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Anal. Chem. 1984, 56, 2776-2781
Resonant Two-Photon Ionization Spectroscopy in Supersonic Beams for Discrimination of Disubstituted Benzenes in Mass Spectrometry Chung Hang Sin, Roger Tembreull, and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
Supersonlc molecular beam spectroscopy has great potential for selectlvlty In chemical analysis. I n thls work we have used resonant two-photon lonlzatlon (RSPI)spectroscopy In supersonic beams of argon as a means of obtalning ultraviolet ionlzatlon spectra wlth sharp features for unlque ldentlflcatlon of species in a mlxture. This technique, therefore, provides a wavelength-selectlve means of forming Ions for detection in tlme-of-fllght mass spectrometry. The spectroscopy of several dlsubstltuted benzenes Including p -cresol, p -toluldlne, p -amlnophenol, p -fluoroanlline, p -fluorophenol, and hydroquinone has been studied near thelr origln transltlons. These molecules have low quantum yields In fluorescence but can be detected wlth great sensitivity (lo0 cm-’) or no discernible structure at all to a spectrum with sharp discrete structure (C6 cm-l). Thus, the unique spectrum of each molecule may serve as an unambiguous means of identification and further as a means of selectively discriminating one molecule from another in a mixture. Supersonic beam techniques can be combined with laser multiphoton ionization spectroscopy and ion detection techniques in order to accomplish sensitive detection and selectivity. In particular, in this work we will use (R2PI) resonant two-photon ionization as the means of ionization (12-30). R2PI is a two-photon ionization process whereby the first photon absorbed pumps a molecule to a real intermediate electronic state of interest, followed by a second photon which excites the molecule above its ionization limit, resulting in ionization. The ionization spectrum produced by collecting the ions generated as a function of wavelength reflects the
absorption spectrum of the intermediate excited state in resonance with the first photon. The second photon only serves to ionize the molecule and does not significantly affect the spectrum since it pumps the molecule into an essentially structureless continuum of states (13). The beauty of this technique is that it provides a wavelength-selective ionization source for mass spectrometry. In effect, the sharp, discrete structure obtained in ionization spectroscopy in combination with the use of supersonic beams affords a means of preselecting ions for analysis in a mass spectrometer, thus enhancing the discrimination capability of this device. This technique is analogous to GC/MS where the laser ionization serves as a real-time separation technique by the production of “color”-selective ions. The selectivity of this technique can in principle allow for high discrimination even in an inexpensive, low-resolution mass spectrometer. In addition to the wavelength-selective capability, R2PI exhibits many advantages that should be pointed out. In particular, R2PI allows a means of interrogating the spectroscopy of molecules that do not fluoresce due to rapid radiationless processes. However, these “ d a r k molecules will ionize provided the second photon can ionize the molecule before relaxation from the excited state can occur. In fact, efficient ionization has been demonstrated for a number of nonfluorescing molecules with very short-lived (picosecond) excited states (14, 15). Thus, many substituted aromatic molecules with floppy groups such a$ CH3and NH2have low fluorescence quantum yields (31) due to the added internal rotational degrees of freedom which open up third-channel pathways through which radiationless processes can occur. Fluorescence techniques have been shown to provide very sensitive detection of molecules that do fluoresce; however, they cannot provide ions for mass analysis in a mass spectrometer. Thus, an extra dimensionality is added to spectroscopic analysis where the new axis monitors the mass of the molecule for exact identification. In this paper, we will demonstrate the use of supersonic beams and laser photoionization as a preselective technique for producing ions for mass spectrometry. In particular, we will examine several disubstituted benzene compounds which have very similar ( f 5 amu) molecular weights. These compounds have essentially no quantum yield in fluorescence so that ionization methods are the most appropriate methods for their study. A time-of-flight mass spectrometer is used to monitor the product ions of the RBPI process. We demonstrate that using the laser-selective technique in combination with a supersonic expansion can provide a significant enhancement in discrimination for these compounds compared to that obtained by using a simple TOF device alone.
EXPERIMENTAL SECTION The supersonic beam time-of-flight mass spectrometer unit used in these experiments is similar to that of our previous work ( I ) . This device consists of a TOFMS in which a supersonic molecular beam crosses the ion acceleration region where ionization is produced by laser radiation. The laser beam, molecular beam,
0003-2700/84/0356-2776$01,50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
and time-of-flight ion path length are orthogonal to one another. The vacuum system consists of a stainless steel chamber pumped by a 641-1.diffusion pump with a liquid-nitrogen-cooled baffle to obtain a background pressure of
