Article pubs.acs.org/ac
Macromolecular Ion Accelerator Yun-Fei Hsu,† Jung-Lee Lin,† Szu-Hsueh Lai,§ Ming-Lee Chu,‡ Yi-Sheng Wang,† and Chung-Hsuan Chen*,† †
Genomics Research Center, and ‡Institute of Physics, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei, 115, Taiwan § Department of Chemistry, National Taiwan University, Taipei, Taiwan (R.O.C.) ABSTRACT: Presented herein are the development of macromolecular ion accelerator (MIA) and the results obtained by MIA. This new instrument utilizes a consecutive series of planar electrodes for the purpose of facilitating stepwise acceleration. Matrix-assisted laser desorption/ionization (MALDI) is employed to generate singly charged macromolecular ions. A regular Z-gap microchannel plate (MCP) detector is mounted at the end of the accelerator to record the ion signals. In this work, we demonstrated the detection of ions with the mass-to-charge (m/z) ratio reaching 30 000 000. Moreover, we showed that singly charged biomolecular ions can be accelerated with the voltage approaching 1 MV, offering the evidence that macromolecular ions can possess much higher kinetic energy than ever before.
ccelerator, which was first introduced in the early 20th century, is one of the primary instruments for research in high-energy and nuclear physics. The first particle accelerator can be traced back to 1932 and was constructed by Cockcroft and Walton, who produced a proton beam of 400 keV and achieved the first nuclear reaction.1 Van de Graaff and tandem accelerators were subsequently developed to attain much higher proton kinetic energies.2−4 Later, these particle accelerators were employed as the most important facilities in the field of atomic and fundamental physics. In addition to the aforementioned apparatuses, Ernest Lawrence and Stanley Livingston also worked on the cyclotron configuration to accomplish acceleration in 1932.5 A few years later, the linear accelerators with electron energies higher than gigavolts were also built.6 During the past several decades, accelerators have been extensively applied to every major scientific field, including material characterization and therapeutic treatments. Various types of electron accelerators have played indispensable roles in many critical discoveries. More recently, particle accelerators have also been used for disease diagnosis7 and cancer treatment.8 Acceleration can be achieved in different ways for the charged particles. For electrostatic acceleration, electrons and ions are produced through discharge. For van de Graaff accelerators, ions are put onto a belt to achieve high voltages, and then, negative ions are converted into positive ions through collision processes. However, molecular ions do not survive well in the process of discharge or collision. In addition, parent biomolecular ions are even more unstable under discharge conditions. As for large intact biomolecular ions, their ion
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beams are also very difficult to produce. Accelerating these large biomolecular ions using a cyclotron configuration is expected to be very difficult because a magnetic field exceeding 20 T would be needed. Thus, a cyclotron has never been used to accelerate biomolecular ions. Due to these challenges, the evolution of accelerators has not included large molecular ions. Although there are more than 10 000 different accelerators worldwide, only electron and atomic ion accelerators are currently available. Few molecular ion accelerators have been reported.9 In this study, we report the first development of a macromolecular ion accelerator (MIA) that can impart high energy to large biomolecules and/or polymer ions. Unlike present accelerators that operate through the use of electrostatics,10 oscillating fields,11 and cyclotron devices, MIA is operated by applying multiple pulsed voltages to achieve stepwise acceleration. The critical role of biomolecules for biological functions has placed their investigation at the forefront of biological research. Detection is preferred to be based on their intrinsic properties. Mass, for example, is a key parameter to identify various sample species, ranging from atoms, molecules, and noncovalent molecular complexes12−15 to nano/microparticles.16,17 Matrixassisted laser desorption/ionization (MALDI)18 has been broadly adopted for the ionization of large molecules. MALDI is operated with a pulsed laser and is often coupled with a time-of-flight (TOF) spectrometer to detect large Received: April 18, 2012 Accepted: May 31, 2012 Published: May 31, 2012 5765
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proteins.19,20 Electron multipliers, such as channeltrons and microchannel plates (MCPs), are widely used as detectors by producing secondary electrons21−23 when the surfaces of these detectors are hit by ions at high velocity. The efficiencies of these electron multipliers, however, are strongly dependent on the velocity of the ions. The detection efficiency approaches zero when the velocity of an ion is significantly lower than 104 m/s.24 Therefore, the demand for improved detection efficiency should be addressed. Progress has been made in detection technology to circumvent the low sensitivity for ions at high mass-to-charge ratios (m/z). Two types of detectors, cryogenic and inductive,25,26 have been used as alternatives for the detection of large biomolecules. For cryogenic detectors, doubly charged ions with the mass reaching 4 MDa have been demonstrated.27 However, the response time is slow for cryogenic detectors, and electronic noise is high for induction detectors.28,29 MIA offers a novel, strategic method by directly enhancing the ion energy, leading to improvement in the detection efficiency.
