Optically Selective Molecular Mass Spectrometry David M. Lubman The University of Michigan Department 01 Chemistry Ann Arbor, Mich. 48109 Mass spectrometry (MS) remains one ofthe must powerful means of chemical analysis based upon exact mass identification of molecular species. This identification can be accomplished through production of the molecular or "parent" ion by soft ionization methods or by extensive fragmentation of a molecule into fragment ions that are characteristic of the structure of that species. The fragmentation patterns that are induced by electron bombardment or collision-induced phenomena can thus he used for identification as well as structural analysis of molecules. The use of high-resolution MS for measurement of the mass of the ions produced, with sufficient accuracy. can provide unique identification of molecular composition. For analysis of mixtures of compounds, however, tandem methods must generally be used to obtain sufficient discrimination for identification US each of the components in MS. These techniques include methods such as gas chrumatography/MS (GC/ MS), liquid chromatography/MS (LC/ M S ) , and tandem M S methods in which two or more mass spectrometers are combined to effect a form of twodimensional separation and analysis. These may be combined to produce GC/MS/MS for further selectivity where needed. In addition, various combinations of magnetic and electric sectors can be used to study mass-analyzed ion kinetic energy spectra (MIKES) and for metastable ion monitoring for both identification and structural analysis. Further, there are a variety of soft and hard ionization 0003-2700/86/0356-031A5O1S O / O c> 1986 American Chemical Society
methods that can alter the fragmentation pattern obtained and can also provide some selectivity in the molecules ionized, as in chemical ionization. A method that ma? add to this impressive array of techniques used to achieve selectivity in MS is optical ionization. In this method, the spectruscopic absorption of a molecule provides identification and serves as a means of selectively producing ions for discrimination in MS. Resonant two-photon ionization Direct photoionization has been used as an ionization source for some time. Ionization is produced if the photon
ihle or U V light source. When the laser frequency is tuned to a real, intermediate electronic state, the cross-section for ionization is greatly enhanced. This techniclue is called resonance- en^ hanced multiphoton ionization (REMPI). When the laser is not tuned to a real state, the probability for MPI is very small. Thus, although ions are produced as the final product for detection in MS, the ionization cross-section reflects the absorption-excitation spectrum of the intermediate state. The unique property of MPI spectroscopy is that it can he used as a means of achieving spectral selection of a com-
energy is greater than the ionization potential (IP) of the molecule so that vacuum ultraviolet (VUV) radiation (-100 nmJ must t,ypically be used. Generally VUV lamps, which are low intensity and noncoherent sources, are used so that the ion yields, and consequently the sensitivity for MS, are also often low. The recent develonment of VUV laser sources may yet greatly increase the possible photoionization yield, although more development uf this emerging technology is needed. Photoionization has found use for soft ionization of molecules because the en^ ergy of the photon can he chosen, so that little excess energy is available for fragmentation, and selectivity can be achieved based on the relative IPS of the molecules examined. The advent of high-peak-power tunable visible and UV laser sources has made multiphoton ionization (MPI) processes feasible. MPI depends on the absorption of several photons by a molecule on irradiation with an intense vis-
pound prior to mass analysis. The MPI met.hod r,hxt.has found extensive applicativn to analytical chemistry is resonant two-photon ionization (RZPIJ.In this process, one photon excites a molecule to an excited electronic state, that is, S,,--S,, and a second photon ionizes the molecule (see Figure 1). Thus, the sum of the two photon energies must be greater than-the IP of the malecule for R2P1, and the two photons can have either the same or different frequencies. Because most organic species of interest have IPS between I and 1 3 eV, R2PI can he achieved using near-UV pulsed laser sources. An 1P of 7 e\' requires two 354-nm photons for ionization, whereas an IP of 13 eV requires twa 191~nmphotons. Thus, broadly tunable Nd:YAG and excirnerpumped frequency-doubled dye lasers in the UV serve as versatile sources for selective resonant ionization of molecules. Other MPI processes such as twophoton resonant ionization (TPRI) or
ANALYTICAL
CHEMISTRY, VOL. 59, NO.
1, JANUARY 1.
