A Two-Color Three-Photon Ionization Scheme for the Efficient and

Anal. Chem. 2004, 76, 2419-2422

A Two-Color Three-Photon Ionization Scheme for the Efficient and Selective Ionization of a Chlorinated Aromatic Hydrocarbon Tomohiro Uchimura, Kei-ichiro Kai, and Totaro Imasaka*

Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka, 812-8581, Japan

A two-color three-photon ionization scheme, for the efficient and selective ionization of a chlorinated aromatic hydrocarbon that has an ionization potential higher than the two-photon energy of the laser used for excitation, is described. In this technique, an ultraviolet (UV) laser, i.e., the second harmonic emission of a fundamental (VIS) laser, is used for excitation and a UV and VIS laser for the subsequent two-photon ionization from the electronic excited state. A sample of o-chlorophenol was used as a model compound to demonstrate the advantage of this technique. The signal in supersonic jet/resonance-enhanced multiphoton ionization/mass spectrometry was increased ∼4 times by the introduction of the VIS beam, when the polarization was adjusted to be parallel to the UV beam. Thus, the two-color three-photon (2UV+VIS) ionization scheme is more sensitive than one-color threephoton (3UV) ionization. The merits of this method over other ionization schemes such as two-color two-photon (UV1+UV2) ionization are discussed in terms of sensitivity and selectivity in spectrometric analysis. Polychlorinated dibenzo-p-dioxin/dibenzofuran (PCDD/F) is known to be one of the most toxic compounds artificially synthesized by humans. It is well recognized that PCDD/F is emitted mainly from waste incineration plants, consists of numerous isomers and congeners, and is present at ultratrace levels. Although gas chromatography combined with mass spectrometry has been utilized for analysis of these compounds, it requires a lengthy procedure for preconcentration and pretreatment. Thus, a selective as well as sensitive analytical method is desirable for the on-line monitoring of PCDD/F. Supersonic jet/multiphoton ionization-mass spectrometry (SSJ/MPI-MS)1-4 may be potentially useful for this purpose because of the high selectivity provided by the high spectral resolution in supersonic jet spectrometry and high sensitivity/selectivity offered by mass spectrometry.5,6 It is, * To whom correspondence should be addressed. Phone: 81-92-642-3563. Fax: 81-92-632-5209. E-mail: [email protected]. (1) Amirav, A.; Even, U.; Jortner, J. Chem. Phys. 1980, 51, 31-42. (2) Hayes, J. M.; Small, G. J. Anal. Chem. 1983, 55, 565A-574A. (3) Hayes, J. M. Chem. Rev. 1987, 87, 745-760. (4) Lubman, D. M. Anal. Chem. 1987, 59, 31A-40A. (5) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (6) Hafner, K.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Kettrup, A. Anal. Chem. 2001, 73, 4171-4180. 10.1021/ac035057t CCC: $27.50 Published on Web 03/13/2004

© 2004 American Chemical Society

however, necessary to solve two problems to achieve this. For example, the concentration of pentachlorodibenzofuran (PCDF) in the flue gas is highly correlated with the toxicity equivalent quantity value, the correlation factor sometimes exceeding 0.99.7 Therefore, this molecule can be used as an indicator for evaluation of the toxicity of the sample. However, it is difficult to efficiently ionize PCDF using a conventional nanosecond laser. This result arises from a short excited-state lifetime of PCDF (∼10-100 ps). It is reported that the ionization efficiency is optimal when a laser pulse width is identical to the lifetime value of the analyte, and then a picosecond laser with a transform-limited line width is suggested being used.8,9 Unfortunately, such a laser has not yet been commercialized. On the other hand, it is well known that the excitation of the analyte molecule at the band origin, i.e., the 0-0 transition peak, is of great importance. Since this transition is located at the longest wavelength, the excitation of small interfering molecules that absorb the light emitting at shorter wavelengths can be minimized, thus improving the spectral selectivity. In addition, the signal of this peak is strong in most cases, thus further improving the sensitivity and selectivity in the analysis. Various excitation/ionization schemes have been proposed to date, as shown in Figure 1. A one-color ionization scheme using a laser emitting at the 0-0 transition (Figure 1a) is less preferable in the case of PCDD/F, since the ionization potential is much higher than twice the excitation energy of the laser used for excitation, and as a result, three photons are necessary for ionization.10 Due to the small cross section for a three-photon process, this scheme provides low ionization efficiency or induces strong fragment ions at higher laser intensities. To solve this problem, a one-color two-photon ionization scheme is proposed (Figure 1b). In this case, the laser wavelength should be relatively short, to excite the analyte molecule to the vibrational level of the electronic excited state. A laser emitting at shorter wavelengths may increase the possibility of ionizing interfering molecules, as described before. It should be noted that numerous polycyclic aromatic hydrocarbons have absorption bands in this spectral region, and as a result, spectral selectivity is substantially degraded. To avoid this undesirable effect, a two-color two-photon (7) (8) (9) (10)

