Performance of a Halo Ion Trap Mass Analyzer with Exit Slits for Axial Ejection
The halo ion trap (IT) was modified to allow for axial ion ejection through slits machined in the ceramic electrode plates rather than ejecting ions radially to a center hole in the plates. This was done to preserve a more uniform electric field for ion analysis. An in-depth evaluation of the higher-order electric field components in the trap was also performed to improve resolution. The linear, cubic and quintic (5th order) electric field components for each electrode ring inside the IT were calculated using SIMION (SIMION version 8, Scientific Instrument Services, Ringoes, NJ, USA) simulations. The preferred electric fields with higher-order components were implemented experimentally by first calculating the potential on each electrode ring of the halo IT and then soldering appropriate capacitors between rings without changing the original trapping plate design. The performance of the halo IT was evaluated for 1% to 7% cubic electric field (A4/A2) component. A best resolution of 280 (m/Δm) for the 51-Da fragment ion of benzene was observed with 5% cubic electric field component. Confirming results were obtained using toluene, dichloromethane, and heptane as test analytes.
Key wordsIon trap Toroidal Higher-order field Microfabrication Miniaturized mass spectrometry
With its inherent high sensitivity, compact analyzer size, and high specificity, ion trap (IT) mass spectrometry (MS)  has been widely accepted for many applications, including cosmic exploration [2, 3], threat and forensic detection [4, 5], identification of chemical and biochemical compounds in pure form or in complex mixtures [6, 7], and environmental monitoring [8, 9]. Other major advantages that enhance the attractiveness of ITMS for field portable applications  include simple construction, relatively high pressure operation compared with other mass analyzers, low power consumption, and the ability to perform multi-stage tandem MS in a single analyzer.
The conventional quadrupole IT was originated by Paul and Steinwedel in 1953 . A hyperbolic ring electrode between two hyperboloidal endcap electrodes produces a time-dependent quadrupole electric field when a megahertz radiofrequency (rf) voltage is supplied to the ring electrode. Current commercial IT mass spectrometers typically have a trap radius (r0) of approximately 1 cm and are operated with an rf trapping voltage on the order of 15 kVp-p. For portable analysis by ITs, it is desirable both (1) to reduce electrical power by operating the trap using a lower voltage, and (2) to reduce the trap dimensions (z0 and r0). At the same time, the rf frequency must be increased to maintain an adequate pseudopotential well depth. However, smaller IT mass analyzers are more subject to space charge performance degradation . One of the solutions to this problem is to increase the ion storage capacity of the trap without increasing its characteristic trapping dimensions (z0 and r0). Recently, several groups [13, 14, 15, 16, 17, 18] have investigated various IT geometry designs to maintain ion capacity while miniaturizing the analyzer. The toroidal IT [19, 20], reported by Lammert et al., is comprised of four hyperboloidal electrodes: two endcaps, an inner ring, and an outer ring, which form a toroidal trapping geometry; rf trapping voltage supplied to the two ring electrodes forces ions to focus in a circular band rather than at a point as in a 3-D quadrupole IT, thus maintaining the ion storage volume while significantly decreasing the characteristic trapping dimensions. This miniature device, with a 2.5-mm trapping field radius, has approximately the same ion storage volume as a commercial IT mass analyzer with a 1 cm radial dimension, but operates at rf voltages around 1 kVp-p instead of the 15 kVp-p typical of commercial quadrupole ITs. Unit mass resolution was obtained for toluene, dibromochloromethane, and diethylphthalate, which is comparable to resolution values observed from most bench-top GC-MS systems . However, improvement in the performance of the toroidal IT is limited to a large degree by the machining tolerances , which are specified to 0.0005 in. (0.013 mm) for the electrodes and which are at or near the limits of current machining capabilities.
The halo IT mass analyzer , which has a circular trapping geometry analogous in shape and size to the toroidal IT, was recently reported by our group. It consists of two patterned parallel ceramic plates made by microlithography. The toroidal trapping volume is located between the two plates, which is produced not by hyperboloidal electrodes, but rather by a number of concentric electrode rings fabricated on the two facing planar surfaces. By the same process, we have also created a conventional Paul ion trapping field between two ceramic plates, which demonstrates better resolution than the halo IT . While the halo IT possesses a similar trapping volume as the toroidal IT, its open structure provides much greater access for electron ionization. The use of microlithographic methods to fabricate the traps resolves the issue of close machining tolerances, and all ITs fabricated in this manner can be further miniaturized without being limited by machining tolerances. By adjusting the array of the electrode rings (i.e., number of rings and spacing between rings) and the voltages applied to the rings, the radius of curvature of the toroidal trapping field can be changed to modify the shape and size of the ion storage volume.