Figure 1. Macromolecular ion accelerator. The probe is mounted on the first plate with 25 kV DC applied, and the sample is exposed to the third harmonic of the Nd:YAG laser (355 nm) for desorption/ ionization. The 2nd, 5th, 8th, ... plates (blue) are wired together and connected to the first switch (SW1). The second switch (SW2) and third switch (SW3) are connected to two other sets of electrodes, which are represented in green and turquoise, respectively.
EXPERIMENTAL SECTION Materials. Sinapinic acid (SA), all-trans retinoic acid, tetrahydrofuran (THF), bovine serum albumin (BSA), lactoferrin, and fibrinogen were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO 63103). Immunoglobulins G, A, and M (IgG, IgA, and IgM) were obtained from Fitzgerald Industries International. Acetonitrile (ACN) and trifluoroacetic acid (TFA) were purchased from Merck & Co., Inc. (USA). Double distilled water (ddH2O) was used throughout the experiment. Sample Preparation. The method employed for MALDI sample preparation was “dried-droplet” instead of a vacuum drying process. In the demonstration of the accelerator, SA was dissolved in 50% ACN in H2O to a final concentration of 50 nmol/μL with 0.1% TFA as an additive. Each protein was dissolved in ddH2O to a final stock concentration of 50 pmol/ μL. IgG, IgA, and IgM were prepared as 20, 20, and 1.8 pmol/ μL stocks, respectively. Typical samples for MALDI analysis were prepared by combining 8 μL of matrix solution with 2 μL of protein in an Eppendorf tube. Then, 2 μL of this mixture was applied to the sample probe and dried with ambient air. The sample probe was then mounted in a homemade vacuum chamber. A frequency-tripled Nd:YAG laser (wavelength = 355 nm, ∼80 μJ/pulse in energy, LOTIS TII, LS-2137U) was used to irradiate the samples. In the experiment with solely all-trans retinoic acid, the sample was prepared at 150 nmol/μL in THF. After air-drying, the sample was exposed to laser firing and subjected to acceleration with six different sets of acceleration pulses. Instrumentation. The experimental schematic of the accelerator is shown in Figure 1. The accelerator consisted of a series of plates with equal spacing between adjacent ones. These plates were powered by time-regulated pulsed voltages to achieve acceleration. The pulses were applied to the electrodes using fast high voltage switches (Behlke Electronics, HTS 65103-GSM/HTS 301-03 GSM) wired to power supplies (Glassman High Voltage Inc., EK60R10). All of the pulses were edited and preset by an arbitrary waveform generator (WW2572, Tabor Electronics Ltd.). When the desorbed ions passed a specific electrode, a pulsed voltage was applied to conduct acceleration. With 40 plates, the ions acquired a maximum of 1.2 MeV by repeatedly experiencing a series of kV pulses. Because the spacing between two electrodes was 2.5 cm,
the length of the entire acceleration region was approximately 1 m. Large molecular ions are produced by MALDI with the sample probe mounted on the first plate. When the laser irradiates the sample, ions are desorbed from the surface of the probe tip and accelerated by a selected DC voltage (e.g., 25 kV). A series of square plates (4 cm × 4 cm) are employed as electrodes. The holes (1 cm in diameter) at the center are welded with 95% transmission meshes. To accelerate the ions, a sequential series of selected pulsed voltages (e.g., 30 kV) was applied stepwise in synchronization with the movement of the ions. The kilovolts regime, rather than Megavolts, is adopted for the safety concern. The breakdown and arcing should be prevented so that the stability of the electric field can be maintained. Thus, the pressure inside of the accelerator is held at 10−7 Torr to ensure the sustainability of the high-voltage pulses. Programmable pulses consisting of a sequence of timevarying fields between two adjacent electrodes are applied to increase the kinetic energy of the selected ions. Calculations on the travel time of specific ions are used as the first-step approximation to determine all pulse durations and the timegaps between pulses. Then, fine-tuning the pulse sequences is executed to optimize the signals detected by the MCP detector, which is mounted at the end of the accelerator. More specifically, the timing of switching on a pulse, which is significantly suited for an efficient acceleration, must be examined empirically besides calculation. Moreover, the timing of a single pulse is in a tight conjunction with the rest of the pulses so that adjusting one pulse leads to further adjustment of other pulses. Thus, the timing of the overall pulses demands careful inspection until the best signal is obtained. In the physical arrangement, the first electrode serves as a sample plate. Electrode 2 is wired to electrode 5 and then to electrode 8. With two electrodes as spacing, the first set of electrodes is composed of electrodes 2, 5, 8 and so on. In this similar way, the second set of electrodes comprises electrodes 3, 6, 9, and so on. The third set of electrodes consists of electrodes 4, 7, 10, and so on. As for the electronics, the first power supply, modulated with the first programmable switch, is connected to electrodes 2, 5, 8, and so on. The second and third modulations are connected to electrodes 3, 6, 9, and so on and 4, 7, 10 and so on, respectively. Two arbitrary waveform generators (AWG)
