Anal. Chem. 1997, 69, 4156-4161
An Ion Trap Interface for ESI-Ion Mobility Experiments Cherokee S. Hoaglund, Stephen J. Valentine, and David E. Clemmer*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A method for improving the sensitivity of ion mobility measurements of ions generated from continuous ionization sources such as electrospray ionization is described. The method involves an ion trap interface between the continuous source and the pulsed experiment. The trap allows the electrosprayed ion beam to be accumulated between pulses of the mobility experiment and provides tightly focused (both spatially and temporally) ion pulses for initiating experiments. Using this method, we have improved signal-to-noise ratios by factors of ∼10-30 compared with those of existing approaches and achieved experimental duty cycles of nearly 100%. With these improvements, it is possible to record complex ion mobility distributions in less than 10 s, and we have obtained detection limits of 1.3 pmol for the oligosaccharide maltotetraose. A detailed description of the experimental arrangement is given. An application of the improved methods in which collision-induced fragmentation of biomolecules is monitored by ion mobility spectrometry is described.
ment performance. This interface is similar to one that has been recently developed for ESI-time-of-flight (TOF) MS experiments.13-15 Here, we show that the ESI-ion trap interface is a simple means of concentrating ions during the drift tube measurements; this provides duty cycles that approach 100% and improvements in signal-to-noise (S/N) ratios (compared with existing methods) by factors of ∼10-30. The ion trap also provides ion pulses that are tightly focused both spatially and temporally, allowing ions to be efficiently injected into the ion mobility instrument. A number of MS-based techniques are currently being used to study the structures of biological ions in the gas phase, including measurements of hydrogen/deuterium exchange levels,16-19 proton-transfer reactivity,20 ion scattering experiments in triple-quadrupole instruments,21-24 microscopy studies of the hillocks formed on surfaces after being exposed to high-energy ion collisions,25,26 measurements of kinetic energy release,27-29 and measurements of ion mobilities.10,30-37 The ESI-ion mobility method is a versatile approach. Direct structural information can
(1) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64. (2) Hagen, D. F. Anal. Chem. 1979, 51, 870. (3) Tou, J. C.; Boggs, G. U. Anal. Chem. 1976, 48, 1351. (4) Karpas, Z.; Cohen, M. J.; Stimac, R. M.; Wernlund, R. F. Int. J. Mass Spectrom. Ion Processes 1986, 83, 163. (5) St. Louis, R. H.; Hill, H. H. Crit. Rev. Anal. Chem. 1990, 21, 321. (6) Wittmer, D.; Chen, Y. H.; Luckenbill, B. K.; Hill, H. H. Anal. Chem. 1994, 66, 2348. (7) von Helden, G.; Hsu, M. T.; Kemper, P. R.; Bowers, M. T. J. Chem. Phys. 1991, 95, 3835. (8) Jarrold, M. F. J. Phys. Chem. 1995, 99, 11. (9) Clemmer, D. E.; Jarrold, M. F. J. Mass Spectrom. 1997, 32, 577. (10) Clemmer, D. E.; Hudgins, R. R.; Jarrold, M. F. J. Am. Chem. Soc. 1995, 117, 10141. (11) Dugourd, P.; Hudgins, R. R.; Clemmer, D. E.; Jarrold M. F. J. Sci. Instrum. 1997, 119, 3558.
