of isolation accuracy. FNF requires 30% higher amplitude as compared to SWIFT, but it can be repeated many times. A broadband waveform of new type is desighned an evaluated. Simulation of isolation in a linear ion trap with a square waveform trapping (digital drive) with the use of a new broadband waveform excitation shows that isolation resolution of 4000 is feacible.
References.
[1] D.Hoekman, P.Kelley, US patent 5,703,358
[2] A.Marshal, T. Ricca, T-C. Wang, US patent 4,761,545
[3] Li Ding, M.Sudakov, S.Kumashiro, Int. J. of Mass Spectrometry, v.221, p. 177
Notch
Figure 5. Model of a linear ion trap for simulation of ion isolation.
Axial trapping
-100V ~
Waveform
Excitation
Radial trapping
Square Wave