Instrumentation Robert J. Cotter Department of Pharmacology The Johns Hopkins University Baltimore, Md. 21205
urn
LASERS
A N D MASS SPECTROMETRY In the late 1960s, a number of arti cles began to appear in the literature on lasers and mass spectrometry. Early studies included the vaporiza tion of graphite by Berkowitz and Chupka (7) and Lincoln (2), laser-in duced vaporization of coal by Knox and Vastola (3), for classifying coals according to rank (4), elemental anal ysis in metals (5), isotope ratio mea surements (6), and pyrolysis (7). Later work saw the extension of these meth ods to biological samples (8), the de velopment of the laser microprobe mass spectrometer by Hillenkamp et al. (8), and the formation of "molecu lar" ions from nonvolatile organic salts by Vastola, Mumma, and Pirone (9). These and other applications have been reviewed (10). 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 be more dissimilar. The multiphoton techniques in volve direct interaction of laser pho tons with individual molecules, atoms, or ions in the gas phase. To date rela tively small molecules have been stud ied. The techniques include multipho ton ionization (MPI), of both the res onance-enhanced (REMPI) and nonresonant variety, resonance ioniza tion (RIMS) for elemental analysis at high sensitivity, and photodissocia tion. Because electronic transitions are involved, selectivity for particular atoms or molecules is possible. On the other hand, the laser de sorption methods involve the interac tion of a laser beam with a samplesubstrate in the solid phase, for the production of molecular ions and 0003-2700/84/0351-485A$01.50/0 © 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, but rather an exploration of the basic principles involved, and a com parison of the kinds of analytical in formation that can be obtained from these methods. Multiphoton Techniques
For spectroscopists, multiphoton techniques offer the possibility for studying neutral molecule electronic transitions that may be forbidden ac cording to the selection rules for sin gle-photon absorbance. MPI can be used either to enhance the sensitivity of such absorbance spectra, or as a
photoionization method using wave lengths of the electromagnetic spec trum, which are less difficult instrumentally than vacuum UV wave lengths. As an ion source in a mass spectrometer, resonant techniques can produce high ionization efficiencies and high selectivity in combination with mass analysis. Nonresonant ion ization, while less sensitive and selec tive, in general produces less fragmen tation. The ability to induce fragmen tation, on the other hand, gives rise to methods in which the laser is mounted on another part of the mass spectrom eter—a collision chamber between an alyzers. Such methods are useful for fundamental and analytical studies analogous to the collision-induced de composition (CID) techniques in ion kinetic energy spectrometry and mass
Single-Photon Processes
(a)
(b)
Multiphoton Processes
IP-
IP-
=2g-
GS UV 259 η m Absorption Spectroscopy
(e)
(d)
(c)
m IP-
-2e—r
1
GS Vacuum UV Nonresonant
GS Visible 391 nm
Visible 391 nm
UV 259 nm
Photoionization
Multiphoton Spectroscopy
Multiphoton Ionization (REMPI)
Multiphoton Ionization (REMPI)
GS
IP
GS
B2u
GS UV Nonresonant Multiphoton Ionization (Nonresonant)
Figure 1 . E x a m p l e s of s o m e of the a b s o r p t i o n - i o n i z a t i o n p r o c e s s e s for t h e b e n z e n e m o l e c u l e , as d e s c r i b e d in text IP = ionization potential
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3,
MARCH 1984 · 485 A
+ 200 V ο
Tunable Dye Laser
El Mass Spectrum
Nitrogen Laser
Absorbance Cell 55
Current Amplifier
Ion Current
J^_JLJ*1_JL-X__JU —ι
1
1 50 50
30 360 370
380
390
400
410 (nm)
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 J B 2 u state forms the strongest band in the UV absorbance spectrum of that mol ecule (11) and occurs at 259 nm (Fig ure la). The Έ 2 Β state is not accessi ble by single-photon techniques (12, 13), but can be reached by a two-pho ton process (14), using laser radiation at 391 nm (Figure lc). The transition is more easily observed as an ion cur rent using a three-photon process (Figure Id). Using a N 2 -pumped dye laser to scan the region from 360 to 400 nm, Johnson recorded an ion yield spec trum of benzene (Figure 2) and ob served a current maximum at a wave length equal to 2X the wavelength (195 nm in the UV), which would cor respond to the energy of the 0-0 (vi brational) transition to the 1 E 2 g state (12, 13). The ion yield spectrum was recorded as total ions striking a collec tor electrode, coaxial with a cylindrical absorbance chamber biased at 200 V. Currents were in the range of 1 0 _ n A. There are two other interesting fea tures of the three-photon ionization (two-photon resonance) spectrum in Figure 2. The first is that transitions between other vibronic states "com plicate" the spectra, so that experi ments 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 in crease in the nonresonant ionization current (toward lower wavelengths), which begins at the point where three
photons just equal the ionization po tential. The great advantage of the MPI technique, however, is that the measurement of ion current is a great deal more sensitive than the measure ment of the attenuation of the laser beam (absorbance) by the gas phase sample.