(12) Discussions of ion traps have been given previously. See, for example: Paul, W.; Steinwedel, H. U.S. Patent 2939952, 1960. March, R.; Huges, R. Quadrupole Storage Mass Spectrometry; Wiley: New York, 1989. Cooks, R. G.; Kaiser, R. E., Jr. Acc. Chem. Res. 1990, 23, 213. Glish, G. L.; McLuckey, S. A. Int. J. Mass Spectrom. Ion Processes 1991, 106, 1. March, R. E. J. Mass Spectrom. 1997, 32, 351. (13) Michael, S. M.; Chien, M.; Lubman, D. M. Rev. Sci. Instrum. 1992, 63 (10), 4277. (14) Michael, S. M.; Chien, B. M.; Lubman, D. M. Anal. Chem. 1993, 65, 2614. (15) Quian, M. G.; Lubman, D. M. Anal. Chem. 1995, 67, 234A. (16) Winter, B. E.; Light-Wahl, K. J.; Rockwood, A. L.; Smith, R. D. J. Am. Chem. Soc. 1992, 114, 5897. (17) Suckau, D.; Shi, Y.; Beu, S. C.; Senko, M. W.; Quinn, J. P.; Wampler, F. M.; McLafferty, F. W. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 790. (18) Wood, T. D.; Chorush, R. A.; Wampler, F. M.; Little, D. P.; O’Connor, P. B.; McLafferty, F. W. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2451. (19) Cassady, C. J.; Carr, S. R. J. Mass Spectrom. 1996, 93, 3143. (20) For a recent review, see: Williams, E. R. J. Mass Spectrom. 1996, 31, 831. (21) Covey, T. R.; Douglas, D. J. J. Am. Soc. Mass Spectrom. 1993, 4, 616. (22) Collings, B. A.; Douglas, D. J. J. Am. Chem. Soc. 1996, 118, 4488. (23) Chen, Y.-L.; Collings, B. A.; Douglas, D. J. J. Am. Soc. Mass. Spectrom., in press. (24) Cox, K. A.; Julian, R. K.; Cooks, R. G.; Kaiser, R. E. J. Am. Soc. Mass Spectrom. 1994, 5, 127. (25) Quist, A. P.; Ahlbom, J.; Reimann, C. T.; Sundquist, B. U. R. Nucl. Instru. Methods in Phys. Res. B 1994, 88, 164. (26) Sullivan, P. A.; Axelsson, J.; Altmann, S.; Quist, A. P.; Sundquist, B. U. R.; Reinmann, C. T. J. Am. Soc. Mass Spectrom. 1996, 7, 329. (27) Kaltashov, I. A.; Fenselau, C. C. J. Am. Chem. Soc. 1995, 117, 9906. (28) Adams, J.; Strobel, F.; Reiter, A. J. Am. Soc. Mass Spectrom 1996, 7, 30. (29) Kaltashov, I. A.; Fenselau, C. C. Proteins 1997, 27, 165. (30) von Helden, G.; Wyttenbach, T.; Bowers, M. T. Science 1995, 267, 1483. (31) Wyttenbach, T.; von Helden, G.; Bowers, M. T. J. Am. Chem. Soc. 1996, 118, 8355. (32) . Shelimov, K. B.; Clemmer, D. E.; Hudgins, R. R.; Jarrold, M. F. J. Am. Chem. Soc. 1997, 119, 2240. (33) Shelimov, K. B.; Jarrold, M. F. J. Am. Chem. Soc. 1997, 119, 2987. (34) Valentine, S. J.; Clemmer, D. E. J. Am. Chem. Soc. 1997, 119, 3558.
4156 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
S0003-2700(97)00526-X CCC: $14.00
Recently, electrospray ionization (ESI)1 sources for mass spectrometry (MS) have been coupled with ion mobility techniques2-9 in order to examine the conformations of large biomolecules in the gas phase.10 An intrinsic limitation of the ESI-ion mobility combination is that ESI is a continuous ion source, while ion pulses are required for mobility measurements. It is typical to discard 99-99.9% of the ion signal during the mobility experiment, making these methods inherently insensitive. Recent improvements in the resolving power of these instruments require the use of shorter pulses or longer times between pulses;11 this reduces the duty cycle. Thus, it has become important to address how to increase the efficiency of these measurements. In this paper, we describe an ion trap12 interface for the ESI-ion mobility experiment that offers significant advantages in instru-
© 1997 American Chemical Society
Figure 1. Schematic diagram of the experimental apparatus.
be obtained by comparing experimentally measured collision cross sections with those that are calculated for trial conformations.10 It is possible to investigate the dynamics of conformational changes32-36 as well as the chemical reactivity of shape-resolved conformations.34-37 For example, when protein ions are injected into the drift tube at elevated kinetic energies, they undergo structural changes that correspond to folding and unfolding transitions,32-36 which can be recorded in the ion mobility distribution. Mobilities are measured at high pressures (∼2-5 Torr), making it possible to vary the buffer gas temperature in order to measure Arrhenius activation energies for structural transitions.9,38 Information about the chemical reactivities of differently shaped protein conformations have been carried out by doping the buffer gas with reagent gasses. By adding deuterated solvents to the buffer gas, it is possible to monitor the number of accessible heteroatom hydrogens at the surface of compact and elongated conformers of cytochrome c.34 Equilibrium binding enthalpies for the first water molecules that associate with these conformers have also been measured.37 Virtually any type of biomolecule can be examined, and these methods have recently been extended to study the gas-phase geometries of oligosaccharides39 and DNA.40 As mentioned above, the present work is closely related to recent advances in ESI-TOFMS measurements.13-15 Lubman and co-workers have shown that the ESI-ion trap combination facilitates duty cycles near 100% for TOFMS, significantly improving S/N ratios, sensitivity, and mass resolution.14 The shortcomings of this approach arise from different pressure requirements for the ion trap (10-4-10-3 Torr) and flight tube (