In the toroidal IT , trapped ions are ejected through three equally spaced 93° arc slits in the endcap electrodes rather than through the inner electrode. The slits in one endcap electrode serve as the entrance for energetic electrons from the filament, and the slits in the other endcap electrode provide an exit passway for resonantly ejected ions in the axial direction (i.e., in the z direction) to the detector. In an attempt to improve the resolution of the halo IT, we simulated the resonant ejection of ions in the axial (z) direction instead of in the radial direction and found that axial ejection should improve resolution and ejection efficiency. Therefore, in this study, three equally spaced 92° arc slits for ion ejection were machined in each of the trapping plates (Figure 1b) at the same radius as in the toroidal IT.
It is well known that the mass resolution in ITMS is affected by the presence of multipole fields. A number of investigations of multipole field components in trapping fields of conventional quadrupole [1, 23], cylindrical , and rectilinear ITs  have been made to understand the behavior of trapped ions and to optimize the performance of the ITs. Certain amounts and types of higher-order multipole fields can increase the speed with which ions accumulate energy during dipole ejection to improve the ITMS resolution. To date, no detailed studies have been made of the effects of higher-order fields on the performance of toroidal trapping fields. In fact, the curvature of the toroidal trapping area contributes additional higher-order field components to the electric field compared to the normal quadrupole IT . Therefore, determination of the higher-order components in the toroidal field is an important step in the optimization of the performance of the halo IT.
In this study, the halo IT plates were modified by machining thin slits in the sixth electrode rings in order to determine if axial ejection of ions would improve ITMS resolution compared with radial ejection. Furthermore, a variety of potential distributions were simulated and then applied across the series of halo IT electrode rings to study the effects of higher-order electric field components on resolution. Both efforts were designed to improve the overall resolution of the halo IT.
Recognizing that exact multipoles do not exist in toroidal systems, it is nonetheless useful to investigate the effects of higher-order fields on ion trapping and ejection in the halo IT. Although some of the effects of higher-order multipoles in conventional ion traps will correspond directly with similar effects created by the multipole-like fields in a toroidal trap, some differences may also be present, particularly as the distance from the trapping center increases. For instance, in a conventional ion trap, the electric field exactly on the ejection axis is collinear with the axis itself due to symmetry. Thus, ions are pushed back exactly to the trapping center. In toroidal traps, this condition is not guaranteed by symmetry and may not exist. The “center” of a cubic, quartic, or other field component may not lie exactly at the trapping center. As another example, in a conventional trap, a linear field in the z direction is also linear in the r direction. In a toroidal trap, a field linear in the z direction may be linear in the r direction only instantaneously at the trapping center. These and other differences between quadrupole and toroidal ion traps doubtless have consequences for ion behavior. Therefore, throughout this paper, higher-order electric field terms are used to describe the multipole-like components in a toroidal trapping system. For example, a quadratic field produces a hexapole-like component, a cubic field produces an octopole-like component, and a quintic (5th-order) field produces a dodecapole-like component.
3.1 Fabrication of the Halo IT with Axial Ejection Slits
Dimensions and positions of the electrode rings on the ceramic plates
Inner radius (mm)
Outer radius (mm)
3.2 Experimental Setup
The experimental setup to test the modified halo IT design was identical to that described previously . Briefly, the halo ITMS system included a custom-built electron gun, halo IT mass analyzer, discrete dynode electron multiplier detector, rf waveform generator, arbitrary waveform generator, and control and data acquisition system. The electron gun was gated by changing the bias voltage from −70 to +120 V on the filament (W5; Scientific Instrument Services, Ringoes, NJ, USA) with a 1.7-A current to control the ionization time. A custom-built rf generator was used to apply a 1.7-MHz sinusoidal waveform with amplitude up to 900 Vp-p to the capacitor network on the PCB for trapping of ions, as shown in Figure 2. For monopole ejection, an AC signal was frequency swept using an arbitrary waveform generator (33250A; Agilent Technologies, Santa Clara, CA, USA), which was amplified up to 10 Vp-p using a custom-built amplifier. For dipole ejection, the signal from the custom-build amplifier was further converted into two signals with 180º phase difference using a custom-built converter. The ejection signal (either before the converter for monopole or one of the two phase different signals after the converter for dipole) was connected to the 6th electrode ring (where the slits are located) on the ceramic plate farthest from the detector, which was spread to the other electrode rings on the same plate by the capacitor network. The 6th electrode ring in the other plate (i.e., the plate closest to the detector through which the ejected ions pass) was grounded for monopole ejection, or connected to the out-of-phase signal after the converter for dipole ejection. The discrete dynode detector (AF138, ETP; SGE Inc., Austin, TX, USA) at −1.9 kV was used to collect positive ions ejected from the trap. The output of the detector was amplified using a custom-built integrating amplifier (128 MΩ) and fed into the DAQ board on a computer. The halo IT system was controlled using a PC through a BNC-2110 data acquisition board (National Instruments, Austin, TX, USA). The control and data handling software was Labview 7.1.
