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Anal. Chem. 1992, 6 4 , 177-180 (12) Eatherton, R. L.; Morrissey, M. A., Siems, W. F.; Hili, H. H., Jr. J . High Resolut. Chromatogr. Chromatogr. Commun. 1986, 9 , 154. (13) Hill, H. H.,Jr.; Morrissey, M. A. I n Modern Supercritical FluU Chromatography; White, C . M., Ed.; Huthig: Heidelberg, 1988. (14) Knorr, F. J.; Eatherton, R. L.; Siems, W. F.; Hili, H. H., Jr. Anal. Chem. 1985, 57, 407. (15) Eatherton, R. L.; Siems, W. F.; Hili, H. H.,Jr. J . High Resolut. Chro-
.
.
matogr Chromatogr Commun. 1986, 9 , 44. (16) Eatherton, R. L. Ph.D. Thesis, Washington State University, 1987. (17) Morrissey, M. A. Ph.D. Thesis, Washington State University, 1988. (18) Aronson, E. A. Sandh Report, Sand 87-0072, UC-32, March 1987.
RECEIVEDfor Review July 1, 1991. Accepted October 4,1991.
Elimination of Axial Ejection during Excitation with a Capacitively Coupled Open Trapped- Ion Cell for Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Steven C. Beu and David A. Laude, Jr.* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712
A Fourier transform Ion cyclotron resonance (FT-ICR) mass spectrometer trapped-Ion cell Is constructed that eliminates the axial ejection of ions during excitation of cyclotron motion. The cell features an open geometry In which the trapping electrodes are extended In the planes of excitation and detection electrodes. A unlque aspect of the cell Is the capacitive coupling of excltatlon electrodes to the trap plates to distribute the excltation fields beyond the boundaries of the trapping-potential well. The axial component of the excitation field that Is responsible for ejection Is ellmlnated by not termlnatlng excitatlon electrlc fields at trap plates that are positioned perplndkular to the magnetlc field and thus bound the potential well. Profiles of broad-band mass spectra acquired at lncreaslng excitatlon energies demonstrate that, even at 0 . 4 4 trap potentials, only mass-Independent radial ejection occurs when the excitation Is dlstrlbuted throughout the open cell. I n contrast, cells of slmllar dimension, Including both a conventional closed cell at 1 . 0 4 trap potentlals and an uncoupled open cell at 0 . 4 4 trap potentlals, exhibit severe lowmass ejection during hlgkpower excitation. The open cell Is used to acquire laser desorptlon/lonlratlon mass spectra as evidence of a design simplicity that facilitates Immediate lntegratlon Into more Intricate FT-ICR experiments.
INTRODUCTION Recently, we described a new electrode configuration for the trapped-ion cell of a Fourier transform ion cyclotron resonance spectrometer that was distinguished by the collinear extension of the trapping electrodes in the plane of excitation and detection electrodes (1,2). This open cell design differed from previous trapped-ion cell designs for FT-ICR that positioned the trap plates perpendicular to the other cell plates (3-18). With respect to trapping the ions, the use of open trapping electrodes is similar in concept to previous efforts by physicists to confine electrons and antiprotons for highprecision measurements (14-24). Our modeling of open cell trapping and excitation fields suggested a modest reduction in z axis ejection during excitation and a slightly increased shift in the cyclotron frequency associated with the radial trapping field, when compared to the conventional orthorhombic closed cell of similar aspect ratio. These predictions were verified experimentally with an open trapped-ion cell constructed to approximate an elongated orthorhombic cell with an aspect ratio of 2. 0003-2700/92/0364-0177$03.00/0
Clearly, the original open cell would be recommended, not for improved FT-ICR performance, but rather for important practical considerations in executing the FT-ICR experiment and for increased design flexibility. Examples of practical advantages include increased conductance, the elimination of charging and contamination on trap plates, and both simplified and more efficient introduction of charged particle beams. This latter factor is important for external source applications. The increased flexibility of the open cell derives from the potential to use any combination of collinear electrode segments as trapping, detection, and excitation plates and thereby increases the dimension or number of trapped-ion cells without physical alteration of the electrode assembly. It was also suggested that with appropriate modification of electrical circuitry, these electrodes could simultaneously serve more than one function, with the potential for significant improvement in cell performance (2). For example, one suggestion given was that the excitation electrodes could be capacitively coupled