τ 70 70 Mass
1
• 90 90
Γ
ι
110 11
MPI Mass Spectrum 558.8 nm
Figure 2. Ion yield spectrum for benzene using a two-photon-resonance/threephoton-ionization process (see Figure 1d) IP = ionization potential. Adapted from Reference 13
spectrometry/mass spectrometry (MS/MS) experiments.
1
57
42
112
—*_ 30
50
70 Mass
90
110
MPI and Mass Spectrometers
Mass spectroscopists have always been interested in separating and characterizing the components of mix tures, and so they have coupled gas chromatographs (GC/MS), liquid chromatography (LC/MS) (16), and even other mass spectrometers (MS/ MS) (17) to the front ends of their in struments. In the REMPI technique, the laser is tuned to a fixed wave length while the mass spectrum is scanned. The object is to ionize one component of a mixture, using a reso nant intermediate state characteristic of that compound. Some of the ear liest experiments, however, used the REMPI technique to provide informa tion about the multiphoton process it self. In the experiments reported by Zandee and Bernstein in 1978, single ion yield spectra for the molecular ion and fragment ions of benzene were ob tained across the "Johnson band" from 380 to 395 nm, and REMPI spec tra were obtained at selected resonant wavelengths (18). They observed frag mentation much more extensive than that produced by electron impact (EI) ionization. Using available appearance potential (AP) data, they noted that the C 2 H 2 + fragment (AP = 18.6 eV) would require up to six 3.2-eV photons and estimated that for Ci + fragments nine photons are absorbed by the ben zene molecule. A comparison of the EI and REMPI spectra in Figure 3 for triethylenediamine illustrates the de-
486 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
MPI Mass Spectrum 425.2 nm
42 56
.. J ^L-iL.] ι 30
50
70 Mass
90
Figure 3. Comparison of electron impact and resonance-enhanced multiphoton ionization mass spectra for triethylenediamine Reproduced with permission from Reference 19
gree of fragmentation, which also de pends on the wavelength selected (19). A study of the REMPI mass spectra of furan by Cooper et al. (20) revealed that molecular ions could be observed in high abundance in the mass spec trum using a three-photon resonance at 376 nm, but only very weakly using a two-photon resonance at 550.5 nm. Fragment ion intensity is also en hanced at high laser power densities (20). Mechanisms
The dependence of the degree of fragmentation on laser power density
110
*
'' **->
χ
Fragment Ions
ι ,
'
^ " ^ " V - - *
IP
.