Kato, M.; Urano, K. Waste Manage. 2001, 21, 55-62. Matsumoto, J.; Imasaka, T. Anal. Chem. 1999, 71, 3763-3768. Yoshida, N. Hirakawa, Y.; Imasaka, T. Anal. Chem. 2001, 73, 4417-4421. Zimmermann, R.; Lenoir, D.; Kettrup, A.; Nagel, H.; Boesl, U. Proceedings of Twenty-Sixth Symposium on Combustion/The Combustion Institute, 1996; pp 2859-2868.

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Figure 2. Experimental apparatus for the SSJ/MPI-TOF-MS.

Figure 1. Schematic diagram of (a) one-color three-photon ionization, (b) one-color two-photon ionization, (c) two-color two-photon ionization using the fourth harmonic beam (266 nm) of a Nd:YAG laser, (d) two-color two-photon ionization through Rydberg states, (e) two-color three-photon ionization using the fundamental beam (1064 nm) of a Nd:YAG laser, and (f) two-color three-photon ionization using the fundamental beam of the dye laser.

ionization scheme can be used.11-14 As shown in Figure 1c, one of the lasers can be the fourth harmonic emission (266 nm) of the Nd:YAG laser that was used as a pump source for a tunable laser (300 nm). However, numerous interfering molecules are also excited and ionized at 266 nm. To increase the spectral selectivity, another tunable laser (e.g., 275 nm) may be used for ionization from the excited state to the high-energy level such as one of the Rydberg states (Figure 1d), followed by ionization with an electric potential. However, the ionization of interfering molecules (275 + 275 nm) cannot be completely eliminated. This problem can be solved by the use of a two-color three-photon ionization scheme. To enhance the ionization efficiency, it is necessary to use a highintensity laser as the second laser. For this reason, the fundamental beam of the Nd:YAG laser can be used for this purpose, as shown in Figure 1e. Unfortunately, it is difficult in practice to align this invisible intense laser temporally and spatially with a weak tunable UV laser and may sometimes increase the possibility of a hazardous accident. As described before, the use of a highpeak-power picosecond laser is necessary for the efficient ionization of an analyte molecule having a short excited-state lifetime, but the nanosecond Nd:YAG laser has a low peak power and the enhancement of the signal might be minimal. In this study, a two-color three-photon ionization scheme, as shown in Figure 1f, was employed, in which the fundamental beam used for the generation of the second harmonic beam was also utilized to enhance the ionization efficiency. For example, a picosecond ultraviolet (UV) laser can be used for excitation, which is followed by two-photon ionization using two photons arising (11) Hager, J. W.; Wallace, S. C. Anal. Chem. 1988, 60, 5-10. (12) Oser, H.; Coggiola, M. J.; Faris, G. W.; Young, S. E.; Volquardsen, B.; Crosley, D. R. Appl. Opt. 2001, 40, 859-865. (13) Lin, C. H.; Hozumi, M.; Imasaka. T. Analyst 1991, 116, 1037-1041. (14) Uchimura, T.; Kanda, H.; Imasaka, T. Anal. Sci. 2003, 19, 387-389.

