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Anal. Chem. 1992, 64, 1501-1570
Electrospray Ionization/Magnetic Sector Mass Spectrometry: Calibration, Resolution, and Accurate Mass Measurements Robert B. Cody,' Jun Tamura, and Brian D. Musselman JEOL USA, Inc., 11 Dearbom Road, Peabody, Massachusetts 01960
Accurate mars measurements were obtained for a number of c O " with~ masse8 inthe 500-16 000 urange analyzed with a doublefocurlng magnotk-mctor mars 8pectrometer equipped with an electrospray lonizatlon(ESI) source. Mars accuracles in the low-ppm range were obtained for positive and negative Ions by wlng either magnetic field scans or acciefathgvdtagescanmethod& Thesema#rmeasurements were Sumclently accurate to permit the determination of elementaicompodtkr#,forsmalermokculee,andtheyprovide confklence in the mass aulgmnento for larger blWlWUle8 wch as hen egg-white lysozyme and equine myogiobln. The utility of poly(ethylene oxide) and poly(propylene oxlde) as posnive-lon reference standards and poly(ethyiene oxlde), poly(propyieneoxide), and poly(propyleneoxide) sulfates as negativekn referencestandards k demonstrated, and factors affecting accurate mass measurements for large molecules are dkussed. The ESI mass spectrum of the lysozyme [M 9H]O+ 8pecles measured at a resoiutlon of 10 000 permits the determlnatlonof the mas8e8of individual kotopesto within 0.15 u (12 ppm) of the theoretical value, and a peak asrigned to water lots k observed.
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INTRODUCTION Electrospray ionization (ESI)l-' is a promising new ionization method for mass spectrometry that has received a great deal of attention as an ionization method for high molecular weight species such as biomolecules. Electrospray ionization of large molecules (e.g., those with molecular weights up to 250 OOO) can produce highly charged species with mass-to-charge ratios that fall within the range of detection for most mass analyzers. The number of charges on each multiply charged species can be determined by solving simultaneous equations or by using deconvolution algorithms.3.8 For a given accelerating potential, the highly charged species produced by ESI can be accelerated to higher kinetic energies than the singly charged species produced by other common ionization methods such as fast atom bombardment. The increased kinetic energy can result in increased sensitivity for the highly charged species, and it has extended the mass range that can be accessed by collision-
* Address correspondence to this author.
(1) Mach, L. L.; Kralik, P.; Rheude, A.; Dole, M. J. Chem. Phys. 1970, 52, 4977. (2) Dole, M.; Mach, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. P.; Alice, M. B. J. Chem. Phys. 1968,49, 2240. (3) Mann, M.; Meng, C.-K.; Fenn, J. B. Anal. Chem. 1989,61,17021708. (4) Wong,S. F.; Meng, C.-K.;Fenn, J. B. J.Phys. Chem. 1988,92,546. (5) Fenn, J. B.; Mann, M.; Meng, C.-K.; Wong, S. F.; Whitehouse, C. M. Science 1989,246,64. (6) Fenn, J. B.; Mann, M.; Meng, C.-K.; Wong, S. F.; Whitehouse, C. M. Moss Spectrom. Reu. 1990, 9, 37. (7) Meng, C.-K.; Mann, M.; Fenn, J. B. 2.Phys. D. 1988, 10, 361. (8) Covey, T. R.;Bonner, R. F.; Shushan, B. I.; Henion, J. Rapid Commun. Mass Spectrom. 1988,2, 24S256.
0003-2700/92/0384-1581$03.00/0
induced dissociation (CID) experimentsP The only disadvantages to the ESI approach appear to be some solubility restrictions,l0 relatively poor ion statistics, and a sensitivity to the presence of alkali metal cations and other contaminants." In addition to its use as an ionization method for large biomolecules, electrospray ionization shows promise ae an interface between liquid chromatography12or capillary electrophoresis13 and mass spectrometry and as an ionization method for a variety of smaller molecules that are difficult or impossible to ionize by other means. We have found that ESI maw spectra can be readily obtainedfor many compounds that cannot be analyzed by using other common "soft ionization" techniques such as desorption chemical ionization (DCI), field desorption (FD), and fast atom bombardment (FAB). Such compounds include ionic compounds, surfactants, and organometallics. In comparison with FAB, ESI produces a relatively low chemical background. Electrospray ionization sources have been implemented on quadrupole,3-8 time-of-flight,l*ion trap,16 Fourier transform116-19and magneticsector mass spectrometers.1'23 Mass accuracies in the 0.01 % range have typically been reported8 for analysis with quadrupole mass spectrometers. Recently, Feng et al. demonstrated that mass accuracy as high as 12 ppm can be obtained with a quadrupole mass spectrometer by using curve-fitting methodsto interpret the data" Despite this encouraging result, the unit resolution commonly available with quadrupole mass spectrometers limits their utility for accurate mass measurements and elemental composition calculations. (9) Loo, J. A.; Edmonds, C. G.; Smith, R. D. Anal. Chem. 1991,63, 2488-2499. (10) Chowdhury, S. K.; Chait, B. T. Anal. Chem. 1991,63,1660-1664. (11) Chowdhury, S. K.; Katta, V.; Beavis, R. C.; Chait, B. T. J. Am. Soc. Moss Spectrom. 1990,1,382-388. (12) Huang, E. C.; Henion, J. D. Anal. Chem. 1991,63, 732-739. (13) Loo, J. A.; Udseth, H. R.; Smith, R. D. Anal. Biochem. 1989,404412. (14) Boyle, J. G.; Gulcicek, E. E.; Shen, S.; Whitehouse, C. M.; Fenn,
J. B. An Electrospray Ionization Time-of-Flight Maes Spectrometer for the Analysis of Biological Molecules. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied T o- ~ i c. sN,ashville.. TN.. May 19-24,1991. (15) Berkel. G. J. V.: Glish. G. L.: McLuckev. S. A. Anal. Chem. 1990. " I
62, '12&1-1295: (16) Henry, K. D.; Williams, E. R.; Wang, B. H.; McLafferty, F. W.; Shabanowitz, J.; Hunt, D. F. Proc. Natl. Acad. Sci. (U.S.A.) 1989, 86, 9075-9078. (17) Loo, J. A.; Quinn, J. P.; Ryu, S. I.; Henry, K. D.; Senko, M. W.; McLafferty, F. W. R o c . Nat. Acad. Sci. (U.S.A.) 1992,89,286-289. (18) Henry, K. D.; Quinn, J. P.; McLafferty, F. W. J. Am. Chem. SOC. 1991,113,5447-5449. (19) Meng, C.-K.; McEwen, C. N.; Larsen, B. S. Rapid Commun. Ma38 Spectrom. 1990,4, 151-155. (20) Meng, C.-K.; McEwen, C. N.; Larsen, B. S.Rapid Commun. Moss Spectrom. 1990,4, 147-150. (21) Gallagher, R. T.; Chapman, J. R.; Mann, M. Rapid Commun. Moss Spectrom. 1990, 4 , 369-372. (22) Larsen, B. S.; McEwen, C. N. J.Am. SOC.Mass Spectrom. 1991, 2,205-211. (23) Wada, Y.;Tamura, J.; Musselman, B. D.; Kaseel, D. B.; Sakurai, T.; Matauo, T. Rapid Commun. Mass Spectrom. 1992, 6, 9-13. (24) Feng, R.; Konishi, Y.;Bell, A. W. J. Am. SOC.Moss Spectrom. 1991,2,387-401.
0 1982 American Chemical Society
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