to trap plates, thereby creating an axially homogeneous excitation field over the entire length of the trapping-potential well. Not only would the axial component of the excitation field effectively vanish with consequent elimination of z axis ejection during excitation, but with the opportunity to now use very low trap potentials, the deleterious effects of the radial trapping electric field would also diminish. In previous efforts to minimize z axis ejection, the ions could either be driven to the bottom of the potential well by collisional cooling (25) or through an adiabatic ramping of the trapping potential (26))or alternatively, the magnitude of the axial excitation fields could be reduced. Examples of this latter approach involve the design of new trapped-ion cells. Caravatti and Allemann distributed the excitation potential over trap plates segmented into 11electrodes shaped to mimic the radial contour of the electric field (16). Wang and Marshall (17) and Hanson and co-workers (18) reduced both axial excitation and radial trapping fields by distributing the excitation potential over guard wires or rings inserted adjacent to the trap and/or detection electrodes in a manner similar to that employed in the original ICR cell (27). In this manuscript we describe the successful assembly of a capacitively coupled open trapped-ion cell in which the excitation pulse is also applied to the trap electrode segments extending from the two central excitation electrodes. The geometry of this new open cell is identical to the original open cell and therefore retains the practical benefits described above for that cell. Also to be presented is a demonstration of the 0 1992 American Chemical Society
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Figure 1. Capacitively coupled open cell configuration and associated circuitry. An effective trapping potential well is created when equal potentials are applied to the two sets of four exterior electrodes. Trappingpotential maxima are located near the center of the trap plate assemblies. The use of excite, detect, and trap electrodes of equal dimensions results in an open cell geometry corresponding to an orthorhombic cell geometry with an aspect ratio of 2. I n the current implementation, actual components of the circuit shown are located outside the vacuum chamber.
open cell in laser desorption/ionization (LD/I) /FT-ICR experiments. Results provide insight into questions concerning the possible role of conventional perpendicular trap plates in mechanisms for trapping of externally generated ions.
EXPERIMENTAL SECTION An open cell that approximates an elongated trapped-ion cell of aspect ratio 2 was employed for this work. The aspect ratio for the open cell is defined as the distance between trapping potential maxima (located approximately at the center of the trapping electrode assemblies) divided by the width of the cell. An aspect ratio of 2 was achieved here by assembling 12 5-cm x 5-cm aluminum plates in an open orthorhombicarray, as shown in Figure 1. The spacing between each plate was 3 mm at common edges. The four interior plates served traditional functions with each of two opposing electrode pairs used for excitation or detection. The two groups of four exterior plates were used as trapping electrodes. In addition, to accomplish the distributed excitation,the trap plates extending from either side of the excitation plates were capacitively coupled, as shown in Figure 1, with a circuit that included 1-kQblocking resistors (1/4-W film type, 1%tolerance) and 0.10-pF coupling capacitors (100-V dc polycarbonate, 5% tolerance). For developmental purposes the circuit elements were mounted outside the vacuum chamber and electrical leads and feedthroughs were required for each of the cell electrodes. Future designs will seek to install the circuit elements at the cell to minimize the length of shielded radiofrequency leads and thus the total reactive load on the excitation amplifier. The open cell was mounted on a conventionalflange assembly and positioned in a diffusion-pumped vacuum chamber maintained in the 10-9-Torrrange. The FT-ICR used was an external source time-of-flight/FT-ICR with a 2.0-T superconducting magnet, described in detail elsewhere (28). For this work a 3/4-in. probe inlet was mounted on one end of the vacuum chamber to allow insertion of either a probe-mounted fiber-optic-based laser desorption interface or a probe-mounted filament assembly for electron ionization experiments. The data system and analog electronics are components of the Extrel InstrumentsFTMS-2000 spectrometer. Of particular interest here, a differential high-power excitation amplifier with 110-V, maximum output