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Robert J. Cotter
Department of Pharmacology The Johns Hopkins University Baltimore. Md 21205
ANDMASS SP-ROMETRY In the late 1960s, a number of articles began to appear in the literature on lasers and mass spectrometry. Early studies included the vaporization of graphite hy Berkowitz and Chupka (I) and Lincoln ( 2 ) .laser-induced vaporization of coal by Knox and Vastola (3),for classifying coals according to rank ( 4 ) , elemental analysis in metals ( 5 ) ,isotope ratio measurements (6),and pyrolysis (7).Later work saw the extension of these metbods to biological samples (8),the development of the laser microprobe mass spectrometer by Hillenkamp et al. (81,and the formation of "molecular" ions from nonvolatile organic salts by Vastola, Mumma, and Pirone (9). These and other applications have been reviewed (IO). In the past several years there has been a great deal of interest in two new broad areas involving laser-mass spectrometer combinations: the multiphoton techniques and the desorption techniques. They could not he more dissimilar. The multiphoton techniques involve direct interaction of laser photons with individual molecules, atoms, or ions in the gas phase. To date relatively small molecules have been studied. The techniques include multiphoton ionization (MPI), of both the resonance-enhanced (REMPI) and nonresonant variety, resonance ionization (RIMS) for elemental analysis a t high sensitivity, and photodissociation. Because electronic transitions are involved, selectivity for particular atoms or molecules is possible. On the other hand, the laser desorption methods involve the interaction of a laser beam with a samplesubstrate in the solid phase, for the production of molecular ions and 0003-27001841035 1485A$0 1 5010 1984 American Chemical Society
structurally significant fragments from more complex, nonvolatile, and (at times) very large molecules. The discussion that follows is not an extensive review of activity in these areas, hut rather an exploration of the basic principles involved, and a comparison of the kinds of analytical information that can he obtained from these methods. Multlphoton Techniques
For spectroscopists, multiphoton techniques offer the possibility for studying neutral molecule electronic transitions that may be forbidden according to the selection rules for single-photon absorbance. MPI can be used either to enhance the sensitivity of such absorbance spectra, or as a
photoionization method using wavelengths of the electromagnetic spectrum, which are less difficult instrumentally than vacuum UV wavelengths. As an ion source in a mass spectrometer, resonant techniques can produce high ionization efficiencies and high selectivity in combination with mass analysis. Nonresonant ionization, while less sensitive and selective, in general produces less fragmentation. The ability to induce fragmentation, on the other hand, gives rise to methods in which the laser is mounted on another part of the mass spectrometer-a collision chamber between analyzers. Such methods are useful for fundamental and analytical studies analogous to the collision-induced decomposition (CID) techniques in ion kinetic energy spectrometry and mass
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Figure 1. Examples of some of the absorption-ionization processes for the benzene molecule, as described in text IP = ionization potential
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3. MARCH 1984
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Figure 2. ion yield spectrum for benzene using a two-photon-resonancelthreephoton-ionization process (see Figure Id) IP = ioniratim potential. Adapted lrmReterem 13
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MPI The benzene molecule has been studied extensively. It offers a good example of the processes involved (Figure 1). The electronic transition from the ground state (GS) to the 'Bz. state forms the strongest band in the UV absorbance spectrum of that molecule ( 1 1 ) and occurs a t 259 nm (Figure la). The 'E, state is not accessible by single-photon techniques (12, 13), but can be reached hy a two-photon process (14),using laser radiation a t 391 nm (Figure IC).The transition is more easily observed as an ion current using a three-photon process (Figure Id). Using a N2-pumped dye laser to scan the region from 360 to 400 nm, Johnson recorded an ion yield spectrum of benzene (Figure 2) and observed a current maximum a t a wavelength equal to 2X the wavelength (195 nm in the UV), which would correspond to the energy of the 0-0 (vibrational) transition to the lEa state (12,I3).The ion yield spectrum was recorded as total ions striking a collector electrode, coaxial with a cylindrical absorbance chamber biased a t 200 V. Currents were in the range of lo-" A. There are two other interesting features of the three-photon ionization (two-photon resonance) spectrum in Figure 2. The first is that transitions between other vibronic states "complicate" the spectra, so that experiments using "cooled" molecules emerging from an expanding carrier gas have been carried out to narrow the absorption band (15).The second is that there is a small but finite increase in the nonresonant ionization current (toward lower wavelengths), which begins a t the point where three 486A
photons just equal the ionization potential. The great advantage of the MPI technique, however, is that the measurement of ion current is a great deal more sensitive than the measurement of the attenuation of the laser beam (absorbance) by the gas phase sample.