GS Neutral Ladder
Ionic Ladder
Figure 4. lonization-fragmentation scheme for the benzene molecule Generally, fragmentation in MPI results from absorption of additional photons by the ionized parent molecule, rather than autoionization of neutral photofragments. Adapted from Reference 21
is not surprising. However, the depen dence of the total ion current is gener ally 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-pho ton ionization (one-photon resonance) of benzene using pulsed-UV laser ra diation at 259 nm (Figure le). At power densities of 106 W/cm 2 , the ab sorption of a third photon is improba ble and little fragmentation occurs. When a second, visible pulse at 518 nm and a power density of 108 W/cm 2 is added, fragmentation oc curs, even when the visible radiation lags the UV pulse by up to 20 ns. From this they concluded that frag mentation occurs separately from ion ization, a "two-ladder process." That is, fragmentation is not the result of autoionization of superexcited neutral molecules or photofragments; instead, ionization occurs first, followed by dis sociation of the parent ion (Figure 4). Assuming a four-level model that in cluded 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 at low laser power, its linear dependence at intermediate power, and the saturation that occurs at high power. Photofragmentation can, however,
precede MPI in some cases. Laser irra diation of Mn 2 (CO)io at 511 and 483 nm by Lichtin et al. produced photofragments: Μη, Μη 2 , and MnCO (23). These were then ionized by ab sorption of additional photons, and the other peaks in the mass spectrum resulted from ion-molecule reactions between Mn + and Mn2(CO)io- The time-of-flight (TOF) mass spectrome ter confirmed the products of bimolecular processes with broader peaks for these species than for those ions pro duced directly by MPI. Nine photons were involved. Photodissociation re quired six. Two were used to produce a resonant excited state for Mn, and the last produced the ionized atom. Nonresonant Techniques Excimer lasers such as the ArF laser (193 nm, 6.42 eV) or the KrF laser (249 nm, 5.0 eV) can be used for twophoton nonresonant ionization of mol ecules with ionization potentials less than 12.8 or 10 eV, respectively (Fig ure If). Like the photoionization pro cess (Figure lb) (24) nonresonant MPI can produce molecular ions with little fragmentation (25), but the ion ization efficiency may be several or ders of magnitude below that of the resonant MPI technique (26). Unlike the photoionization technique, how ever, the differential vacuum pumping associated with vacuum UV is avoid ed, and very high fluxes can be used. DeCorpo et al. (26) have obtained ArF excimer laser mass spectra for a num ber of organic molecules. Ionization takes place by the usual photoioniza tion channels of direct ionization or autoionization from a superexcited state, except that two photons are in volved. However, in addition to disso ciation of the molecular ion or super excited molecules into ion pairs, frag ment ions may also result from ab sorption of additional photons by the molecular ion or neutral photofrag ments. 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 focus ing" technique first described by Wiley and McLaren (27) improves resolution for ions produced with an energy spread somewhat larger than EI methods. Schlag et al. (28) have used a reflectron TOF to examine metastable decomposition of molecu lar ions produced by MPI. MPI tech niques also have been used successful ly with Fourier transform mass spec trometers (29, 30).
488 A · 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 re sults in high sensitivity due to the ion ization of all (or nearly all) of the atoms of a particular element in a par ticular 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 be used for elemen tal analysis or to measure the popula tion of an electronic state (31-35). When combined with mass spectrome try it is known as resonance ioniza tion mass spectrometry (RIMS), and isotopic abundances of elements can be measured in extremely small con centrations or where there are isobaric interferences (36-38). The four-level model can be used to describe the saturation condition used in the RIS technique. The rate equa tions 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 in creases until both states are in equilib rium. For the two-photon ionization of Cs atoms, for example (32), using a 2-μβ laser pulse at 459 nm and an en ergy density of 0.2 J/cm 2 , this condi tion will be 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 background gas, are not photon dependent, so that high photon fluxes can be used to favor the ionization channel (flux con dition). 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 ioniza tion continuum. Hurst et al. have given a detailed treatment of the ki netics, as well as a review of applica tions (31,32). The RIS technique can detect a sin gle atom. This is not merely a detector response, but 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 ion ized and will emit a detectable elec tron (32). A particularly elegant dem onstration of this ability to detect a single atom was carried out by Kramer et al. (39), who looked for neutral Cs atoms following a 2,,2Cf fission event
