Anal. Chem. 2008, 80, 7379–7382
Automated Liquid Injection Field Desorption/ Ionization for Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Donald F. Smith,†,‡ Tanner M. Schaub,† Ryan P. Rodgers,†,‡ Christopher L. Hendrickson,†,‡ and Alan G. Marshall*,†,‡ National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, and Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306 We describe automation of liquid injection field desorption/ionization (LIFDI) for reproducible sample application, improved spectral quality, and high-throughput analyses. A commercial autosampler provides reproducible and unattended sample application. A custom-built field desorption (FD) controller allows data station or front panel control of source parameters including high-voltage limit/ramp rate, emitter heating current limit/ramp rate, and feedback control of emitter heating current based on ion current measurement. Automated LIFDI facilitates ensemble averaging of hundreds of Fourier transform ion cyclotron resonance mass spectra for increased dynamic range, mass accuracy, and S/N ratio relative to singleapplication FD experiments, as shown here for a South American crude oil. This configuration can be adapted to any mass spectrometer with an LIFDI probe. Field desorption (FD) ionizes a variety of nonvolatile analytes that are difficult to ionize by other means.1-13 Further, FD is a “soft” ionization technique, yielding predominantly intact M+• molecular ions for a wide range of compound classes, such as paraffins, cycloparaffins, aromatic hydrocarbons, and nonpolar (e.g., thiophenic) sulfur not accessible by electrospray or matrix* To whom correspondence should be addressed. Telephone: +1-850-6440529. Fax: +1-850-644-1366. E-mail:
[email protected]. † National High Magnetic Field Laboratory. ‡ Department of Chemistry and Biochemistry. (1) Van der Greef, J.; Leegwater, D. C. Biomed. Mass Spectrom. 1983, 10, 1–4. (2) Schiebel, H. M.; Schulten, H. R. Mass Spectrom. Rev. 1986, 5, 249–311. (3) Van der Greef, J. TrAC, Trends Anal. Chem. (Pers. Ed.) 1986, 5, 241–246. (4) Selva, A.; Redenti, E.; Ventura, P. Org. Mass Spectrom. 1991, 26, 170. (5) Malhotra, R.; McMillen, D. F.; Watson, E. L.; Huestis, D. L. Energy Fuels 1993, 7, 1079–1087. (6) Evans, W. J.; DeCoster, D. M.; Greaves, J. J. Am. Soc. Mass Spectrom. 1996, 7, 1070–1074. (7) Carr, R. H.; Jackson, A. T. Rapid Commun. Mass Spectrom. 1998, 12, 2047– 2050. (8) Komori, M.; Ghosh, R.; Takaichi, S.; Hu, Y.; Mizoguchi, T.; Koyama, Y.; Kuki, M. Biochemistry 1998, 37, 8987–8994. (9) Gross, J. H.; Vekey, K.; Dallos, A. J. Mass Spectrom. 2001, 36, 522–528. (10) Schaub, T. M.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Green, L. A.; Olmstead, W. N. Energy Fuels 2005, 19, 1566–1573. (11) Gross, J. H.; Nieth, N.; Linden, H. B.; Blumbach, U.; Richter, F. J.; Tauchert, M. E.; Tompers, R.; Hofmann, P. Anal. Bioanal. Chem. 2006, 386, 52–58. (12) Qian, K.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Energy Fuels 2007, 21, 1042–1047. (13) Qian, K. N.; Edwards, K. E.; Siskin, M. Energy Fuels 2001, 15, 949–954. 10.1021/ac801085r CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
assisted laser desorption/ionization. However, the pulsed nature of the FD ionization source, combined with the need to reapply sample after every experiment, made FD analysis exceptionally difficult. The development of liquid injection field desorption/ ionization (LIFDI) enabled in vacuo sample application; i.e., sample is applied without the need to break vacuum.14 LIFDI has been used for many sample types including petroleum mixtures,10,15-18 metal complexes,11,19-21 and fullerenes.22,23 Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) requires externally generated LIFDI ions to be accumulated in an accumulation octopole (with helium gas for collisional cooling) before transfer to the ICR cell. Accumulation periods as short as 5 s (here, 5-30 s accumulation) are sufficient to fragment n-alkanes in the NHMFL LIFDI FTICR mass spectrometer.24 Thus, external LIFDI FTICR MS has been generally used to target the aromatic hydrocarbons, nonpolar sulfur compounds, and other more stable species. The complexity of petroleum mixtures requires ultrahigh mass resolving power (m/∆m50% > 300 000) and sub-ppm (8 σ above baseline noise)
100 averaged experiments
single experiment
437 000
410 000
104
166
3 591
1 215