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from the UV laser and the fundamental (VIS) laser producing the UV laser. The laser wavelength is adjusted at the 0-0 transition as in the case of two-color two-photon ionization (e.g., Figure 1c,d). However, a VIS laser and a UV laser emitting at longer wavelengths are used, thus substantially reducing the ionization of interfering molecules (cf. Figure 1c,d). Several studies using twocolor three-photon ionization schemes have been reported, e.g., a study of the excited and autoionization states of an erbium atom15 and that of the superexcited states of phenol.16 To our knowledge, no study has been reported to date in which the fundamental and the second harmonic beams are simultaneously used to enhance the sensitivity without losing selectivity in analytical spectroscopy. Although this can be accomplished simply by removing the interference filter for the separation of the SHG and VIS beams, we investigate herewith the effect of beam polarization in observing signal enhancement. The advantage of this technique in applications to the on-line analysis of PCDD/F in flue gas is also discussed in this study. EXPERIMENTAL SECTION Figure 2 shows the experimental setup used in this study. The SSJ/MPI-MS instrument has been described in detail elsewhere.17,18 A home-built picosecond laser system (UV: 270-290 nm, 400 µJ, 80 ps) consisting of a tunable distributed-feedback dye laser (VIS: 540-580 nm, 2 mJ, 60 ps) and a second-harmonic generator (β-barium borate) was used for the excitation and subsequent ionization of the analyte molecule. The VIS and UV beams are separated using an interference filter, and the direction of polarization is rotated 90° using two reflective mirrors. In the case of a one-color ionization experiment, either the VIS beam or the UV beam was shut off by placing a beam stop in the beam path. The sample was heated and then entrained into a jet with a carrier gas of argon from a pulsed nozzle into a vacuum. The induced ions are accelerated by an electric potential toward a linertype time-of-flight tube and are detected by an assembly of (15) Song, K.; Cha, H.; Lee. J.; Kolpakov, I. Microchem. J. 1997, 57, 265-273. (16) Schick, C. P.; Weber, P. M. J. Phys. Chem. A 2001, 105, 3725-3734. (17) Takeyasu, N.; Deguchi, T.; Tsutsumikawa, M.; Matsumoto, J.; Imasaka, T. Anal. Sci. 2002, 18, 1-4. (18) Matsumoto, J.; Nakano, B.; Imasaka, T. Anal. Sci. 2003, 19, 383-386.

Figure 3. Mass spectrum of o-chlorophenol: (a) one-color fivephoton ionization (557.22 nm only), (b) one-color three-photon ionization (278.61 nm only), and (c) two-color three-photon ionization (278.61 + 557.22 nm).

microchannel plates. The mass spectrum is measured using a digital oscilloscope (LeCroy 9360). The resolution of the mass spectrometer was 330 at m/z ) 112. The mass-selected MPI spectrum is measured by means of a boxcar integrator interfaced with a personal computer. The sample, i.e., o-chlorophenol, was utilized because it is one of the precursors of PCDD/F19 and three photons are required for ionization at the wavelength of the 0-0 transition.20 The reagent was purchased from Kanto Chemical and was used without further purification. RESULTS AND DISCUSSION Figure 3 shows the mass spectrum of o-chlorophenol. The wavelengths of the VIS and UV lasers were adjusted to 557.22 and 278.61 nm, respectively, the latter of which corresponds to the wavelength of the 0-0 transition for the cis isomer.21 No signal is observed when only a VIS laser is introduced (Figure 3a), although the pulse energy of the VIS laser is several times larger than that of the UV laser. It should be noted that two photons are required for the excitation and additional three photons for the ionization of o-chlorophenol at the wavelength of the above laser. The present result is quite reasonable, since the ionization efficiency decreases with an increase in the number of photons required for excitation/ionization. On the other hand, a single peak arising from a molecular ion of o-chlorophenol is observed when a UV beam is used instead, as shown in Figure 3b. This is based on one-color three-photon ionization (see Figure 1a). It may be possible to improve the ionization efficiency by increasing the laser pulse energy. However, it requires a large laser system and may increase the fragments due to a large excess energy absorbed in the molecule/ion. To improve the ionization efficiency further, the use of a two-color ionization scheme is recommended, since it requires a fewer number of photons, thus increasing the ionization yield. For example, two-color two-photon ionization is reported to be 5 times more efficient than one-color three-photon ionization, for the case of p-dichlorobenzene.14 However, it is necessary to use a laser that emits at shorter wavelengths (see (19) Blumenstock, M.; Zimmermann, R.; Schramm, K.-W.; Kettrup, A. Chemosphere 2001, 42, 507-518. (20) Uchimura, T.; Hafner, K.; Zimmermann, R.; Imasaka, T. Appl. Spectrosc. 2003, 57, 461-465. (21) Yamamoto, S.; Ebata, T.; Ito. M. J. Phys. Chem. 1989, 93, 6340-6345.

Figure 4. MPI spectrum of o-chlorophenol: (a) one-color threephoton ionization (278.61 nm only) and (b) two-color three-photon ionization (278.61 + 557.22 nm). The asterisk in the figure indicates the 0-0 transition peak for cis-o-chlorophenol.