capable of !c under computer control was stepwise attenuation through 31B used. For axial ejection studies, the trap potential was set to 0.5 V for various cells including the capacitively coupled open cell, the identical open cell without the distributed excitation, and a closed orthorhombic cell of 5-cm X 5-cm X 10-cm dimension. Perfluorotributylamine (PFTBA)was introduced through a precision leak valve to 5 X lop8Torr. Electron beam currents of a few microamps and beam periods of a few milliseconds were selected to generate ion populations well below the space charge limit of the cell. A linear frequency sweep at 3200 Hz/ps over a full 2.667-MHz bandwidth was used to excite the ions. In successive experiments, excitation power was varied over a range from 110 V,, at O-dBattenuation to 20 V at 15-dB attenuation. Excitation was followed by broad-band ietection with a 2.667-MHz bandwidth and acquisition of 16K data points. Transients were processed by applying a sine bell apodization function, appending
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16K zerm to the transient, and then performing a magnitude mode Fourier transform. Peak intensities were extracted from each spectrum for ions at m / z 69,131, 219, 264,414, and 502. Laser desorption spectra were acquired with the open cell by using a probe-mounted fiber-optic-basedlaser desorption assembly (29) that was inserted to within a few centimeters of the open cell. Samples of dilaurylthiodipropionate (DLTDP) mixed with KBr were deposited on a stainless steel probe tip. A single 8-ns, 23-mJ pulse of 1064-nm radiation from a Q-switched Nd:YAG laser (Model DCR-11, Spectra Physics) was transmitted through a fiber optic onto the sample. Calculated irradiance for a 0.9-mm2 spot size was about 3 X lo8 W/cm2. Cation-attached DLTDP spectra were acquired with 3.0-V potentials applied to the trap electrodes, broad-band swept excitation as above, and 64K data points for an 8 0 0 - Ebandwidth. Subsequentfiringsat the same spot on the probe yielded stainless steel ions that were trapped in a 4.0-V well and detected under standard broad-band conditions.
RESULTS AND DISCUSSION In the original paper on the open cell (2), SIMION calculations of axial electric field components associated with potentials applied during excitation were generated. The data indicated that an elongated open cell of aspect ratio 2 exhibited axial fields about a factor of 2 lower in magnitude than its closed cell counterpart. Similar SIMION calculations were performed for the capacitively coupled open cell. Shown in Figure 2 are comparison isopotential contours with f100 V applied to excitation plates of the conventional elongated closed cell, uncoupled open cell, and capacitively coupled elongated open cell. For the closed cell and uncoupled open cell, most of the excitation field lines must ultimately terminate at the trapping electrodes positioned adjacent to the excitation plates. This necessarily introduces field curvature and thus an axial component to the excitation field. In contrast, for the coupled open cell the excitation is also applied to the trap plates and a uniform collinear series of isopotential contours is observed to extend well beyond the trap potential maxima located near the center of the trapping electrodes. In effect, with respect to the axial dimension of the cell, the excitation field conforms to the infinite electrode approximation of an ideal trapped-ion cell. Presented in Figure 3a are comparison SIMION-generated profiles of the axial electric field at a radial displacement that is 10% of the way from the center line of the cell to the excitation plates for the same cell configurations described in Figure 2. As was discussed, the maximum amplitude of the uncoupled open cell electric field is approximately half that of the closed cell profile, which translates experimentally
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
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to a modest reduction in axial ejection for a given effective excitation power. In contrast, the theoretical coupled open cell profile suggests that axial ejection during the distributed excitation event should be nonexistent since the responsible axial electric field component is absent. Experimental data to verify the predictions based upon the Figure 3a data are presented in Figure 4, in the form of absolute abundances of PFTBA ions as excitation energy is increased. Presented in Figure 4a is the result typical of broad-band excitation in closed trapped-ion cells, in this case for a 1.0-V trapping potential in an elongated cell at 2.0-T magnetic field strength. Approximately correct relative abundances are observed only a t