Anal. Chem. 2005, 77, 1317-1324
Instrumentation and Method for Ultrahigh Resolution Field Desorption Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry of Nonpolar Species Tanner M. Schaub,† Christopher L. Hendrickson,† John P. Quinn, Ryan P. Rodgers,† and Alan G. Marshall*,†
Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005
We describe the construction and application of a 9.4-T FT-ICR mass spectrometer interfaced to a commercial field desorption ion source for high-resolution, high-mass accuracy measurements of nonpolar species. The FT-ICR MS instrument includes a liquid injection field desorption ionization source, octopole ion guides, external octopole ion trap capable of an axial potential gradient for ion ejection, capacitively coupled open cylindrical ion trap, and pulsed gas valve for ion cooling. Model compound responses with regard to various source and instrument conditions provide a basis for interpretation of broadband mass spectra of complex mixtures. As an example, we demonstrate broadband speciation of a Gulf Coast crude oil, with respect to numerous heteroatomic classes, compound types (rings plus double bonds), and carbon number distributions. Field ionization/field desorption mass spectrometry (FI/FD MS) has been demonstrated for analysis of a variety of samples such as fullerenes,1 aliphatic polyamides,2 polyethylene,3 and various biological substances.4 The FI/FD emitter consists of a small-diameter tungsten wire coated with a pyrolyzed carbon layer on which an array of carbon microneedle structures is formed by electrodeposition. The electron cloud of an analyte near a needle tip is polarized by an intense electric field (>108 V/cm) formed between the microneedle tip and the nearby counter electrode. That polarization causes an electron to tunnel from the analyte to the surface of the emitter (in positive ion formation). The internal energy of the nascent ions is less than 1 eV, and little fragmentation results from ion formation.5 Sample introduction in which vaporized sample ionizes as it flows past the microneedle tips is * To whom all correspondence should be addressed. E-mail: marshall@ magnet.fsu.edu. † Also members of the Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32310. (1) Wang, J.; Fu, H.; Wu, Y. Fenxi Ceshi Xuebao 1995, 14, 54-56. (2) Schulten, H. R.; Plage, B. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 2381-2394. (3) Evans, W. J.; DeCoster, D. M.; Greaves, J. J. Am. Soc. Mass Spectrom. 1996, 7, 1070-1074. (4) Van der Greef, J. TrAC, Trends Anal. Chem. 1986, 5, 241-246. (5) Beckey, H. D. Int. J. Mass Spectrom. Ion Phys. 1969, 500-507. 10.1021/ac048766v CCC: $30.25 Published on Web 02/01/2005
© 2005 American Chemical Society
termed field ionization. In field desorption, a condensed-phase sample is applied to the emitter, which is then heated with a small ( 50 000, in which (13) Larsen, B. S.; Fenselau, C. C.; Whitehurst, D. P.; Angelini, M. Anal. Chem. 1986, 58, 1088-1091. (14) Bricker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Malhotra, R.; Coggiola, M. A.; Yong, S. E. Energy Fuels 2001, 15, 996-1002. (15) Bricker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; P. M., R.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Yong, S. E. Energy Fuels 2001, 15, 23-37. (16) Malhotra, R.; Goggiola, S. E.; Yong, S. E.; Spiondt, C. A.; Hsu, C. S.; Dechert, G. J.; Rahimi, P. M.; Bricker, Y. In Chemistry of Diesel Fuels; Hsu, C. S., Song, C., Mochida, I., Eds.; Taylor & Francis: New York, 2000; pp 77-92. (17) Ueda, K.; Matsui, H.; Malhotra, R.; Nomura, M. Sekiyu Gakkaishi 1991, 34, 62-70. (18) Liang, Z.; Hsu, C. S. Energy Fuels 1998, 12, 637-643. (19) Qian, K.; Dechert, G. J. Anal. Chem. 2002, 74, 3977-3983. (20) Hsu, C. S.; Green, M. Rapid Commun. Mass Spectrom. 2001, 15, 236-239. (21) Jaffe, S. B. Proceedings of 222nd ACS National Meeting, Chicago, IL, 2001; PETR-032. (22) Quann, R. J.; Jaffe, S. B. Ind. Eng. Chem. Res. 1992, 31, 2483-2497. (23) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53-59. (24) Guan, S.; Marshall, A. G.; Scheppele, S. E. Anal. Chem. 1996, 68, 46-71. (25) Hsu, C. S.; Liang, Z.; Campana, J. E. Anal. Chem. 1994, 66, 850-855. (26) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74. (27) Wu, Z.; Jenstrom, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2003, 17, 946-953.