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are used to pre-edit the waveforms, which are then transmitted to the switches for acceleration. The typical wiring of an MIA is displayed in Figure 2. When the desorbed ions left the sample plate and reached the second
Figure 3. Calculation of acceleration with IgG ions examined. This simulation is shown on the potential energy surface (green). At the beginning of the simulation, DC 25 kV is applied to the first plate while the second plate is grounded, as the contour lines (red) shows. When the calculation of the trajectory (blue) is triggered, IgG ions start to travel toward the detector. Pulsed voltages are switched on in synchronization with the motion of ions. As seen, the trajectories of ions are computed by applying 26 pulses of 30 kV. Ions possessing the initial kinetic energy of 0.2 eV are generated with a cone angle of ∼15°.
Figure 2. Electronics. A series of accelerations are programmed and initiated by the firing of the laser. SW1, SW2, and SW3 are wired to the first, second, and third power supplies (PS1, PS2, and PS3), respectively. An arbitrary function generator (AWG) is used to edit the output sequences. CH1 and CH2 of AWG1 represent the output channels for the first and second switches (SW1 and SW2), respectively. CH1 of AWG2 indicates the output channel for the third switch (SW3).
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RESULTS AND DISCUSSIONS Experiments were conducted to evaluate the performance of the MIA system. For this purpose, we selected several immunoglobulins (Igs) and glycoproteins to examine the effect of increased kinetic energy, which is determined by the number of pulses. Six spectra of IgG (MW: ∼150 kDa) were obtained, and the results indicated signal increase and arrival time shortening (Figure 4a). Good agreement was obtained between calculation results of the terminal velocities and experimental measurements of the arrival times. It verified the efficiency of the acceleration. Similar examination of the acceleration was conducted with fibrinogen (MW: ∼340 kDa)30 (Figure 4b). Figure 4a,b demonstrated the ability of the method to accelerate biomolecular ions up to 200 kV. By taking into account the acceleration voltage we applied to the electrodes, the position of the detector, the timing sequences of the acceleration pulses, and the simulation data, we can calculate the ion energies based on their arrival time. Figure 4c shows the evolution of the signal for secretory IgA (sIgA) (MW: ∼385 kDa)31 as acceleration energy increased from 145 to 325 kV. An actual traveling time of 67 μs was measured with a loaded accelerating energy of 325 kV, which closely matched the calculated traveling time of 66.7 μs. With this approach, we further investigated the acceleration of IgM (MW: ∼980 kDa)32,33 using 565 kV (Figure 4d). A clear peak that agreed well with the calculated result of 159 μs was observed. Therefore, the successful acceleration of molecular ions with m/z approaching one million and detected by a regular MCP detector was demonstrated with MIA. It is apparent that the width of the arrival time becomes narrower as the acceleration voltage increases. This phenomenon is attributed to the high-selectivity of the acceleration electric field. The ions can only be detected after they successfully pass through the exactly controlled pulsed electric fields. In addition, the more pulses we apply, the higher extent of synchronization of the ion motion with the electric fields is required. Thus, the narrow width of the arrival time distribution reflects that these detected ions are the most effectively accelerated. Furthermore, we performed experiments on IgG and fibrinogen ions with energies up to 1 MeV (Figure 4e,f). Narrower peaks were observed for ions experiencing higher acceleration than those with lower acceleration. The full width half-maximum (fwhm)
