Planar Geometry for Trapping and Separating Ions and Charged

Anal. Chem. 2007, 79, 6857-6861

Planar Geometry for Trapping and Separating Ions and Charged Particles S. Pau,*,† W. B. Whitten,‡ and J. M. Ramsey§

The University of Arizona, Tucson, Arizona 85721, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and The University of North Carolina, Chapel Hill, North Carolina 27599

A planar quadrupole ion trap is proposed. We have demonstrated an extremely large operating range by trapping ions and particles with mass-to-charge ratio ranging from 102 to 109 at frequencies from 2.8 × 106 to 60 Hz at an operating pressure of 1.1 × 10-4 to 760 Torr, respectively, using a trap radius of r1 ) 1 mm. We have also performed mass spectrometry with a resolution of 1.2 amu with mass-to-charge range from 50 to 150. Our geometry is simple enough to be integrated into existing integrated circuits and microelectromechanical system devices, opening up the possibility of many novel hybrid applications and experiments. Miniaturized devices to store and to separate ions and charged particles are important for applications in atomic clocks,1 precision spectroscopy,2 isotope separation,3 mass spectrometry,4-7 quantum computing,8-13 ion-molecule chemistry,14 and micropumping.15 While it is not possible to trap an ion using a pure electrostatic field according to Earnshaw’s theorem, it has been demonstrated in many experiments that ions and charged particles can be stored * To whom correspondence should be addressed. E-mail: spau@ optics.arizona.edu. † The University of Arizona. ‡ Oak Ridge National Laboratory. § The University of North Carolina. (1) Jefferts, S. R.; Monroe, C.; Barton, A. S.; Wineland, D. J. IEEE Trans. Inst. Meas. 1995, 44, 148. (2) Brewer, R. G.; DeVoe, R. G.; Kallenbach, R. Phys. Rev. A. 1992, 46, R6781. (3) Alheit, R.; Enders, K.; Werth, G. Appl. Phys. B 1996, 62, 511. (4) Badman, E. R.; Cooks, R. G. J. Mass Spectrom. 2000, 35, 659. (5) Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M. Science 2006, 311, 1566. (6) Blain, M. G.; Riter, L. S.; Cruz, D.; Austin, D. E.; Wu, G.; Plass, W. R.; Cooks, R. G. Int. J. Mass Spectrom. 2004, 236, 91. (7) Pau, S.; Pai, C. S.; Low, Y. L.; Moxom, J.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Phys. Rev. Lett. 2006, 96, 120801. (8) Seidelin, S.; Chiaverini, J.; Reichle, R.; Bollinger, J. J.; Leibfried, D.; Britton, J.; Wesenberg, J.; Blakestad, R. B.; Epstein, R. J.; Hume, D.; Itano, W. M.; Jost, J. D.; Langer, C.; Ozeri, R.; Shiga, N.; Wineland, D. J. Phys. Rev. Lett. 2006, 96, 253003-1. (9) Chiaverini, J.; Blakestad, R. B.; Britton, J.; Jost, J. D.; Langer, C.; Leibfried, D.; Ozeri, R.; Wineland, D. J. Quant. Inf. Comput. 2005, 5, 419. (10) Madsen, M. J.; Hensinger, W. K.; Stick, D.; Rabchuk, J. A.; Monroe, C. Appl. Phys. B 2004.78, 639. (11) Stick, D.; Hensinger, W. K.; Olmschenk, S.; Madsen, M. J.; Schwab, K.; Monroe, C. Nat. Phys. 2006, 2, 36. (12) Kim, J.; Pau, S.; Ma, Z.; McLellan, H. R.; Gates, J. V.; Kornblit, A.; Slusher, R. E.; Jopson, R. M.; Kang, I.; Dinu, M. Quant. Inf. Comput. 2005, 5, 515. (13) Pearson, C. E.; Leibrandt, D. R.; Bakr, W. S.; Mallard, W. J.; Brown, K. R., and Chuang, I. L. Phys. Rev. A 2006, 73, 032307. (14) Alvarez, E. J.; Vartanian, V. H.; Brodbelt, J. S. Anal. Chem. 1997, 69, 1147. (15) Roth, J. R. Phys. Plasmas 2003, 10, 2117. 10.1021/ac0706269 CCC: $37.00 Published on Web 08/03/2007

