Anal. Chem. 2006, 78, 6133-6139
Characterization of Laser-Induced Acoustic Desorption Coupled with a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer Ryan C. Shea,† Christopher J. Petzold,†,§ J. Larry Campbell,†,‡ Sen Li,† David J. Aaserud,| and Hilkka I. Kentta 1 maa*,†
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, and The Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe, Ohio 44092
Several experimental factors have been investigated that influence the efficiency of desorption and subsequent chemical ionization of nonvolatile, thermally labile molecules during laser-induced acoustic desorption/Fourier transform ion cyclotron resonance mass spectrometry (LIAD/FT-ICR) experiments. The experiments were performed by using two specially designed LIAD probes of different outer diameters (1/2 and 7/8 in.) and designs. Several improvements to the design of the “first generation” (1/2 in.) LIAD probe are presented. The larger diameter (7/8 in.) probe provides a larger surface area for desorption than the smaller diameter probe. Further, it was designed to desorb molecules on-axis with the magnetic field of the instrument. This is in contrast to the smaller probe for which desorption occurs 1.3 mm offaxis. This improved alignment, which provides better overlap between the desorbed molecules and trapped reagent ions, results in a substantial increase in the sensitivity of LIAD analyses. The thickness of the sample layer deposited on the irradiated metal foil and the number of laser shots fired on the backside of the foil were found to have a significant effect on the overall signal and the relative abundances of the ions formed in the experiment. Evaporation of a tetrapeptide, Val-Ala-Ala-Phe (VAAF), from Ag, Al, Au, Cu, Fe, and Ti foils, followed by protonation by protonated pyridine, revealed that the titanium foil provides the greatest signal. The importance of the laser power density was examined by desorbing a low MW polymer, polyisobutenyl succinic anhydride, at power densities ranging from 5.40 × 108 to 9.00 × 108 W/cm2 at the backside of the foil. Higher laser power densities resulted in greater signals and an improved distribution for the higher molecular weight oligomers. Laser-induced acoustic desorption1-3 (LIAD) was recently demonstrated4-7 to be a valuable technique for the evaporation of nonvolatile, thermally labile, neutral molecules into Fourier * To whom correspondence should be addressed. E-mail:
[email protected]. † Purdue University. ‡ Current address: University of Western Ontario, London, ON, N6A 5C1, Canada. § Current address: University of CaliforniasBerkeley, Berkeley, CA 94720. | The Lubrizol Corp. 10.1021/ac0602827 CCC: $33.50 Published on Web 08/02/2006
© 2006 American Chemical Society
transform ion cyclotron resonance mass spectrometers (FT-ICR). In this technique, the analyte is deposited onto a thin metal foil. Irradiation of the backside of the foil with a series of short (3 ns), high-energy (532 nm) laser pulses generates an acoustic wave that propagates through the foil and causes the desorption of neutral analyte molecules from the opposite side of the foil. The acoustically desorbed molecules have unprecedented low kinetic and internal energies.4,8 Additional advantages associated with LIAD include the ability to use a wide array of methods to ionize the analyte and the ability to optimize independently the evaporation and ionization processes. Neither is possible for matrixassisted laser desorption/ionization or electrospray ionization, which are restricted to simple ion attachment/removal reactions (e.g., protonation or deprotonation) and involve convoluted desorption/ionization mechanisms.9 Several recent examples in the literature illustrate the ability of LIAD to evaporate a wide variety of analytes, including nonpolar hydrocarbon polymers,6,10 petroleum distillates,7 nucleic acid components,5,11,12 and peptides.4 The ionization methods that have been utilized include electron ionization (EI) and chemical ionization (CI) using Brønsted acids,4 distonic radical cations,5 and organometallic ions as the reagent ions.6,7,10 In addition to the LIAD experiments conducted with an FT-ICR mass spectrometer, LIAD has also been successfully adapted to a flowingafterglow guided ion beam apparatus.13,14 To enhance the perfor(1) Lindner, B. Int. J. Mass Spectrom. Ion Processes 1991, 103, 203-218. (2) Golovlev, V. V.; Allman, S. L.; Garrett, W. R.; Taranenko, N. I.; Chen, C. H. Int. J. Mass Spectrom. Ion Processes 1997, 169/170, 69-78. (3) Golovlev, V. V.; Allman, S. L.; Garrett, W. R.; Chen, C. H. Appl. Phys. Lett. 1997, 71, 852-854. (4) Pe´rez, J.; Ramire´z-Arizmendi, L. E.; Petzold, C. J.; Guler, L. P.; Nelson, E. D.; Kentta¨maa, H. I. Int. J. Mass Spectrom. 2000, 198, 173-188. (5) Petzold, C. J.; Ramire´z-Arizmendi, L. E.; Heidbrink, J. L.; Pe´rez, J.; Kentta¨maa, H. I. J. Am. Soc. Mass Spectrom. 2002, 13, 192-194. (6) Campbell, J. L.; Crawford, K. E.; Kentta¨maa, H. I. Anal. Chem. 2004, 76, 959-963. (7) Crawford, K. E.; Campbell, J. L.; Fiddler, M. N.; Duan, P.; Qian, K.; Gorbaty, M. L.; Kentta¨maa, H. I. Anal. Chem. 2005, 77, 7916-7923. (8) Shea, R. C.; Petzold, C. J.; Liu, J.; Li, S.; Kentta¨maa, H. I. Unpublished work 2003. (9) Knochenmuss, R.; Zenobi, R. Chem. Rev. 2003, 103, 441-452. (10) Campbell, J. L.; Fiddler, M. N.; Crawford, K. E.; Gqamana, P. P.; Kentta¨maa, H. I. Anal. Chem. 2005, 76, 959-963. (11) Guler, L. P.; Yu, Y.-Q.; Kentta¨maa, H. I. J. Phys. Chem. A 2002, 106, 67546764. (12) Liu, J.; Petzold, C. J.; Ramire´z-Arizmendi, L. E.; Pe´rez, J.; Kentta¨maa, H. I. J. Am. Chem. Soc. 2005, 127, 12758-12759.