very low excitation energies that generate spectra with significantly reduced signal-bnoise ratios. At higher excitation voltages, severe axial ejection is evident from the selective ejection of low-mass ions. Presented in Figure 4b are abundance profiles for the uncoupled open cell at an effective trap potential of 0.4 V. At these low trap potentials the closed cell used to acquire the data in Figure 4a was incapable of generating spectra with mlz 69 as the base peak at any excitation level that generated detectable spectra. In contrast, relative abundances for the uncoupled open cell achieve fairly good ratios at excitation levels below 50% of optimum levels. Presented in Figure 4c are PFTBA ion abundance profiles acquired at a 0.4-V effective trapping potential with the ca-
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pacitively coupled open cell. Near ideal abundance ratios are observed with proportional increases in absolute mass abundance observed over the entire range of excitation levels. The rapid truncation of the signal beyond about 80-V excitation potential corresponds to the expected mass-independentradial ejection of all ions. Profiles exhibiting this behavior are not observed with conventional closed cells unless trap potentials are increased and efforts are taken to collapse the ion popu-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
a)
b)
In the first case a gas-phase reaction between neutrals and preformed ions occurs in the cell, and in the second case, plasma ignition a t the metal surface is followed by plasma shielding of the trapping electric fields to introduce low-energy ions. Both processes are easily accommodated with the open cell and suggest the need for additional studies to contrast injection efficiency for the open cell compared to the closed cell. What the open cell data in Figure 5 also indicate is that the physical properties of the trapping electrodes are not necessarily important in defining mechanisms by which externally generated ions are collected in cells with static trapping potentials. The open cell arrangement simplifies a n evaluation of possible trapping mechanisms by eliminating complications from trap plate surfaces or materials.
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REFERENCES
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lation to the bottom of the potential well (25, 26). An obvious defect of the original uncoupled open cell design is poor excitation field uniformity over the z axis length of the cell (2). This is apparent from the isopotential contours in Figure 2b and is verified from the sIMrON-generated profiles of the effective radial excitation field presented in Figure 3b. An advantage of distributed excitation in the capacitively coupled open cell is that the excitation power is not attenuated a t the trapping-well boundaries, as the profile in Figure 3b shows. A rough estimate of the gain in effective excitation power is obtained by integrating the area under the curves in Figure 3b over the length of the trapping-potential well. Such an integration suggests a relative effective excitation power in the ratio of 1.00:0.81:0.49 for the ratio of coupled open cel1:closed celluncoupled open cell. Although this ratio is difficult to verify experimentally with the data in Figure 4 because of competing axial ejection processes, the decreased excitation power necessary for optimum signal level in the coupled cell is evident. For example, the excitation potential maxima in Figure 4b,c of 87 and 70 V, respectively, indicate that the same effective excitation power is achieved with a 20% lower excitation voltage for the coupled versus uncoupled open cells. As mentioned in the Introduction, other cell configurations and experimental procedures have been suggested to alleviate axial ejection of ions. However, we believe the capacitively coupled open cell arrangement offers several advantages including design simplicity because of the reduced number of electrodes. In addition, detection electrodes are not screened by the trap plates or additional electrodes; so in principle, detection sensitivity should be improved. Finally, perpendicular trapping electrodes that may behave as physical barriers to injection of charged and neutral species, or worse, as Russell has suggested (30),alter ion trajectories with consequent deterioration of FT-ICR performance, are eliminated. As evidence that the open cell is indeed compatible with injection of externally generated ions, laser desorption spectra were acquired for both cation-attached organic molecules and for metals, as shown in Figure 5. These LDI/FTICR spectra result from very different ionization and trapping mechanisms.