© 2007 American Chemical Society

for days in ultrahigh vacuum and at ambient pressure, respectively, using a time-varying electric field. Many such traps share the common characteristic of a nonvanishing quadrupole component of the alternating electric field. While the conventional quadrupole ion trap, made of a ring and two end caps, has dimensions of 1-10 mm, the requirements for many of the aforementioned applications necessitate a reduction in size of the ion trap and an increase of the number of traps to form an array. A planar array of ion traps can potentially trap more ions per unit area and thus increase signal-to-noise ratio for mass spectrometry applications.7 The main motivations are to reduce the size, voltage, and operating power of the device and to manipulate large arrays of ions in a controlled environment. In this respect, there is a need to design an ion trap geometry that is scalable and conducive to planar microfabrication, a technique that is used extensively to make silicon integrated circuits and microelectromechanical systems. In this paper, we propose and demonstrate a planar trap geometry that will be useful to trap and to separate ions and charged particles and can be potentially applied to spectroscopy of particles or molecules isolated from an external reservoir. All ring electrodes are coplanar and can be fabricated and arrayed by patterning a single layer of conductors, which simplifies the fabrication process. A Paul-Straubel trap made of a single ring electrode and large ground plane or planes located far away has been demonstrated.16-18 We propose here a planar ring structure shown in Figure 1a, consisting of a circular ring electrode on top of a circular ground plane. An approximate analytical solution for the potential exists for this geometry. The potential for a flat circular disk of radius r0 and potential V0 is given by VD(R, z, r0) )

2V0 -1 tan π

[x

]

x2r0

(1)

R2 - r02 + x(R2 - r02)2 + 4z2r02

where R)(x2+y2+z2)1/2.19 The potential for a ring of inner and outer radius, r1 and r2, sitting on top of a ground plane with a small finite gap can be approximated by VR(R,z) ) VD (R,z,r2) - VD(R,z,r1). By setting the voltage of the ring to be periodic in time, in the form, V ) V0cos(Ωt), it is possible to (16) Straubel, H. Naturwissenshaften 1955, 42, 506. Straubel, H. Z. Elektrochemie 1956, 60 (9/10), 1033-0136. (17) Hartung, W. H.; Avedisian, C. T. Proc. R. Soc. London 1992, 437, 237. (18) Yu, N.; Nagourney, W.; Dehmelt, H. J. Appl. Phys. 1991, 69, 3779. (19) Smythe, W. R. Static and Dynamic Electricity, 2nd ed.; McGraw-Hill Co.: York, PA, 1950; p 124.

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Figure 1. (a) Schematic showing the dimensions of the single ring electrode trapping configuration. Equal potential contours for (b) one-ring, (c) two-ring, and (d) three-ring electrode configurations with the presence of a ground plane. The spacings of the contour are chosen to illustrate the presence of the potential minimum.

show that this configuration of a flat ring on a ground plane creates a trap potential above the ring as shown in Figure 1b. By setting ∂VR/∂z|x)0,y)0 ) 0, we can also show that the local potential minimum is centered along the z-axis at a distance h1 ) (r1r2)1/2 above the ring (inset of Figure 1a). We note that the result can also be obtained by transformation of the Poisson equation to flat-ring cyclide coordinates.20 Our analysis can be generalized to multiple ring configurations with two and three rings. By increasing the number of rings, it is possible to eliminate the ground plane, such that the entire device can be fabricated using a single layer of patterned metal on top of an insulating layer. It is also possible to use multiple rings to generate a nonvanishing hexapole potential along three orthogonal directions to trap particles with a permanent or induced dipole moment.21 For geometry with more than one ring, there exists a toroidal potential, which is shown in Figure 1c,d and has been successfully utilized as a mass analyzer.22 One of the important parameters of the trap is the depth of the potential well. Two relevant trap depths are ∆Vz and ∆Vr, corresponding to the directions along the z-axis and the r-axis respectively where r ) (x2 + y2)1/2. For simplicity, we choose to analyze the depths along these axes. Figure 2 shows the potential of a ring, as in Figure 1a, for the parameters, V ) V0cosΩt and r2/r1 ) 3 along the z-axis and (b) the r-axis for different voltages V0. Unless explicitly stated, the potential is calculated from fixed dc potential on the electrodes and is not the same as the pseudopotential from the adiabatic approximation of the Mathieu equation. The normalized depths are also shown as a function of normalized ring radius. We see that the maximum depth along (20) Moon, P., Spencer, D. E. Field Theory Handbook, 2nd ed.; John T. Zubal Inc.: Cleveland, 2003; p 102. (21) Pau, S. Appl. Phys. Lett. 2005, 87, 134104. (22) Austin, D. E.; Wang, M.; Tolley, S. E.; Maas, J. D.; Hawkins, A. R.; Rockwood, A. L.; Tolley, H. D.; Lee, E. D.; Lee, M. L. Anal. Chem. 2007, 79, 2927-2932.