MPI and Mass Spectrometers Mass spectroscopists have always been interested in separating and characterizing the components of mixtures, and so they have coupled gas chromatographs (GC/MS), liquid chromatographs (LC/MS) (113,and even other mass spectrometers (MSI MS) ( 1 7 )to the front ends of their instruments. In the REMPI technique, the laser is tuned to a fixed wavelength while the mass spectrum is scanned. The ohject is to ionize one component of a mixture, using a resonant intermediate state characteristic of that compound. Some of the earliest experiments, however, used the REMPI technique to provide information about the multiphoton process itself. In the experiments reported by Zandee and Bernstein in 1978, single ion yield spectra for the molecular ion and fragment ions of benzene were obtained acrws the "Johnson hand" from 380 to 395 nm, and REMPI spectra were obtained a t selected resonant wavelengths (18).They observed fragmentation much more extensive than that produced by electron impact (EI) ionization. Using available appearance potential (AP) data,.they noted that the C2H2+fragment (AP = 18.6eV) would require up to six 3.2-eV photons and estimated that for C1+ fragments nine photons are absorbed by the henzene molecule. A comparison of the E1 and REMPI spectra in Figure 3 for triethylenediamine illustrates the de-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3. MARCH 1984
Figure 3. Comparison 01 electron impact and resonanceenhanced multiphoton ionization mass spectra for triethylenediamine R e w o d ~ w dwilh wmission horn Reference 19
gree of fragmentation, which also depends on the wavelength selected (19). A study of the REMPI mass spectra of furan by Cooper et al. (20) revealed that molecular ions could he observed in high abundance in the mass spectrum using a three-photon resonance a t 376 nm, hut only very weakly using a two-photon resonance a t 550.5 nm. Fragment ion intensity is also enhanced a t high laser power densities (20) . Mechanisms
The dependence of the degree of fragmentation on laser power density
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precede MPI in some cases. Laser irradiation of Mnz(CO)loa t 511 and 483 nm by Lichtin et al. produced photofragments: Mn, Mnz, and MnCO (23).These were then ionized by absorption of additional photons, and the other peaks in the mass spectrum resulted from ion-molecule reactions between Mn+ and Mnn(CO),o. The time-of-flight (TOF) mass spectrometer confirmed the products of bimolecular processes with broader peaks for .hese species than for those ions proluced directly by MPI. Nine photons were involved. Photodissociation reluired six. Two were used to produce 3 resonant excited state for Mn, and .he last produced the ionized atom.
Vonresonant Techniques
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Generally. fragmentation in MPI results from absorption Of additional photons by the lonlzed parent molecule. rather than autoimizalionof neutral photofragments. Adapt64 ham Reference 21
is not surprising. However, the dependence of the total ion current is generally of low order, even though a large number of photons may be required to produce the fragment ions (18). Schlag et al. (21) studied the two-photon ionization (one-photonresonance) of benzene using pulsed-UV laser radiation a t 259 nm (Figure le). At power densities of 106 W/cm2, the absorption of a third photon is improbable and little fragmentation occurs. When a second, visible pulse at 518 nm and a power density of 108 W/cm2 is added, fragmentation occurs, even when the visible radiation lags the UV pulse by up to 20 ns. From this they concluded that fragmentation occurs separately from ionization, a “two-ladder process.” That is, fragmentation is not the result of autoionization of superexcited neutral molecules or photofragments; instead, ionization occurs first, followed hy dissociation of the parent ion (Figure 4). Assuming a four-level model that included a population-trapping state into which the intermediate state may relax but cannot ionize, Reilly and Kompa (22) constructed a set of rate equations that correlated well with the observed quadratic dependence of the total ion current a t low laser power, its linear dependence a t intermediate power, and the saturation that occurs at high power. Photofragmentation can, however, 488 A
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Excimer lasers such as the ArF laser ,193 nm, 6.42 eV) or the KIF laser 249 nm, 5.0 eV) can he used for two>hoton nonresonant ionization of molxules with ionization potentials less chan 12.8 or 10 eV, respectively (Figure 10.Like the photoionization process (Figure lb) (24) nonresonant MPI can produce molecular ions with little fragmentation ( W ) ,but the ionization efficiency may he several orders of magnitude helow that of the resonant MPI technique (26). Unlike the photoionization technique, however, the differential vacuum pumping associated with vacuum UV is avoided, and very high fluxes can he used. DeCorpo et al. (26) have obtained ArF excimer laser mass spectra for anumher of organic molecules. Ionization takes place by the usual photoionization channels of direct ionization or autoionization from a superexcited state, except that two photons are involved. However, in addition to dissociation of the molecular ion or superexcited molecules into ion pairs, fragment ions may also result from absorption of additional photons hy the molecular ion or neutral photofragments. DeCorpo and co-workers’mass spectrum of benzene is most similar to low-energy electron impact spectra (26).