Figure 1c,d). In this case, other interfering species would be excited and ionized, thus providing poor selectivity in the spectrometric analysis. To avoid this problem, a two-color threephoton ionization scheme can be used, as shown in Figure 1f. Unfortunately, no signal enhancement was observed when this experiment was performed by removing the interference (VIS transmission) filter placed in the beam path of the UV and VIS lasers (see Figure 2). This arises from a mismatch in the polarization for these two lasers. Because of this, the UV and VIS beams were separated and then recombined after changing the direction of the polarization in parallel with each other. The signal increased ∼4 times as shown in Figure 3c. This signal enhancement may be explained as follows. The two-photon ionization process from the electronic excited state to the ionized level is not saturated (Figure 3b), and the ionization yield may then be proportional to (or dependent on) the intensities of the UV and VIS lasers (Figure 3c). The intensity of the VIS laser is several times larger than that of the UV laser. Therefore, the ionization efficiency could be substantially improved by introduction of a VIS laser. Such an enhancement in ionization efficiency would be provided only when two (UV and VIS) beams, linearly polarized in parallel with each other, are used in the ionization process and their polarization directions are adjusted in parallel with the electric dipole moment of the electronic excited state prepared in parallel with the polarization direction of the excitation (UV) beam. It should be noted that the energy of the VIS laser is exactly half that of the UV laser. As a result, it provides no appreciable effect except assisting the single-photon UV excitation with two VIS photons (see Figure 1f). The MPI spectrum of o-chlorophenol was measured with and without the use of a VIS laser, and the results are shown in Figure 4. The spectrum was measured by either one-color three-photon ionization or two-color three-photon ionization. The largest peak in the spectrum arises from the excitation of cis-o-chlorophenol at the band origin: cf. the band origin for trans-o-chlorophenol Analytical Chemistry, Vol. 76, No. 8, April 15, 2004

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appears at 280.10 nm21 and is not shown here. As in the case of the mass spectrum (Figure 3), the 0-0 transition peak obtained by two-color three-photon ionization increases by ∼4 times that obtained by one-color three-photon ionization, allowing a more sensitive detection of o-chlorophenol. This is in contrast to other ionization schemes such as two-color two-photon ionization (see Figure 1c,d), in which a laser emitting at shorter wavelengths is used, since substantial background arising from interference species may be unavoidable in such cases. The direction of polarization of the fundamental (VIS) beam is perpendicular to that of the second harmonic (UV) beam, produced by second harmonic generation. As a result, the direction of polarization must be changed so as to be parallel with each other. In this study, the UV and VIS beams were separated and the polarization direction of the VIS beam was changed using a pair of mirrors before recombination. However, this procedure is tedious and time-consuming because of the difficulties in the alignment of the mirrors for spatial and temporal overlaps of the laser beams/pulses. In addition, the above scheme induces energy losses for both the UV and VIS lasers. It should be noted that the direction of polarization can be rotated only for a single (e.g., VIS) component using a specially designed wave plate, which acts as a half-wave (λ/2) plate for the VIS beam to rotate the direction of polarization 90° and acts as a wave (λ) plate for a UV beam to retain the polarization direction (or rotate 180°). Signal enhancement can then simply be achieved by removing the beam separator and by placing a wave plate in the beam path of the UV and VIS beams. This approach provides no energy loss and has a 100% temporal and spatial coupling efficiency. As a result, further increases in ionization efficiency would be expected.

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CONCLUSIONS A two-color three-photon ionization scheme was investigated for the sensitive and selective detection of o-chlorophenol. This molecule has a higher ionization potential than the two-photon energy of the laser used for resonance excitation at the wavelength of the 0-0 transition. The signal intensity increased by ∼4 times by introducing the fundamental beam remaining in the process of the second harmonic generation, since this second-color photon assists the ionization of o-chlorophenol from the electronic excited state. This ionization scheme may be useful for analysis of a sample in a gas mixture, e.g., a flue gas emitted from an incinerator, compared to a two-color two-photon ionization scheme, since no excitation/ionization of interfering species occurs, in theory, by the addition of the second-color photon. This may substantially reduce interference by other molecules such as polycyclic aromatic hydrocarbons that are also present as major components in flue gas. Therefore, the use a two-color threephoton ionization scheme for the on-line analysis of PCDD/F in a complex gas mixture emitted from an incineration plant represents a promising solution to this environmental problem. ACKNOWLEDGMENT This work is supported by Grants-in-Aids for Scientific Research and for the 21st Century COE Program, “Functional Innovation of Molecular Informatics”, from the Ministry of Education, Culture, Science, Sports and Technology of Japan. Received for review September 10, 2003. Accepted February 26, 2004. AC035057T