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the r axis is around V0/5 for r2/r1 ∼ 10. In general, we have ∆Vr < ∆Vz. It is instructive to compare this with that of a pure quadrupole trap. In this case, where the end caps are grounded and the ring potential is V0, the potential is given by V(R, z) )

V0(R2 - 2z2) 2r0

2

+

V0 2

(2)

where r0/z0 ) 21/2 and r0 and z0 are the radius and height of the trap.23 The depth of the potential along the r-axis is ∆V ) V0/2. The depth of the pseudopotential along the axial direction for a pure quadrupole potential is given by Dz=V0qz/8 ) ∆Vqz/4, where 0 eqz e0.908 is the axial Mathieu parameter. While the potential is cylindrically symmetric for the planar ring trap, it is highly asymmetric along z with different multipole contributions. By changing the relative diameter and width of the ring, one can design a trapping pseudopotential that minimizes and maximizes the different multipole contributions. In general, the potential depth decreases with increasingly open electrode geometry. Figure 2 also shows a finite element analysis of a cylindrical ion trap (CIT) with two end caps (Figure 2d), with one end cap (Figure 2e), with no end cap (Figure 2f), and a ring planar trap of identical radius (Figure 2g). There is a reduction of trapping potential from 100 to 61% for a fixed applied voltage when we truncate the two end caps of the CIT. By going to a completely open planar ring structure, Figures 1b and 2g, we find that the trapping potential has reduced to approximately a third of the closed CIT geometry. In our experiments, we utilized planar ring traps of different geometries, made from a two-layer printed circuit board. The top (23) Dawson, P. H., Ed. Quadrupole Mass Spectrometry and Its Applications; AIP Press: New York, 1995; p 102.

Figure 2. Plot of the normalized potential as a function of normalized coordinates for a single ring and one ground plane geometry along (a) the z-axis at r ) 0 and along (b) the r-axis at z ) r1r21/2 for V0 ) (1, (0.6, and (0.2 V. The plot also shows the definition of the potential depth ∆Vz and ∆Vr, which is not the same as the pseudopotential depth D. (c) The depths are plotted for different ring radius ratios. (d) Finite element analysis of cylindrical ion trap and planar trap showing reduction of potential depth with identical ring diameter. GND denotes ground electrode and AC denotes alternating current electrode. Figure 2(g) shows AC voltage applied to the top ring layer electrode and GND connected to the bottom layer electrode.

and bottom copper layers are nominally 43 µm thick and have holes that are plated through to connect top layer electrode of circuit board to the bottom layer electrode. The dielectric material is the industry standard FR-4 epoxy glass, with a thickness of 1.57 mm and a dielectric constant of 4.2-5. Two types of geometries are used, a single ring with a ground24 and a three-ring configuration. In the former case, the trapping potential is generated by applying a sinusoidal voltage to the single ring. The ring electrode is located on one side of the circuit board and the ground plane is located on the other side. In the three-ring configuration, the trapping potential is generated by applying ground or constant voltages to the inner and outer rings and a sinusoidal voltage to the middle ring. The relative dimensions of the rings determine the shape of the pseudopotential and are listed in Table 1. Calculations using finite element analysis and expansion of the potential in Legendre polynomials show that this geometry provides a relatively large quadrupole component and large trap potential.25 We have constructed ring traps with r1 ranges from 0.5 to 3.3 mm. We note that, in the design of the electrodes, we try to minimize the area of exposed dielectric, which can charge up and potentially create long-term drifting effects. The experiments described here cover two different regimes, charged microparticles in air and ions in vacuum, as described in (24) In principle, this can be replaced by two conducting planes, one of which has a circular hole and the other serves as ground. (25) Jachowski, M. D. A.; Low, Y. L.; Pau, S. Planar Micro-miniature ion trap devices. U.S. Patent 7217922.