1987 *
31 A
more generally, n-photon resonant ionization, could he used. In this method two or more photons are needed to reach the first resonance. Because a t least one photon is nonresonant and interacts with a very short-lived virtual state s), the efficiency for ionization in these processes is far less than that achieved in RZPI. Totally nonresonant ionization is also possible, but as expected, very high laser power is needed to drive this very inefficient process, and, of course, the wavelength selectivity desired with the MPI process is lost. It should he noted that in all cases, the wavelength range used allows the light source to operate in air as opposed to VUV sources, which must operate in vacuum. In addition, the coherent nature of laser radiation allows the laser source to be located a conveniently long distance from the ion source without significant loss of light intensity. With noncoherent sources, intensity decreases as l/(distance)* so that the lamp must he as close to the ion source as possible. Thus, laser-induced RZPI can be produced in several instruments, even in different rooms, with one laser source. RZPI has several important attributes for MS in addition to its potential for selectivity. R2PI can provide very efficient soft ionization of molecules, yielding the molecular ion with little or no fragmentation (2-5). Although the full capability of this property has not been explored, soft ionization generally appears to occur at modest laser energies in a wide range of organic species, including thermally labile biological species and pharmaceuticals (6-7). Although there are distinct exceptions such as Fe(C0I5 and other inorganic complexes (8), where ahsorption of the first photon causes photodissociation, the general results appear quite encouraging. Although an electron beam can provide soft ionization, it can do so only with a significant decrease (often several orders of magnitude) in ionization efficiency. RZPI can typically provide ionization efficiencies of several percent or higher within the laser beam volume while the laser beam is on (P 106-107 W/cm2) ( I d , 9-10),although of course the efficiency is limited by the duty cycle of current laser sources. The ultimate limits to efficiency are fundamental considerations, such as the ahsorption cross-section of the molecule at a particular wavelength and the radiationless transition rate (generally due to internal conversion), Le., the rate a t which energy leaks out of SI before the second photon can induce ionization. Thus, molecules with groups that induce radiationless transitions, such as chlorinated and brominated groups on aromatic rings, will generally exhibit less efficient ionization than their un-
-
32A
substituted counterparts. Theories that model the competing processes in MPI and discuss the effect of substitution on the efficiency of RZPI have appeared in the literature (12-12). Although laser-induced fluorescence is a very sensitive detection method for molecules that fluoresce, the addition of substituent groups on aromatic rings often reduces the fluorescence quantum yield to negligible levels. These “dark” molecules will ionize, however, provided that the up-pumping rate is increased so that the second photon ionizes the molecule before relaxation from the excited state can occur. Efficient ionization has been demonstrated for a number of nonfluorescing molecules with very short-lived (picnsecond) excited states (5,22). Thus, RZPI serves as a means of generating an optical absorption spectrum of nonfluorescent compounds, for which a fluorescence excitation spectrum obviously cannot be measured. MPI can also induce fragmentation in the ionization process, the extent of which can be controlled by the laser power. Although the molecular ion is initially produced, subsequent ahsorption of additional photons may occur, resulting in excitation to a state that dissociates, producing ionic fragments. These may absorb subsequent photons, producing yet smaller ionic fragments (23). Extensive fragmentation occurs as the power density is increased, and fragments as small as C+ have been observed in hydrocarbons ionized at very high power densities (>lo8 W/cm2) (5-9). Thus, the laser serves as a versatile ionization source for producing either soft or hard ionization with great efficiency. It should be noted that the fragmentation patterns produced hy laser ionization are initiated by light typically -300 nm in wavelength, i.e., much larger than the dimension of the molecule, so that the transitions occurring are Franck-Condon controlled. According to the Franck-Condon principle, electronic transitions occur rapidly with respect to nuclear motions, involving a change to a vibrational state in a new electronic configuration in which the positions and momenta of the nuclei are essentially the same as in the initial state. Thus, light-induced transitions occur vertically without a change in the internuclear distance, and only transitions to certain states are readily allowed. Also, because of spin conservation from molecule to ion, fewer states are accessible than hy electron impact (EI). Ionization by 70-eV E1 corresponds to light of -0.15-nm energy, so that strong perturbation of the molecule occurs, and many more states are accessible with the potential for much more interesting fragmentation for analysis. Thus, laser fragmentation
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1. 1987
Flgure 1. R2PI process IP
= ionization potential
patterns generally are not as useful for structural analysis as EI-induced fragmentation. But, the laser source can produce fragments with high appearance potentials with relative ease through increases in the laser energy. Supersonic beam The potential optical selectivity of R2PI is limited hy the gas-phase UVvis spectra of polyatomic molecules, which are generally broad and structureless a t room temperature. This hroad structure results from thermal population of a large manifold of internal states, that is, rotations and vibrations of a molecule, thus causing congestion in the spectrum. To make RZPI a viable tool for optical analysis, this hroad contour must be “cooled out” so that the structure collapses down to several sharp peaks. This can be accomplished using the supersonic beam technique in which a small amount of a large polyatomic molecule is seeded into a large bath of a light carrier gas, such as Ar, and expanded through an orifice (where X [mean free path]
