Selective Multiphoton Ionization of Coplanar Polychlorobiphenyls

Anal. Chem. 2004, 76, 5534-5538

Selective Multiphoton Ionization of Coplanar Polychlorobiphenyls Using 266-nm Laser Emission by Gas Chromatography/Mass Spectrometry Tomohiro Uchimura, Kunihiro Sakai, and Totaro Imasaka*

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka, 812-8581, Japan

Polychlorinated biphenyls (PCBs), used as a heat exchange oil (Kanechlor, KC-300), can be measured by gas chromatography combined with multiphoton ionization mass spectrometry (GC/MPI-MS). Several compounds, e.g., 4,4′-dichlorobiphenyl and 3,4,4′-trichlorobiphenyl, have nearly the same retention time but can be selectively determined by MPI-MS. Coplanar PCBs are more efficiently ionized using the fourth harmonic emission of a Nd:YAG laser (266 nm), compared to noncoplanar PCBs. Thus, the approach reported herein may be useful in the selective as well as the sensitive analysis of toxic PCBs, contained in old transformers and capacitors that have been mandated by the government to be properly disposed of within 10 years in Japan. Polychlorinated biphenyls (PCBs) are widely used in numerous electric devices such as transformers and capacitors in industries, because of their superior electric performance and excellent chemical stability. However, it is well known that some PCBs are carcinogenic and are suspected to be one of the candidates responsible for causing birth defects. In particular, several PCBs having no chlorine atoms in the ortho position, referred to as “coplanar PCBs”, have higher toxicities. The production, use, and even transport of PCBs are strictly prohibited by law in Japan. Some PCBs have been stored in containers more than 30 years in factories in Japan. However, the leakage of PCBs into the environment presents a potentially serious hazard, and the government has determined the elimination of PCBs in 10 years. However, the environmental hazard is a serious concern of citizens living near the facility where the decomposition of PCBs will be carried out. For this reason, the development of a method for monitoring PCBs is an urgent priority. A gas chromatograph combined with a flame ionization detector (GC-FID) or more preferentially an electron capture detector can be used for the measurement of trace hydrocarbons. It is, however, difficult to resolve complex mixtures of PCBs using a capillary column for analyte separation. In addition, a selective and sensitive detector is essential for the analysis of PCBs. A low-resolution or highresolution (HR) mass spectrometer (MS) based on electron impact ionization can be used for this purpose. However, 209 congeners are present in PCBs, and as a result, it is difficult to determine all the isomers of PCBs even using GC/HRMS, especially when many * To whom correspondence should be addressed. Phone: 81-92-642-3563. Fax: 81-92-632-5209. E-mail: [email protected].

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impurities, e.g., aliphatic compounds, are present in the sample. Therefore, it would be desirable to develop a new type of spectrometer for the detection of PCBs, especially toxic coplanar PCBs. Multiphoton ionization mass spectrometry (MPI-MS) is one of the candidates for this purpose.1-4 This technique provides superior selectivity, since each congener can be selectively ionized via resonance transition. Such an approach would be useful for the determination of specific aromatic hydrocarbons in a sample containing numerous interfering compounds. For example, nontoxic aliphatic hydrocarbons in the sample are not ionized and, as a result, do not interfere with a spectrometric analysis of PCBs. Since 209 congeners are present in PCBs and their toxicities differ greatly, it is necessary to determine the concentration of each isomer, if the toxicity of the sample is to be evaluated. In MPIMS, selectivity can be substantially improved by adjusting the wavelength of the laser at the resonance transition, e.g., the 0-0 transition. Unfortunately, only a few studies have reported on the wavelength of the 0-0 transition for PCBs.5-7 This is due to the fact that the signal intensity of the peak at the 0-0 transition is negligibly small and the spectrum is seriously congested for PCBs. A laser ionization technique has been applied to several PCBs.8,9 However, for the above reason, a wavelength-fixed laser is used for ionization and an isomer-selective analysis has not been demonstrated. To improve selectivity, this technique can be interfaced with GC.10,11 However, the laser wavelength must be fixed during the measurement of the analytes that are to be separated by GC. A suitable wavelength for the efficient ionization of toxic PCBs has not yet been reported. (1) Tembreull, R.; Sin, C. H.; Li, P.; Pang, N. M.; Lubman, D. M. Anal. Chem. 1985, 57, 1186-1192. (2) Lubman, D. M. Anal. Chem. 1987, 59, 31A-40A. (3) Boesl, U.; Weinkauf, R.; Weickhardt, C.; Schlag, E. W. Int. J. Mass Spectrom. Ion Processes 1994, 131, 87-124. (4) Boesl, U.; Zimmermann, R.; Weickhardt, C.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Schlag, E. W. Chemosphere 1994, 29, 1429-1440. (5) Takei, Y.; Yamaguchi, T.; Osamura, Y.; Fuke, K.; Kaya, K. J. Phys. Chem. 1988, 92, 577-581. (6) Im, H.-S.; Bernstein, E. R. J. Phys. Chem. 1988, 88, 7337-7347. (7) Zimmermann, R.; Weickhardt, C.; Boesl, U.; Schlag, E. W. J. Mol. Struct. 1994, 327, 81-97. (8) Deguchi, Y.; Dobashi, S.; Fukuda, N.; Shinoda, K.; Morita, M. Environ. Sci. Technol. 2003, 37, 4737-4742. (9) Mahajan, T. B.; Ghosh, U.; Zare, R. N.; Luthy, R. G. Int. J. Mass Spectrom. 2001, 212, 41-48. (10) Imasaka, T.; Tashiro, K.; Ishibashi, N. Anal. Chem. 1986, 58, 3244-3248. (11) Zimmermann, R.; Boesl, U.; Heger, H.-J.; Rohwer, E. R.; Ortner, E. K.; Schlag, E. W.; Kettrup, A. J. High Resolut. Chromatogr. 1997, 20, 461-470. 10.1021/ac049496j CCC: $27.50

© 2004 American Chemical Society Published on Web 08/13/2004

Figure 2. Gas chromatogram of KC-300 measured by GC-FID.