Analytical Chemistry, Vol. 78, No. 17, September 1, 2006 6133
mance of the LIAD technique, a new LIAD probe has been designed and built. A comparison of the performance of the two probes, as well as the characterization and optimization of several experimental parameters, is described here. EXPERIMENTAL SECTION All experiments were performed by using either a Nicolet model FTMS 2000 dual-cell FT-ICR or an Extrel model FTMS 2001 dual-cell FT-ICR. Each instrument was equipped with a 3-T superconducting magnet and a differentially pumped dual cell. Both instruments have been described previously.6,15 The Nicolet FT-ICR utilized two Edwards Diffstak 160 diffusion pumps (700 L/s), each backed by an Alcatel 2010 (3.2 L/s) dual rotary-vane pump, to maintain a nominal baseline pressure of 109 W/cm2).25 This is supported by Figure 6, wherein the relative abundances of the fragment ions relative to that of the protonated molecule decrease significantly with increased numbers of laser shots applied to the foil. If fragmentation of the evaporated molecules occurred during desorption through a thermal process, these lower m/z signals would be expected to continue to dominate the mass spectra even with the desorption of more material through the application of many laser shots. Additional experimental evidence that shows desorption does not
thermal processes are unlikely to play a role in the desorption of the analyte.
Figure 9. Three-dimensional plot of the signal intensities of PIBSA oligomers detected after LIAD/CI (deprotonation with bromide ion). Distributions are shown as a function of laser irradiance (at foil).
occur by a thermal process has been obtained by demonstrating that thermally labile biomolecules are evaporated intact by LIAD,12 as well as by examining the performance a LIAD probe capable of delivering much higher energy laser pulses than the LIAD probes described in this work.33 Spectra obtained with the use of power densities of up to ∼5 × 109 W/cm2 are identical to those obtained with the laser irradiances used here. Additional details of these experiments will be presented elsewhere.33,34 Given the conditions under which these LIAD experiments were performed, (33) Shea, R. C.; Habicht, S. C.; Vaughn, W. E.; Kentta¨maa, H. I. Unpublished results 2006. (34) Shea, R. C.; Gqamana, P. P.; Yang, L.; Fiddler, M. N.; Kenttamaa, H. I. Unpublished results 2006. (35) Lide, D. R., Ed. Handbook of Chemistry and Physics, 71st ed.; CRC Press: Cleveland, OH, 1990/91.
CONCLUSIONS The LIAD technique has proven to be a valuable tool for the mass spectrometric studies of thermally labile, nonvolatile molecules.4-7,10,12 This investigation into the influence of some experimental variables to the LIAD process facilitates the understanding of LIAD and aids in the optimization of experimental conditions. Comparison of a recently designed, improved LIAD probe with the previous version demonstrated the importance of proper alignment of the probe with the magnetic field axis and the center of the FT-ICR cell. The amount of analyte deposited on the metal foil and the number of laser shots applied to the backside of the foil were found to significantly influence both the intensity of the overall signal and the relative abundances of the protonated analyte and its fragment ions. As more laser shots were applied, or a thicker sample layer was used, increased protonated analyte signal was observed. This is rationalized by unwanted ionization of analyte molecules by fragment ions of the protonated analyte. The physical properties of the metal foil influence the formation of acoustic waves within the foil. The metal foil that demonstrated the best performance in this study was 12.7-µmthick Ti foil. Higher laser irradiances were found to yield greater numbers of gaseous analyte molecules (more signal) and more efficient desorption of higher MW molecules from the foil’s surface. ACKNOWLEDGMENT The authors thank the National Science Foundation and the Lubrizol Corporation for their financial support of this work. We also thank Weldon Vaughn, Mark Carlsen, and Dr. Hartmut Hedderich for their invaluable assistance with instrumentation and helpful discussions. Received for review February 14, 2006. Accepted May 25, 2006. AC0602827
Analytical Chemistry, Vol. 78, No. 17, September 1, 2006
6139