electrode, the energy of the ions was approximately 25 keV. After the ions just passed through the second plate, the pulsed voltage of 30 kV was applied while the third plate was kept grounded. When these ions approached the third plate, the total energy of the ions was expected to be approximately 55 keV. With a kinetic energy of 55 keV, these ions would undergo one more identical step of potential gradient between the third and fourth plates. As they arrived at the fourth electrode, a kinetic energy of approximately 85 keV was expected to be achieved. Similar accelerations were repeatedly conducted by subsequently synchronizing the pulses with the motion of the ions. Thus, an accelerating gradient of 1.2 MV/m was established. Only four power supplies were employed for the whole apparatus instead of each electrode being connected to a separate power supply to dramatically reduce the cost. Simulation. To simulate the behavior of macromolecular ions under acceleration, a numerical program (SIMION 8.0) is utilized to compute the electric fields and the trajectories of ions for the schematic shown in Figure 1. The simulation trajectory is given in Figure 3. The simulation of acceleration is conducted with DC 25 kV at the sample plate and 26 pulses of 30 kV for the subsequent electrodes, corresponding to the total energy of 805 keV. This simulation is shown on the potential energy surface, allowing the observation of acceleration gradients. The accelerating gradient built up between the first and the second plate is characterized by the red contour lines. To allow the visibility of the trajectories, all pulsing electric fields are not shown in this figure. On the basis of the simulation, the travel time and the final kinetic energy can be recorded when the ions reach the detector. In this study, for those effectively accelerated ions, the total energy that they obtain is around 795 keV, which exhibits the energy loss is within 2%. Thus, this simulation depicts fully explicit acceleration of biomolecular ions. 5767
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Figure 5. The acceleration of IgM multimer ions at 500 kV. (a) Monomer, (b) dimer, (c) trimer, and (d) tetramer. All of the multimers were singly charged and naturally produced using MALDI, so no chemical bonding existed among individual IgM ions. The horizontal axes represent the time scale; the vertical axes represent the signal intensity. The arrival times for the oligomeric species were approximately 126.6, 176.0, 213.1, and 244.7 μs, respectively. The slight decline in signal intensities could be attributed to the different abundances of the multimers.
that the signal remained detectable; even, the mass reached approximately 4 MDa. In general, the detection of molecular ions with several megadaltons is not possible by electron multipliers under normal conditions because of the extremely low velocity. With the aid of the MIA, the detection of fairly large molecular ions can be easily accomplished. To extend the mass detection capability of MIA, the detection of molecular weights exceeding 10 MDa was attempted. Although protein complexes, such as the nuclear pore complex, possess the molecular weights more than 10 MDa,34 they are not suited for the MALDI process. To overcome this barrier, we utilized all-trans retinoic acid as the sample, which is known for self-polymerization in MALDI.35 The agglomeration of all-trans retinoic acid is generated without a special sample pretreatment. Results displayed in Figure 6 show the successful acceleration for 0.5, 1, 5, 10, 20, and 30 MDa. For each molecular weight, the programming of acceleration pulses in advance can be regarded as a specific selection of mass. Only the molecular ions that move in synchronization with the movement of the accelerating fields can survive and be detected. These results demonstrate that MIA can be used for the analysis of very heavy ions. With agglomerated molecular ion acceleration, we demonstrated the first detection of ions with a mass-to-charge ratio (m/z) reaching 30 000 000, which has not previously been reported. Thus, the acceleration and detection of molecular ions up to 30 MDa can be a milestone not only in the development of accelerators but also in mass spectrometry detection technology.
Figure 4. Temporal evolution displaying the effect of acceleration. (a) The acceleration of IgG with various acceleration energies from 25 to 205 kV. (b) The acceleration of fibrinogen with energies growing from 55 to 205 kV. (c) The examination of IgA with acceleration energies up to 325 kV. (d) The acceleration of IgM ions at 565 kV. (e) The acceleration of singly charged IgG ions at 325, 585, and 985 kV. (f) The acceleration of fibrinogen ions at 585, 745, and 985 kV. The horizontal axes represent the time scale; the vertical axes represent the signal intensity. It is obvious that the width of the arrival time distribution exhibits significant reduction by increasing the acceleration voltages. For example, it is found that the width is reduced by more than one-half under the comparison of the acceleration voltage with 585 and 985 kV.
