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Exchange of Comments on “Lasers and Mass Spectrometry” Sir: I would like to comment on a recent article entitled “Lasers and Mass Spectrometry” by Robert Cotter (Anal. Chem. 1984,56,485-504 A), which brings together several ideas in using lasers for chemical analysis. However, it also contains several misconceptions that I would like to correct. In particular, the article leaves the impression that resonance-enhanced multiphoton ionization (REMPI) is necessarily a high-fragmentation technique producing fragmentation more extensive than electron impact ionization (EI). This is not true, and the opposite is borne out hy Cotter’s reference to the work of Schlag et al. ( I ) . In this experiment UV light (259 nm) was used to softly ionize benzene to produce only the molecular ion in a resonant two-photon ionization (RZPI) process. The point is that REMPI, when performed as R2P1, is generally a very efficient soft ionization method. Although only a limited number of molecules have been studied thus far, soft ionization generally appears to he the rule for the aromatic compounds studied to date. In the case of molecules such as aniline, naphthalene, toluene, benzene, and other aromatic compounds ( 2 4 ) ,soft ionization has been achieved in RZPI with efficiencies ranging from 0.05 to as high as 100%. Of course, this efficiency applies only to the time that the laser pulse is on and to the volume the laser beam intersects. Extensive fragmentation may always he produced in RZPI by focusing the laser beam or by simply increasing the laser power. The molecular ion produced even in RZPI can further absorb photons and he excited to an excited ionic state that dissociates, thus producing fragmentation. As shown by Schlag et al. ( 1 ), this process can continue as the power is raised so that the ionic fragments produced can absorb more photons, resulting in further fragmentation. However, the laser power can easily he controlled to minimize this problem. Using the R2PI technique more complex molecules such as laser dyes and large polynuclear aromatics
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( 2 ) have been softly ionized with high efficiencies. Of course there will always he some counterexamples, such as the metal carbonyls (5),which appear to dissociate upon absorption of light. The question as to whether delicate hiological molecules such as those that might interest Cotter can he softly ionized in RZPI still remains to be investigated and is a problem of current interest in our own work. However, RZPI as studied to date is indeed a process which can produce soft ionization with high efficiency, and fragmentation is generally produced when one or more nonresonant steps are present in the up-pumping process, as in Reference 6. Finally, the fragmentation patterns produced by MPI have yet to he proven as useful for identifying molecules as those of EI. We believe that MPI will eventually prove to be a valuable technique for its fragmentation capabilities. However, RZPI has already shown great capability as a spectroscopic tool, based upon its wavelength selectivity and its ability to effect soft ionization, and I believe it will he an increasingly valuable tool in chemical analysis.
David M.Lubman Department of Chemistry The University of Michigan Ann Arbor, Mich. 48109
References Boesl, U.; Neusser, H. J.;Schlag, E. W . J.Chem. Phys. 1980,72,4321. (2) Lubman, D. M.; Kroniek, M. N. Anal. (1)
Chem. 1982,54,660. Dietz, T. G.; Duncan, M. A,; Liverman, M.G.; Smalley, R. E. J. Chem. Phys.
(3)
1980,73,4816. (4) Leotwyler,S.; Even, U.; Jortner, J. J. Chem. Phys. 1983,79,5769. ( 5 ) Duncan, M.A.; Dietz, T.G.;Smalley, R.E. Chem. Phys. 1979,44,415. ( 6 ) Zandee, L.; Bernstein, R. B. J. Chem. Phys. 1979.71,1359.
Reply: David Luhman points out quite correctly that lowering the laser power density in multiphoton ionization increases molecular ion formation. This occurs since (generally, for
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aromatic compounds) fragmentation follows ionization, upon the absorption of additional photons, i.e., the “two ladder process” (I).Hence he suggests the possibilities of using the R2PI technique for soft ionization of biomolecules. There is a parallel in the development of laser desorption of organic solids. Earlv work emdoyed hiahpowered lasers for the vaporization of eraDhite ( 2 ) .elemental analysis (3), and pyrolysis ( 4 ) .while only recently has there been interest (and success) in the desorption of intact molecular ions of rather heavy compounds (5). Lower power denstties have been responsible for the production of molecular rather than atomic ions ( 6 ) .while shorter pulses reduce pyrolytic effects. Thus the in ANALYTI..._ . . recent review ~ ~ CAL CHEMISTRY(7)may reflect only a historical emphasis on fragmentation in multiphoton experiments, and as Lubman states: “The question as to whether delicate biological molecules ..can be softly ionized in R2PI still remains to be investigated . .” Assuming that the analysis of large molecules is a direction that Lubman and others will follow, it is interesting to consider some aspects of multipho-
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ton ionization in comparison with current soft ionization techniques. The emphasis up till now on relatively small, simple aromatic compounds which are stable, volatile, and easily ionized by electron impact has made it difficult to evaluate the potential of MPI for large, unstable, highly functionalized molecules, which are usually solids. Larger, polynuclear aromatic compounds, chosen for their chromo. phoric properties, do respond well to multiphoton ionization ( B ) , just as quaternary ammonium salts and easily protonatable compounds are particularly well suited for analysis by laser desorption (LD) and fast atom bombardment (FAB). Therefore, in addition M obvious compounds such as chlorophyll, it is conceivable that peptides conwining Dhenvlalanine tv-~ . ~ ~ rosine, for =& e, be approp;iate candidates for chemical analysis by MPI and may possibly produce complementary sequence information, resulting from fragmentation patterns which differ somewhat from those observed in other techniques. It will be interesting to see if the fragment ions are of the structurally informative type that, in other methods, accompany the localization of the charge on a
heteroatom adjacent to easily fragmented single bonds. Current MPI studies have generally focused on aromatic ring fragmentation. Finally, it is not inconceivable that chromophoric deriuatiration, attachment of aromatic groups to specific sites on a molecule, may become a useful technique for increasing ionization efficiency and directing fragmentation.
Robert J. Cotter Department of Pharmacology The Johns Hopkins University Baltimore, Md. 21205 References (1) Boesl, U.;Newer, H.J.; Schlag, E.W. J . Chem. Phys 1980,72.4327. (2)_.. Lincoln, K.A. Anal. ~ ~ Chem. 1565.57, ~ 241.
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(3) F ~N. ~D ~ IN. ~~R., J.~.water., Sei. 1968.3,259. (4) Wiley, R. H.; Veeranrager, P. J.Phys. Chem. 1968,72,2417. (5) Posthumus, M. A.; Kistemaker,P. G.;
Meuzelaar, H.L.C.; Ten Noever De Brauw, M. C. Anal. Chem. 1978,50,985. (6)Cotter, R. J.; Tabet, J.-C. Am. Lnb. (7)1984,16,86. cotter, J, Anal, Chem, 1984,56, 485 A.
(8) Lubman. D. M.; Kronick, M. N. Anal. Chem. 1982,54,660.
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