The halo IT was enclosed in a custom-built vacuum chamber pumped using a 520 L/s turbomolecular pump (model TMH 520-020; Pfeiffer, Asslar, Germany). Sample headspace vapors and helium buffer gas were introduced directly into the vacuum chamber via precision needle leak valves (Nupro/Swagelok, Solon, OH, USA) at pressures of 10−5 mbar and 10−4 mbar, respectively, which were measured using a full-range cold cathode vacuum gauge (model PKR 251/261; Pfeiffer). All pressure readings reported in this paper are uncorrected.
3.3 Computation Methods
According to the superposition principle in electric field theory, the coefficients for each order (i.e., linear, cubic and quintic fields in equation 8) for all rings in the halo IT can be summed to provide the overall higher-order field component to the total electric field. SIMION 8.0 and MATLAB R2008b were used to calculate the A2, A4, and A6 dimensionless higher-order coefficients as described in a previous paper . The resultant higher-order components of the total electric field in the halo IT were calculated by multiplying the voltages applied to each ring by the various higher-order coefficients, and summing all of the products of each higher-order type. The various summed higher order component percentages were obtained by dividing each summed higher-order coefficient by the summed linear field coefficient, and then multiplying by 100% (i.e., A4/A2 for cubic field, and A6/A2 for quintic field). Finally, the SOLVER function in Microsoft Office Excel (Microsoft Corp., Bellevue, WA, USA) was used to specify new voltages to apply to the electrode rings in order to adjust the higher-order component percentages to the desired values.
4 Results and Discussion
4.1 Performance of the Halo IT with Axial Ejection
In the halo IT, ions are formed, trapped, and collisionally cooled within the toroidal field. In the original halo IT design , a 9.14-mm-diameter center hole served as an exit for ions ejected from the trap during mass analysis (Figure 1a). During ejection, the trapped ions absorbed energy from the ejection signal and resonated in a direction parallel to the trapping plates (i.e., in the r direction), then were drawn out through the center hole by the voltage on the detector. As shown in Figure 1b, in the modified halo IT, three 92º arc (6-mm radius) slits with 0.4-mm width were fabricated in the ceramic plates to allow direct axial ejection from the toroidal trapping field. In this new design, trapped ions resonate in a direction perpendicular to the trapping plates (i.e., in the z direction), and ultimately escape through the slits to the detector. From simulation using SIMION, we observed that axial ejection should improve mass resolution compared to radial ejection.
Mass spectral resolution was increased from approximately 100 (m/Δm) to 280 (m/Δm) when ion ejection was changed from radial to axial. The higher-order electric field components along the ejection (radial) direction in the original halo IT were calculated using SIMION as described in the experimental section, which gave a cubic field (A4/A2) percentage of 24.3% and a quintic field (A6/A2) percentage of 1.6%. Such a high percentage of quintic field, which has the greatest influence on ion trapping among all of the higher-order fields, was much higher than needed or desired. It lengthened the resonant ejection process, keeping the ions in the trapping field longer than necessary, and producing poor resolution as a result.
In the new design, trapped ions are ejected in the z direction through the slits. Therefore, the trapping center must be exactly in-line with the ejection slits. Simulation has shown that ions should be trapped in a very thin circular band exactly between the slits in the two ceramic plates under the electric field conditions we have considered in this work. In the toroidal IT , geometric modification was made from symmetric to asymmetric geometry to intentionally add a slight nonlinear (mainly cubic field) field to improve the performance of the device. Since the halo IT has the same trapping geometry, the higher-order fields also play a very important role in the ejection of ions. Accordingly, they were studied in detail in order to optimize the performance of the new halo IT design.