of peaks in Figure 4e,f are ∼1000 and ∼770 ns, respectively, for the ion energy reaching 985 keV. These values depict the promise of the use of these ion beams for further applications due to their short time distributions. These results revealed that singly charged biomolecular ions could experience acceleration energies up to approximately 1 MeV. It should be mentioned that the configurations used for the demonstrations of Figure 4a,c are different in length. In Figure 4a, the accelerator was composed of 10 electrodes, and the maximal voltage was applied at 205 kV. This result was our initial demonstration of MIA. Later on, the accelerator was extended by including 37 electrodes to reach the maximal voltage of 985 kV, as the result in Figure 4c. Thus, the length of the accelerator tripled, leading to the difference between Figure 4a,c. Similarly, this variation of the length gave rise to the difference of the results in Figure 4b,f. We also evaluated a similar approach for heavier ions. Singly charged multimers of IgM that could be generated using MALDI were also examined. Figure 5 depicts the acceleration of monomers, dimer, trimers, and tetramers of IgM ions. In Figure 5a−d, the signals peaked at 126.6, 176.0, 213.1, and 244.7 μs, respectively. One apparent benefit of the use of our MIA when analyzing multimeric ions of more than 1 MDa was
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CONCLUSION In summary, we have experimentally demonstrated the acceleration of large biomolecular/molecular ions for the first time. This study has moved the evolution of accelerators forward toward producing energetic biomolecular/polymeric ions, as the acceleration energy can be optimized by carefully adjusting the pulses. MIA also increases the detection efficiency for high-mass ions, which have long suffered from poor detection. In general, MIA introduces the ability to analyze a sample of the molecular weight at 107 Da by imparting sufficiently high energy. Overall, this approach provides a new 5768
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(7) Chang, S. D.; Meisel, J. A.; Hancock, S. L.; Martin, D. P.; McManus, M.; A., J., Jr. Neurosurgery 1998, 43, 28−34. (8) Sheng, K.; Molloy, J. A.; Read, P. W. Int. J. Radiat. Oncol. Biol. Phys. 2006, 65, 917−923. (9) Saitoh, Y.; Mizuhashi, K.; Tajima, S. Nucl. Instrum. Methods Phys. Res., Sect. A 2000, 452, 61−66. (10) Elmore, D.; Fulton, B. R.; Clover, M. R.; Marsden, J. R.; Gove, H. E.; Naylor, H.; Purser, K. H.; Kilius, L. R.; Beukens, R. P.; Litherland, A. E. Nature 1979, 277, 22−25. (11) Geddes, C. G. R.; Toth, C.; van Tilborg, J.; Esarey, E.; Schroeder, C. B.; Bruhwiler, D.; Nieter, C.; Cary, J.; Leemans, W. P. Nature 2004, 431, 538−541. (12) Peng, W.-P.; Lin, H.-C.; Lin, H.-H.; Chu, M.; Yu, A. L.; Chang, H.-C.; Chen, C.-H. Angew. Chem., Int. Ed. 2007, 119, 3939−3943. (13) Chingin, K.; Frankevich, V.; Balabin, R. M.; Barylyuk, K.; Chen, H.; Wang, R.; Zenobi, R. Angew. Chem., Int. Ed. 2010, 122, 2408− 2411. (14) Barrera, N.; Di Bartolo, N.; Booth, P.; Robinson, C. Science 2008, 321, 243−249. (15) Stengel, F.; Aebersold, R.; Robinson, C. V. Mol. Cell. Proteomics 2012, 11, R111.014027. (16) Lin, H.-C.; Lin, H.-H.; Kao, C.-Y.; Yu, A. L.; Peng, W.-P.; Chen, C.-H. Angew. Chem., Int. Ed. 2010, 122, 3538−3542. (17) Uetrecht, C.; Heck, A. Angew. Chem., Int. Ed. 2011, 50, 8248− 8262. (18) Jespersen, S.; Niessen, W. M. A.; Tjaden, U. R.; van der Greef, J.; Litborn, E.; Lindberg, U.; Roeraade, J.; Hillenkamp, F. Rapid Commun. Mass Spectrom. 1994, 8, 581−584. (19) Boesl, U.; Grotemeyer, J.; Walter, K.; Schlag, E. W. Instrum. Sci. Technol. 