Table 1. Relative Dimensions of the Single- and Three-Ring Electrodesa r1

r2

r3

r4

r5

r6

1 1

4.3 1.75

2.15

2.95

3.38

4.22

a r and r are inner and outer radii of smallest ring. r and r are 1 2 3 4 inner and outer radii of middle ring. r5 and r6 are the inner and outer radii of the outer ring.

two comprehensive review articles.26-27 For the former, charged microparticles were levitated by a combination of dc and audio frequency fields and monitored by observing scattered laser illumination.26,28 Trapping and mass-selective ejection of ions in vacuum was observed via the techniques with radio frequency fields and an electron multiplier detector.27 Assuming that the dc contribution is small, the stability conditions are described by dimensionless relationships between the ac voltage, trap dimensions, frequency, charge, and mass. It is then possible to constrain charged particles ranging from atomic ions to micrometer-sized aerosol particles in traps of similar dimension by varying the voltage and frequency. For this study, we present results for charged diamond microcrystals levitated at 60 Hz at atmospheric pressure and demonstrate mass spectrometry of ions at low (26) Davis, E. J. Aerosol Sci. Technol. 1997, 26, 212-254. (27) March, R. E. J. Mass Spectrom. 1997, 32, 351-369. (28) Arnold, S.; Hessel, N. Rev. Sci. Instrum. 1985, 56, 2066.

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Figure 3. Photograph of 25 micron diamond in a three-ring planar trap at 1 atm illuminated by HeNe laser. Vrms ) 500 V at 60 Hz and r1 ) 1.12 mm. Inset shows the circuit board layout of the one- and three-ring geometries. Green color denotes top layer electrode and pale gray color denotes bottom layer electrode. Black color denotes epoxy glass between the two electrode layers.

pressure with the planar circuit board traps described above. We note that the presence of multiple higher orders in the trapping potential will create nonlinear resonance conditions and distort the stability diagram in comparison to that of a pure quadrupole potential.27 For microparticle levitation, the planar trap was oriented with the symmetry axis vertical so that the gravitational force on the particle could be opposed by a vertical dc electric field. The trap was enclosed by a metal chamber to reduce drafts and the effects of stray electric fields and with appropriately placed glass windows for optical access. An ac voltage of ∼500 Vrms at 60 Hz was applied between the two planar electrodes or to the middle ring of the three-ring configuration, while a common-mode dc voltage of 0-100 V was applied to the assembly to counterbalance the weight of the particle. A vertical HeNe laser beam directed through the hole in the trap electrodes was used to illuminate the particles in the trap. The scattered light could be easily observed with a telescope or camera as described in the literature. The major difference between this experiment and those previously reported is that the pseudopotential minimum is located above the plane of the electrodes in the present case. The particles used here were crystals of diamond emory, nominally 10 and 25 µm in diameter. The particles were charged by pushing them from a Teflon tube with a Teflon-insulated wire. They were dropped through the hole in the cover through which the laser beam excited the chamber and occasionally, one or more particles would be trapped. The dc potential could then be adjusted to center the particle while it was being observed visually. The photograph in Figure 3 shows a 25-µm diamond crystal levitated in a three-ring planar trap with inner radius of 1.12 mm. The exact charge of the particle is not known. Simultaneous confinement of up to four charged particles has been observed. To demonstrate the stability of our traps, we have successfully trapped a crystal for a duration of 1 week, after which the experiment was terminated. We have also obtained comparable results using a single ring planar trap to levitate 10µm diamond crystals in the same manner. 6860

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Figure 4. Mass spectrum of (a) Xe isotopes and (b) DMMP using a single-ring trap with r1 ) 1.0 mm. The bar graphs show the relative natural abundances of the isotopes and the fragment ion masses.