4.2 Effect of Percentage of Cubic Field Component on Performance of the Halo IT
It was reported that the optimum performance of cylindrical ITs  and rectilinear ITs  was achieved when the sum of the percentages of the positive octopole component (cubic field) and the negative dodecapole component (quintic field) was approximately −10% in the axial direction. Since the curvature of a toroidal trapping geometry inherently produces higher-order field components, as indicated in equation 3, it became necessary to determine their compositions in different electric fields and to investigate their effects on trap performance in order to optimize the performance of the halo IT. When we tried to constrain the percentages of the cubic and quintic field components, the quintic field component could not be more positive than −45% when the cubic field was kept below 10%. This was mostly due to the curvature of the toroidal geometry in the halo IT . The electric fields in a toroidal trapping volume are not symmetric with respect to radial motion toward or away from the rotational axis. This asymmetry, which does not exist in other types of ITs, affects both the ion motion and the higher-order fields. The influence of fields higher than quintic, such as heptic (A8), nonic (A10), and undecic (A12), on the performance of more conventional ITs have not previously received much attention because they are typically very small. Likewise, we did not consider their contributions to halo IT performance because the cubic and quintic field components were likely much more important. In this work, we fabricated four sets of PCBs to generate 1%, 3%, 5%, and 7% cubic fields (A4/A2) with constant −50% quintic field (A6/A2) to investigate the influence of cubic field percentage in the axial direction.
4.3 Performance of the Halo IT with 5% Cubic Field
Figure 5a shows an average of 10 spectra of dichloromethane taken sequentially. The fragment peaks at m/z 49 and 51 were clearly resolved with a mass resolution of 0.18 Da (WHM for m/z 49). However, the molecular ions at m/z 84/86/88 could not be resolved. Figure 5b shows an average of 10 spectra of heptane with resolution of 0.20 and 0.36 Da (FWHM) for the fragment peaks at m/z 43 and 57, respectively. However, the peaks at m/z 71 and 100 are relatively wide. From these and other mass spectra not shown here, good resolution was only observed for ions with m/z values less than 70. The largest and smallest ions observed in this work were m/z 198 and 38, respectively. Fragments with m/z values lower than 38 were below the low-mass cut-off and could not be detected.
Simulations of the halo IT using SIMION 8.0 also indicated that good mass resolution could be obtained in the low mass region, as shown in Figure 3c. In this simulation, ions with m/z at 63, 65, 91, and 92, matching the m/z values and relative abundances of the typical fragment ions of toluene, were generated between the two trapping plates at random positions, random energies, and random directions. For collisions between ions and helium atoms, only elastic collisions were taken into account. Two out-of-phase frequency-sweeping ejection signals were applied, one to each plate, with a DC offset voltage to eject ions out of the trap. The conditions in the simulation were close to those in the real experiment, except no neutral sample molecules were added. In the simulation, space charge effects were not considered. The simulation was terminated when all ions hit the trapping plates, the boundary walls, or the detector.
From the simulated mass spectrum in Figure 3c, peaks at m/z 63 and 65 are well resolved (0.55 Da FWHM for m/z 63). Much worse resolution (1.9 Da FWHM) was obtained for m/z 91/92. Since the depth of the pseudopotential well is dependent on ion m/z value, higher mass ions are trapped in shallower wells and may be more easily ejected from oscillations stimulated by the ejection signal, even though their matched secular frequencies have not yet been reached . Therefore, there essentially exists a wider range of ejection frequencies to move ions of specific m/z values out of the trap, which broadens the time interval for ejecting ions and, hence, lowers the resolution. This may possibly be a result of using lower operating voltages in miniaturized ion traps, and will need to be addressed if operating power is to be reduced for portable analysis.
The performance of the halo IT mass spectrometer was improved by axial ejection of ions from the trapping region through slits machined in the ceramic plates because a more linear field was obtained in the axial ejection design than in the original trap. The resolution was improved by a factor of 2.3. The resolution and abundance of the mass spectral peaks were greatly affected by percentages of high-order components of the electric field. The best resolution (280 m/Δm for m/z 51) was obtained using an electric field with 5% cubic component. It was found important to consider higher-order components in the trapping field to optimize the performance of the halo IT. The overall trapping field shape and higher-order field composition can be changed by applying different potentials to the electrode rings. This is accomplished by soldering the appropriate capacitor network onto the PCB without having to change the ceramic plates and the structure of the halo IT. This design should allow straightforward evaluation of the effects of higher-order fields other than linear and cubic fields on ion trapping and the effects of all higher-order field components on resonant ejection [29, 30] to further enhance the performance of the halo IT. Improvements in mass resolution may also be possible with higher frequency or higher amplitude rf, or with optimization of other trapping parameters.
The authors acknowledge financial support for this work by the National Aeronautics and Space Administration (NASA) grant no. NNH06ZDA001N-PIDDP, by Torion Technologies, and by Brigham Young University.
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