(1) Beu, S.C.; Laude. D. A., Jr. Am. SOC.Mass Spectrom. Annu. Conf. Mass Spectrom. Allied Topics. 39th 1991; p 130-131. (2) Beu, S . C.; Laude, D. A., Jr. Znt. J. Mass Spectrom. Zon Processes, in Dress. (3) Cgmisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282-283. (4) Comisarow, M. B. Znt. J . Mass Spectrom. Zon Phys. 1981, 37, 251-257. (5) Hunter, R. L.; Sherman, M. G.: McIver, R. T., Jr. Znt. J. Mass Spectrom. Ion Phys. 1983, 5 0 , 259-274. (6) Hofstadler, S.A.; Laude, D. A.. Jr. Znt. J. Mass Spectrom. Zon Processes 1990, 101, 65-78. (7) Lee, S . H.; Wanczek, K.-P.; Hartmann, H. Adv. Mass Spectrom. 1980, 88,1645. ( 8 ) Kofel, P.; Allemann, M.: Kellerhals, Hp.; Wanczek, K. P. Znt. J. Mass Spectrom. Zon Processes 1988. 74,1-12. (9) Van Dyck, R. S . ; Schwinberg, P. 8. Phys. Rev. Lett. 1981, 47, 395-398. (IO) Rempel, D. L.; Ledford, E. B., Jr.; Huang, S. K.; Gross, M. L. Anal. Chem. 1987, 59. 2527-2532. (11) Grosshans, P. B.; Wang, M.; Marshall, A. G. Am. SOC.Mass Spectrom. Annu. Conf. Mass Spectrom. Allied Topics, 36th 1988, 592-593. (12) (a) Cody, R. B.; Kinsinger, J. A.; Ghaderi, S.; Amster, I.J.; McLafferty, F. W.; Brown, R. S. Anal. Chim. Acta 1985, 178,43-66. (b) Nicolet Analytical Instruments, US. Pat. No. 4581533. (13) Naito, Y.; Inoue, M. Am. SOC.Mass Spectrom. Annu. Conf. Mass Spectrom. Allied Topics, 36th 1988, 608-609. (14) Yang, S. S . ; Rempel, D. L.; Gross, M. L. Am. SOC.Mass Spectrom. Annu. Conf. Mass Spectrom. Allied Topics 36th 1988, 586-587. (15) Wang, M.; Marshall, A. G. Anal. Chem. 1989, 67, 1288-1293. (16) Caravatti, P.; Allemann, M. Org. Mass Spectrom. 1991, 26, 514-518. (17) Wang, M.; Marshall, A. G. Anal. Chem. 1990, 62,515-520. (18) Hanson, C. D.; Castro, M. E.; Kerley, E. L.; Russell, D. H. Anal. Chem. 1990, 52,520-526. (19) Byrne, J.; Farago, P. S. Proc. Phys. SOC.1985, 86,801-815. (20) Malmberg, J. H.: deGrassie, J. S. Phys. Rev. Lett. 1975, 35, 577. (21) deGrassie, J. S . ; Malmberg, J. H. Phys. Rev. Lett. 1977, 3 9 , 1077. (22) deGrassie, J. S.; Malmberg, J. H. Phys. Nuids 1980, 63-81. (23) Gabrielse, G.;Mackintosh, F. C. Znt. J. Mass Spectrom. Zon Processes 1984, 57, 1-17. (24) Gabrielse, G.; Haarsma, L.; Rolston, S. L. Znt. J . Mass Spectrom. Zon Processes 1989, 88,319-332. (25) Smalley, R. E. Anal. Znstrum. 1988, 77, 1-12. (26) Gross, M. L.; Hwng, S. K.: Rempel, D. L. Znt. J . Mass Spectrom. Zon Processes 1988, 70,163-184. (27) Sommer, H.; Thomas, A.; Hipple, J. A. Phys. Rev. 1951, 82, 697-702. (28) Beu, S. C.: Laude, D. A., Jr. Znt. J. Mass Spectrom. Zon Processes 1991, 704, 109-127. (29) Hogan, J. D.; Beu, S . C.; Laude, D. A,, Jr.; Majidi, V. Anal. Chem. 1991, 83,1452-1457. (30) Hanson, C. D.; Kerley, E. L.; Hanson, C. D.; Castro, M. E: Russell, D. H. Anal. Chem. 1989, 67, 2528-2534. ~
RECEIVED for review August 12, 1991. Accepted October 31, 1991. This work is supported by the Arnold and Mabel Beckman Foundation, Welch Foundation (Grant F-1138), and National Science Foundation (Grants CHE9013384 and CHE9057097). The FT-ICR was constructed with funds provided by the Texas Advanced Technology and Research Program (No. 4515).