1987, 16, 151−171. (20) Ludwig, K.; Habbach, S.; Krieglstein, J.; Klumpp, S.; König, S. PLoS One 2011, 6, e23612. (21) Shockley, W.; Pierce, J. R. Proc. IRE 1938, 26, 321−332. (22) Tang, X.; Beavis, R.; Ens, W.; Lafortune, F.; Schueler, B.; Standing, K. G. Int. J. Mass Spectrom. Ion Process. 1988, 85, 43−67. (23) Geno, P. W.; Macfarlane, R. D. Int. J. Mass Spectrom. Ion Process. 1989, 92, 195−210. (24) Fraser, G. W. Int. J. Mass Spectrom. 2002, 215, 13−30. (25) Twerenbold, D. Rep. Prog. Phys. 1996, 59, 349. (26) Benner, W. H. Anal. Chem. 1997, 69, 4162−4168. (27) Aksenov, A.; Bier, M. J. Am. Soc. Mass. Spectrom. 2008, 19, 219− 230. (28) Giaever, I.; Hart, H. R.; Megerle, K. Phys. Rev. 1962, 126, 941. (29) Twerenbold, D.; Gerber, D.; Gritti, D.; Gonin, Y.; Netuschill, A.; Rossel, F.; Schenker, D.; Vuilleumier, J.-L. Proteomics 2001, 1, 66−69. (30) Everse, S. J.; Spraggon, G.; Veerapandian, L.; Riley, M.; Doolittle, R. F. Biochemistry 1998, 37, 8637−8642. (31) Macpherson, A. J.; Slack, E. Curr. Opin. Gastroenterol. 2007, 23, 673−678. (32) Wiersma, E. J.; Collins, C.; Fazel, S.; Shulman, M. J. J. Immunol. 1998, 160, 5979−5989. (33) Nelson, R. W.; Dogruel, D.; Williams, P.; Beavis, R. Rapid Commun. Mass Spectrom. 1994, 8, 627−631. (34) Alber, F.; Dokudovskaya, S.; Veenhoff, L. M.; Zhang, W.; Kipper, J.; Devos, D.; Suprapto, A.; Karni-Schmidt, O.; Williams, R.; Chait, B. T.; Sali, A.; Rout, M. P. Nature 2007, 450, 695−701. (35) Nielen, M. W. F. Mass Spectrom. Rev. 1999, 18, 309−344.
Figure 6. The acceleration of polymeric ions formed by all-trans retinoic acid at 500 kV. Owing to its self-polymerization, all-trans retinoic acid can naturally form large polymers (a) 0.5 MDa, (b) 1 MDa, (c) 5 MDa, (d) 10 MDa, (e) 20 MDa, and (f) 30 MDa. All of the polymers were singly charged and spontaneously created in the MALDI process. The acceleration pulses were preprogrammed to specifically select these polymers. The horizontal axes represent the time scale; the vertical axes represent the signal intensity. The arrival times for the polymeric ions were approximately 115, 128, 293, 415, 590, and 796 μs, respectively. These times were in close agreement with the expected arrival times.
perspective on the issue of inspecting complex biomolecular and/or molecular ions at high-energy regime. This approach can also have the potential to provide a new method to study the structure of intact large biomolecules by collision-induced dissociation (CID) at this energy regime.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 011-886-2-27871200. Fax: 011-886-2-27899923. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully thank Professor Yuan T. Lee for his valuable discussions and suggestions. We also thank Ming-Hsin Li and Chiu-Wen Chen for their help with instrument design and component testing. This work was financially supported by Genomics Research Center in Academia Sinica, the National Science Council (grant no. NSC 99-2113-M-001-002-MY3), and the National Health Research Institutes (grant no. NHRIEX101-9803EI) of Taiwan, R.O.C.
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REFERENCES
(1) Cockcroft, J. D.; Walton, E. T. S. Proc. R. Soc. London, Ser. A 1932, 136, 619−630. (2) Doyle, B. L.; Peercy, P. S. Appl. Phys. Lett. 1979, 34, 811−813. (3) Cho, A. Science 2006, 312, 190−191. (4) White, M. G.; Isaila, M.; Prelec, K.; Allen, H. L. Science 1971, 174, 1121−1123. (5) Lawrence, E. O.; Livingston, M. S. Phys. Rev. 1932, 40, 19−35. (6) Ginzton, E. L.; Hansen, W. W.; Kennedy, W. R. Rev. Sci. Instrum. 1948, 19, 89. 5769
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