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Exchange of Comments on "Lasers and Mass Spectrometry" Sir: I would like to comment on a re cent 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 mis conceptions that I would like to cor rect. In particular, the article leaves the impression that resonance-enhanced multiphoton ionization (REMPI) is necessarily a high-fragmentation tech nique producing fragmentation more extensive than electron impact ioniza tion (EI). This is not true, and the op posite is borne out by Cotter's refer ence 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 (R2PI) process. The point is that REMPI, when performed as R2PI, is generally a very efficient soft ioniza tion method. Although only a limited number of molecules have been studied thus far, soft ionization generally appears to be the rule for the aromatic compounds studied to date. In the case of mole cules such as aniline, naphthalene, tol uene, benzene, and other aromatic compounds (2-4), soft ionization has been achieved in R2PI with efficien cies 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 be produced in R2PI by focusing the laser beam or by simply increasing the laser power. The molecular ion produced even in R2PI can further absorb photons and be excited to an excited ionic state that dissociates, thus producing frag mentation. As shown by Schlag et al. (i ), this process can continue as the power is raised so that the ionic frag ments produced can absorb more pho tons, resulting in further fragmenta tion. However, the laser power can easily be 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 be some counterexamples, such as the metal carbonyls (5), which appear to disso ciate upon absorption of light. The question as to whether delicate biolog ical molecules such as those that might interest Cotter can be softly ionized in R2PI still remains to be in vestigated and is a problem of current interest in our own work. However, R2PI as studied to date is indeed a process which can produce soft ioniza tion with high efficiency, and frag mentation 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 be prov en as useful for identifying molecules as those of EI. We believe that MPI will eventually prove to be a valuable technique for its fragmentation capa bilities. However, R2PI has already shown great capability as a spectro scopic tool, based upon its wavelength selectivity and its ability to effect soft ionization, and I believe it will be an increasingly valuable tool in chemical analysis. David M. Lubman Department of Chemistry The University of Michigan Ann Arbor, Mich. 48109 References (1) Boesl, U.; Neusser, H. J.; Schlag, E. W. J. Chem. Phys. 1980, 72, 4327. (2) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982,54,660. (3) Dietz, T. G.; Duncan, Μ. Α.; Liverman, M. G.; Smalley, R. E. J. Chem. Phys. 1980, 73, 4816. (4) Leutwyler, S.; Even, U.; Jortner, J. J. Chem. Phys. 1983, 79, 5769. (5) Duncan, Μ. Α.; 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 Lubman points out quite correctly that lowering the laser power density in multiphoton ioniza tion increases molecular ion forma tion. This occurs since (generally, for
Letters aromatic compounds) fragmentation follows ionization, upon the absorp tion of additional photons, i.e., the "two ladder process" (2). Hence he suggests the possibilities of using the R2PI technique for soft ionization of biomolecules. There is a parallel in the develop ment of laser desorption of organic solids. Early work employed highpowered lasers for the vaporization of graphite (2), elemental analysis (5), 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 densities have been re sponsible for the production of molec ular rather than atomic ions (6), while shorter pulses reduce pyrolytic effects. Thus the recent review in ANALYTI CAL C H E M I S T R Y (7) may reflect only
a historical emphasis on fragmenta tion 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
ton ionization in comparison with cur rent 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 usual ly solids. Larger, polynuclear aromatic compounds, chosen for their chromophoric properties, do respond well to multiphoton ionization (S), just as quaternary ammonium salts and easi ly protonatable compounds are partic ularly well suited for analysis by laser desorption (LD) and fast atom bom bardment (FAB). Therefore, in addi tion to obvious compounds such as chlorophyll, it is conceivable that pep tides containing phenylalanine or ty rosine, for example, will be appropri ate candidates for chemical analysis by MPI and may possibly produce complementary sequence information, resulting from fragmentation patterns which differ somewhat from those ob served in other techniques. It will be interesting to see if the fragment ions are of the structurally informative type that, in other methods, accompa ny the localization of the charge on a
heteroatom adjacent to easily frag mented single bonds. Current MPI studies have generally focused on aro matic ring fragmentation. Finally, it is not inconceivable that chromophoric derivatization, attachment of aromat ic groups to specific sites on a mole cule, 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.; Neusser, H. J.; Schlag, E. W. J. Chem. Phys. 1980, 72, 4327. (2) Lincoln, K. A. Anal. Chem. 1965,37, 541. (3) Fermer, Ν. Ci Daly, Ν. R. J. Mater. Sci. 1968, 3, 259. (4) Wiley, R. H.; Veeranrager, P. J. Phys. Chem. 1968, 72, 2417. (5) Posthumus, Μ. Α.; 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. Lab. 1984,16, 86. (7) Cotter, R. J. Anal. Chem. 1984,56, 485 A. (8) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982,54,660.
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