Identical planar traps were used to trap ions at low pressure. The particular application that we have demonstrated here is mass spectrometry of trapped ions using mass-selective instability to eject the ions.27 The experimental arrangement is one we have used to explore ion trap mass spectrometry with millimeter-sized cylindrical traps.29 The planar trap was placed in line with a 70-eV electron gun and a channel electron multiplier ion detector. There was a focusing anode between the electron-emitting filament and the trap that could be used to gate the electron beam and a screen grid maintained at ground potential to shield the ions in the trap from the detector bias. The trap was oriented with the pseudopotential minimum toward the detector and with the beam from the electron gun passing through the hole in the electrodes. An rf voltage of programmed amplitude was applied to the upper electrode of the planar trap while the lower electrode was at ground potential. The stability of ions in a purely quadrupole trap in the axial direction is determined by

4eVac/mr02ω2 e 0.908

(3)

where Vac and r0 determine the quadrupole electric field. For a planar trap, where the potential has higher order components, we can use (3) with an effective r0 to get an understanding of the scaling relationships between the different parameters. Thus, for a given applied voltage, there is a lower mass limit for stable ion trapping. For the results reported here, the rf voltage was constant for 35 ms, during which time the electron gun was gated on and the gain of the ion detector reduced. At the end of this period, the electron gun was turned off, the detector gain was restored, (29) Moxom, J.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 2003, 75, 3739-3743.

and the rf voltage was ramped to a multiple of the initial value in 8 ms. As the voltage amplitude passed the limit of stability for ions of a given mass, those ions were ejected from the trap along the axis of symmetry, producing a current in the ion detector. The dependence of the detector current on rf voltage was thus a representation of the mass spectrum of the ions that were trapped during the storage period. A mass spectrum of xenon ions obtained in this way is shown in Figure 4a. Xenon gas was admitted to the vacuum chamber at a pressure of 7 × 10-6 Torr. Helium buffer gas was present at a pressure of 1.1 × 10-4 Torr for this measurement. A radio frequency voltage at 2.8 MHz was used with an amplitude of 190 V peak for ion trapping, ramped to 380 V peak. The mass axis was calibrated with respect to 129Xe. A mass spectrum of dimethyl methylphosphonate (DMMP; mw ) 124), is shown in Figure 4b. DMMP is a commonly used surrogate for a chemical warfare nerve agent. The sample was placed in a glass vial connected through a needle valve to the vacuum chamber. The sample pressure was ∼2 × 10-5 Torr. The rf voltage for these measurements was ramped from 88 to 350 V peak in 8 ms. We observed molecular fragmentation of DMMP caused by the electron impact ionization. Relative abundances of the fragments are also shown.30 Differences between the observed and expected mass values were probably due to nonlinearity in the voltage ramp caused by partial saturation of the ac amplifier. There (30) Holtzclaw, J. R.; Wyatt, J. R.; Campana, J. E. Org. Mass Spectrom. 1985, 20, 90.

are extra peaks in both spectra that are believed to be due to substances outgassing from the circuit board dielectric. Their relative intensity increased with rf voltage and with time as the circuit board temperature increased due to dielectric losses. The mass resolution is ∼1.5 amu. By optimizing the current electrode configuration, i.e., diameters and shapes, we expect to achieve higher resolution with such a planar geometry. There are several advantages associated with the planar ring trap. The capacitance of the ring trap is to first-order linearly proportional to the radius of the ring and the dielectric constant of the materials between the rings. By the nature of the planar geometry, most of the materials between the rings are air. The trap potential is located a fixed distance away from the planar electrodes, allowing direct access to the trapped particles or ions by laser or other probing devices. As mentioned before, the geometry is scalable and can be constructed using a single-layer metal deposition and etch at low temperature, which is compatible with CMOS technology. Finally, the relative dimensions of the rings can be manipulated to give a large ranges of quadrupole, hexapole, and octapole components depending on the applications. Controlled motion and confinement of particles can be performed with precision in high vacuum, atmospheric pressure, and liquid. Generalization of circular ring to elliptical or arbitrary planar geometry remains to be explored. Received for review March 29, 2007. Accepted June 25, 2007. AC0706269

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