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m/∆m50% denotes the mass spectral peak full width at halfmaximum peak height) FD mass analysis of nonpolar petroleum constituents.28 That work was performed with a home-built passively shielded 9.4-T FT-ICR system typically configured for ESI analysis.29,30 To further understand the utility of high-resolution FD FT-ICR MS, we have built a complete actively shielded 9.4-T FT-ICR instrument that accommodates a field desorption source and is now available daily at the NSF High-Field FT-ICR MS Facility at the National High Magnetic Field Laboratory. EXPERIMENTAL METHODS Chemicals. Standard compounds were obtained from Aldrich Chemical and were dissolved in methylene chloride to 1 mg/mL. All petroleum samples were supplied by ExxonMobil and were prepared by dissolution in methylene chloride. The petroleum distillation cut is a 468-482 °C boiling range crude oil fraction. The sample crude oil is a moderate maturity crude from the U.S. Gulf of Mexico, with 0.8 wt % sulfur (determined by pyrolysis) and a relative density of 41 deg API (degrees API ) (141.5/ (specific gravity at 60 °C)) - 131.5). Instrumentation. The new FD FT-ICR system operates in a 9.4-T, 155-mm horizontal bore diameter actively shielded superconducting solenoidal magnet (Magnex Scientific, Oxford, England). The magnetic field inhomogeneity is less than 8.5 ppm peak to peak within a 60 mm diameter × 60 mm long cylinder, and the magnetic field drift is 200 ng) that we now know overloads the FD emitter.) Scanning Electron Microscopy. We have also investigated the integrity of the nanoscale structure of the carbon microneedle dendrites on the commercial emitters after repeated petroleum use. For this purpose, we prepared emitters in various states (e.g., new, with sample applied, heavily used, etc.) and collected a series of environmental scanning electron micrographs (ESEM, at the NHMFL Microanalysis Facility) to reveal the condition of the microneedle structures during routine use. Figure 8 (top) 1322
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shows the effect of overloading the emitter in quantitatively the same manner as discussed above. The thick layer of condensed phase sample is clearly evident in the SEM image of an overloaded emitter (Figure 8, top left). It is understandable that field-induced dehydrogenation reactions, which occur through a multimolecular process, are greatly enhanced under this type of sample loading condition. The 10-fold magnification (Figure 8, top right, inset) illustrates that, even under moderate sample loading, some of the outer microneedle dendrites become matted. However, if a -10-kV extraction voltage is applied to the counter electrode, those matted dendrites align with the electric field and are straightened. In fact, provided that the emitter is not overloaded, the structure of the emitter is preserved even after extended use with petroleum, as demonstrated by comparing a new emitter and one used for 30 routine petroleum analyses (see Figure 8, middle). Routine overloading of the emitter leads to permanent deterioration of the emitter dendrites. Figure 8, bottom, shows heavy deposits (probably graphite) on the emitter surface after 30 cycles of overloading the emitter with crude oil. Petroleum. This limited model compound study has several important implications for interpretation of broadband FD FT-ICR mass spectra of “real” mixtures of nonpolar compounds. First, saturated hydrocarbons are observed in our analyses only with relatively short ( 4) are relatively stable. The carbon number distribution for molecules of the DBE ) 4 type is shown as an inset in Figure 10. Finally, it is possible that relative abundances of the saturated hydrocarbon series may be reconstructed from fragment ion data, as in prior FIMS analyses of saturates.18
ACKNOWLEDGMENT The authors thank Daniel McIntosh for the manufacture of all custom system components, Bernhard Linden for technical assistance with the FD source, Steven Beu for the SIMION modeling, and Kuangnan Qian for helpful discussions regarding petroleum mass spectrometry. This work was supported by ExxonMobil Research and Engineering, NJ, NSF (CHE-99-09502), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL.
CONCLUSION The present work illustrates the complementarity of field desorption/ionization and high-resolution FT-ICR MS. Specific instrument development goals include implementation of greater (∼5-10× gain) peak-to-peak voltage on the rf-only multipole ion
Received for review August 18, 2004. Accepted December 8, 2004.
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AC048766V