Instrumentation The experiments discussed above have involved TOF instruments (19, 21-23) or quadrupole analyzers (18, 26). Lichtin et al. (23)have noted that for TOF analyzers the “time lag focusing” technique first described by Wiley and McLaren (27) improves resolution for ions produced with an energy spread somewhat larger than E1 methods. Schlag et al. (28) have used a reflectron TOF to examine metastable decomposition of molecular ions produced by MPI. MPI techniques also have been used successfully with Fourier transform mass spectrometers (29,30).
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
RIS and RIMS Resonance ionization spectroscopy (RIS) is the multiphoton ionization process driven to saturation using high photon flux. This condition results in high sensitivity due to the ionization of all (or nearly all) of the atoms of a particular element in a particular electronic state found within the laser volume during the time of the laser pulse. The electrons given off in the ionization process are detected. The technique can he used for elemental analysis or to measure the population of an electronic state (31-35). When combined with mass spectrometry it is known as resonance ionization mass spectrometry (RIMS), and isotopic ahundances of elements can be measured in extremely small concentrations or where there are isobaric interferences (36-38). The four-level model can he used to describe the saturation condition used in the RIS technique. The rate equations are solved and integrated over the entire time of the laser pulse (31, 32). When the photon flux from the laser is high, stimulated (phOton.de. pendent) emission from the excited state back to the ground state increases until both states are in equilibrium. For the two-photon ionization of Cs atoms, for example (321, using a 2 . ~ 9 laser pulse a t 459 nm and an energy density of 0.2 J/cmZ, this condition will he reached within the first nanosecond of the duration of the pulse. As in any kinetic scheme, where an equilibrium assumption can be made, the rate equations are greatly simplified, depending here only on the depletion of the equilibrium mixture into ions and into states that are lost to the ionization process. The latter processes, transitions to inaccessible states or reactions with hackground gas, are not photon dependent, so that high photon fluxes can be used to favor the ionization channel (flux condition). If the pulse duration is long enough (fluence condition) then very nearly all of the atoms in the initial state will find their way to the ionization continuum. Hurst et al. have given a detailed treatment of the kinetics, as well as a review of applications (31,32). The RIS technique can detect a single atom. This is not merely a detector response, hut rather it means that if a single Cs atom in a gas pressure of 100 torr is in the volume intersected by the laser beam and the gas at the time of the laser pulse, it can be ionized and will emit a detectable electron (32).A particularly elegant demonstration of this ability to detect a single atom was carried out by Kramer et al. (39),who looked for neutral Cs atoms following a 252Cffission event
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where two fission products are produced simultaneously. The first partid e was detected by a surface barrier detector, which triggered the laser pulse a t an appropriate wavelength to nhserve the second particle if it was a neutral cesium atom. In 100 fission events, they observed eight Cs atoms. The combination of resonance ionization with mass spectrometry adds mass selectivity to elemental selectivity. An application of the RIMS technique currently being developed at Oak Ridge National Laboratory is the analysis of neodymium in nuclear fuel in a reactor as an indicator of the numher 01fissions that are occurring. Nd-143, -145, -146, -148, and -150 are fission products. Nd-142 is naturally occurring but is not a fission product and can he used to correct for hackground. However, Sm-144, -148, -150, and Ce-142 produce interferences in mass spectrometric techniques that use thermal ionization, since it is difficult to separate these elements chemi