number of ions produced with automated LIFDI experiments minimizes m/z discrimination in the multipole ion guides and results in transfer of ions of a wider m/z range to the ICR cell. In addition, higher EHC allows a wider range of desorption temperatures to facilitate ionization of higher boiling components. Finally, S/N ratio lost to low ion population is recovered by ensemble averaging and results in detection of low-abundance ion signals at low and high m/z. Insets in Figure 3 show increased S/N ratio for high m/z (as well as low m/z) species with automated LIFDI. Table 1 shows the improvements realized by signal averaging versus a single experiment. The mass accuracy for all assigned elemental compositions is improved through replicate measurement and increased S/N for low abundance species.40 Dynamic range improvement is dramatic for automated LIFDI; evidenced here by an almost 3-fold increase in the number of observed peaks. Coaddition of multiple measurements increases the level of detail and dynamic range for LIFDI of petroleum samples. Figure 4 shows the heteroatom class distribution for South American crude oil derived from the spectra shown in Figure 3. The hydrocarbon class has the highest relative abundance for both experiments, followed by the S1 class. Class distributions are similar for both experiments; however, low-abundance classes (O2 and S1O1) that are not seen in a single experiment are rendered observable with ensemble averaging. Isoabundance plots of double bond equivalents (DBE ) number of rings plus double bonds (40) Marshall, A. G.; Verdun, F. R. Fourier Transforms in NMR, Optical, and Mass Spectrometry: A User’s Handbook; Elsevier: Amsterdam, 1990.
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Figure 4. Heteroatom class distribution for a South American crude oil derived from the LIFDI FTICR mass spectra in Figure 3. Signal averaging enhances detection of the low-abundance classes O4 and S1O1.
5 bottom). Plots derived from the coadded spectrum (Figure 5 left) show wider carbon number distributions than from a single measurement (Figure 5 right) due to the increase in S/N for lowabundance species at low and high mass. The hydrocarbon class extends from carbon number 17-55 for the coadded spectrum (Figure 5, top left), whereas a single experiment yields a narrower carbon number distribution of 18-48 (Figure 5, top right). Similarly, the S1 carbon number distribution for the coadded spectrum ranges from 21 to 52 (Figure 5, bottom left) and the single-experiment carbon number ranges from 21 to 44 (Figure 5, bottom right). More aromatic species (higher DBE) are also enhanced in the coadded spectrum due to the ability to collect ions over an extended heating range. Hydrocarbons in the coadded spectrum range from DBE 1 to 16 (Figure 5, top left) and the single-experiment spectrum ranges from DBE 1 to 14. The S1 class for the coadded spectrum extends from DBE 2 to 17 (Figure 5, bottom left), whereas the single-experiment spectrum ranges from DBE 2 to 16 (Figure 5, bottom right). CONCLUSIONS Automated LIFDI FTICR MS enables the ensemble averaging of multiple mass spectra for improved dynamic range, mass resolving power, and mass accuracy relative to a single-application experiment. Thus, more detailed compositional analysis of nonpolar and low-polarity species in complex mixtures becomes possible. Samples can be analyzed unattended, and the autosampler allows the analysis of multiple samples per experiment. The new FD controller and the use of an autosampler allow fine control of FD ionization parameters for improved reproducibility. The automation setup is robust and allows analysis of any sample that can be analyzed by traditional single-application FD. Any system equipped with a LIFDI probe can easily be configured to accommodate the presently described automation.
Figure 5. Color-coded isoabundance contours for plots of DBE versus carbon number for the hydrocarbon class (top) and S1 class (bottom) derived from the LIFDI FTICR mass spectra in Figure 3. Automated LIFDI FTICR MS (left) enables much more detailed characterization than single-experiment LIFDI FTICR MS (right).
involving carbon) versus carbon number are shown in Figure 5 for the hydrocarbon class (Figure 5 top) and the S1 class (Figure
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ACKNOWLEDGMENT This work was supported by the NSF National High Field FTICR Facility (DMR-06-54118), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL.
Received for review May 28, 2008. Accepted August 4, 2008. AC801085R