where two fission products are pro duced simultaneously. The first parti cle was detected by a surface barrier detector, which triggered the laser pulse at an appropriate wavelength to observe the second particle if it was a neutral cesium atom. In 100 fission events, they observed eight Cs atoms. The combination of resonance ion ization with mass spectrometry adds mass selectivity to elemental selectivi ty. An application of the RIMS tech nique currently being developed at Oak Ridge National Laboratory is the analysis of neodymium in nuclear fuel in a reactor as an indicator of the number of fissions that are occurring. Nd-14;}, -145, -146, -148, and -150 are fission products. Nd-142 is naturally occurring but is not a fission product and can be used to correct for back ground. However, Sm-144, -148, -150, and Ce-142 produce interferences in mass spectrometric techniques that use thermal ionization, since it is diffi cult to separate these elements chemi cally and their ionization behavior is similar. Donohue et al. (36) have used the RIMS technique to produce mass spectra and to determine isotope ratios from Nd in mixtures with Sm. Classically, thermal ionization MS has been used for elemental analysis where thermal filament sources have been used to vaporize and ionize the elements. The ratio of ions (n+) to neutrals (n°) in the gas phase as pre dicted by the Langmuir-Saha equa tion n+/n"«
exp[(w -
Ip)/kT]
(1)
(where w = work function of the metal filament, Ip is the ionization potential of the neutral atom, k is the Boltzmann constant, and Τ is the ab solute temperature) is typically very
Ion Source
Quad 1
Quad 2
small for elements with high ioniza tion potentials, e.g., Re (/,, 7.9 eV), Mo (/„ 7.1 eV), and V (I,, 6.7 eV). Fassett et al. (38) have combined reso nance ionization with a thermal ion ization source that acts as a "neutral atom pool" from which the RIMS technique can be used to selectively ionize an element. Using a Nd-YAGpumped dye laser system and a twophoton resonance scheme, they have developed a fairly convenient tech nique, which can be used to analyze many of the elements of the periodic table. Photodissociation of Ions
The tendency for high photon fluxes to produce extensive fragmen tation forms the basis for a technique known as photodissociation. In this case sample molecules have already been ionized, usually by EI, so that absorption of photons causes the ions to climb the "second energy ladder" leading to fragmentation. The process is not generally a resonant one, as the excess energy absorbed simply distrib utes itself among the degrees of free dom of the ion. While photodissocia tion can be carried out in the ion source, more useful information can be obtained by focusing the laser beam into a field-free region after mass analysis. The results are ob served by recording product ions fil tered through a second mass (or ener gy) analyzer. From these techniques one can obtain fragmentation spectra for mass-selected ions or information about the electronic states of ions pro duced in the source and about the manner in which absorbed photon en ergy is distributed. Two instruments are described here that are quite different in their design,
Quad 3
•
Tunable Flashlamp Dye Laser
Photodissociation Spectrum
Figure 5. The triple-quadrupole instrument of McGilvery and Morrison Scanning the laser wavelength produces a photodissociation spectrum. Adapted from Reference 40
490 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
the variables they scan, and the infor mation they obtain. The first, re ported by McGilvery and Morrison in 1978 (40), uses a triple quadrupole mass spectrometer. Primary ions are produced by EI and mass selected by the first quadrupole filter. The second quadrupole is "rf only." It transmits ions of any mass and is the collision chamber when photodissociation oc curs. The third quadrupole selects product ions. The laser beam is actu ally focused through all three of the quadrupoles. However, ions fragment ing in either of the mass filters are not transmitted. If the mass filters are set to select a particular parent ion and photofragment ion (e.g., CH3l + and CH3 + , respectively), then the wave length is scanned to produce a photodissociation spectrum (Figure 5). The laser in this experiment is a flashlamp-pumped dye laser. Twelve dyes are used to cover the region from 440 to 670 nm. As in the multiphoton methods, measurement of mass-selected ion current is a more sensitive way of monitoring absorption phenomena. For example, in this experiment (40) the effective partial pressure of the ion beam through the middle quadru pole was 10 - 1 1 torr (or 109 ions/s). From each laser flash, 100-200 photofragment ions were produced. Mea surement of the attenuation of the laser beam would be impractical, while detection of 103 ions/s from repetitive laser pulses is easily accomplished by counting methods. Photodissociation processes compete with unimolecular and bimolecular dissociations that may be of comparable or even greater magnitude. In this instrument an u p down counting system is used in which ions are counted and added for a peri od after each laser flash, and back ground dissociation is subtracted for an equal period 1 ms later. A double-focusing, reversed-geometry sector mass spectrometer has been used by Beynon and co-workers (4144) to study the photodissociation of ions. An argon ion laser beam is fo cused into the field-free region be tween the magnetic (B) and electro static (E) sectors. The beam is coaxial with the ion path in the field-free re gion (collision chamber). The magnet ic sector is used to mass-select an ion, usually the molecular ion. The laser wavelength is fixed, and the electro static analyzer (energy filter) voltage is scanned. The output is an ion kinet ic energy (IKE) spectrum, as shown in Figure 6 (41). Again, unimolecular de composition (Figure 6a) competes with photodissociation. Therefore the continuous laser beam is mechanically chopped at 2.4 kHz and phase-sensi tive detection using a lock-in amplifier allows one to look only at the addi-
Argon Ion Laser
generally less than unity. In at least one case this has been used to distin guish between two isomers that other wise have similar fragmentation spec tra. For the loss of NO'2 from 4-fluoronitrobenzene, a = 0.86, while for the 3-fluoro isomer a = 0.54. Fewer de grees of freedom are involved in the distribution of excess energy for 3-fluoronitrobenzene (42).
Collision Region
/
Ion
ι 3
I r
i\' (
/
f Λ