Figure 1. Experimental apparatus for GC/MPI-MS. In the case of GC-FID, the end of the capillary inserted into the vacuum chamber was connected to the FID in the GC.

In this study, MPI-MS combined with GC (GC/MPI-MS) was employed for the isomer-selective analysis of PCBs present in a mixed sample of Kanechlor that had been used as heat-transfer fluids, dielectric fluids, flame retardants, etc. The ionization laser, i.e., the fourth harmonic emission of a Nd:YAG laser emitting at 266 nm, was found to be useful for the efficient and selective ionization of toxic coplanar PCBs. EXPERIMENTAL SECTION Sample. Kanechlor (GL Sciences, KC-300) was used as a sample in this study. The contents of Kanechlor are reported to be as follows (from the manufacturer); dichlorobiphenyl 17%, trichlorobiphenyl 60%, tetrachlorobiphenyl 23%, pentachlorobiphenyl 1%. We submitted a certificate to the government for declaration of a proper use of Kanechlor as a standard chemical for analytical purposes only. The Kanechlor was diluted with methanol and the concentration adjusted to 10 µg/µL. GC-FID. A GC-FID system (Agilent Technologies, 6890GC version 4.08), composed of a 60-m-long, 0.32-mm-i.d. fused-silica capillary was used for separation of the analytes. The sample solution (2 µL in FID, 1 µL in MPI-MS) was injected into the GC column using a microsyringe. The pressure of helium used as a carrier gas was adjusted to 25.4 psi. The temperatures of the sample injection port and the FID were maintained at 300 °C. The temperature of the oven containing the capillary column was programmed as follows; 150 °C (1 min), 20 °C/min to 185 °C, 2 °C/min to 245 °C (3 min), 6 °C/min to 290 °C (hold), according to the protocol reported in the literature.12 The gas chromatogram was measured on a strip chart recorder (HP 3396, Series II integrator). GC/MPI-MS. Figure 1 shows a block diagram of the experimental apparatus used for the GC/MPI-MS. Details of the MPIMS instrument have been described elsewhere13-15 and are only briefly described here. The end of the capillary attached to the FID was disconnected and was directly introduced into a vacuum (12) Takasuga, T.; Inoue, T.; Ohi, E. J. Environ. Chem. 1995, 5, 647-4742. (13) Uchimura, T.; Imasaka, T. Anal. Chem. 2000, 72, 2648-2652. (14) Onoda, T.; Saito, G.; Imasaka, T. Anal. Chim. Acta 2000, 412, 213-219. (15) Matsumoto, J.; Nakano, B.; Imasaka, T. Anal. Sci. 2003, 19, 379-382.

chamber. The top of the capillary was not restricted, and the sample was then allowed to continuously flow as an effusive (nonjet) molecular beam. For this reason, the analyte molecule is not cooled. The conditions used in the GC separation were similar to those described in the previous section (GC-FID), and the capillary between the GC and the vacuum chamber was heated to 200 °C to prevent the analytes from condensing and clogging. The fourth harmonic emission of a Nd:YAG laser (New Wave Research, Inc., Tempest, 266 nm, 4 ns) was used for the ionization of the PCBs. An induced ion is accelerated by a repulsive potential into a time-of-flight tube and is detected by an assembly of three microchannel plates (Hamamatsu, F4655-10). A mass spectrum was measured by means of a 350-MHz digital oscilloscope (Iwatsu, Wave Runner, DS-4264M) or a 100-MHz digitizer (National Instrument, NI5112) programmed by the LabView software program. The mass resolution was typically 330 at m/z ) 112 when a digital oscilloscope was used for signal measurements. The resolution was, however, reduced to ∼50 in the mass spectra shown in Figure 4 when a digitizer was used instead. This arises from a smaller number of data for recording a mass spectrum repetitively at different retention times during 30 min. A mass chromatogram was measured either by a boxcar integrator interfaced with a personal computer or directly using the above digitizer. The retention time obtained in GC/MPI-MS is slightly smaller than the value obtained in GC-FID, even though the same capillary was used for analyte separation. This arises from a lower pressure in the capillary for GC/MPI-MS, since the sample is directly introduced into a vacuum chamber for MPI-MS. The assignment of the peaks in the mass chromatogram was accomplished using the reference data by taking account of the above difference in the retention times. RESULTS AND DISCUSSION GC-FID. Figure 2 shows the gas chromatogram for a sample of KC-300 obtained by GC-FID. More than 30 peaks are observed, which were assigned using the reported data of the retention times and the peak intensities observed in a chromatogram of authentic samples containing KC-300, -400, and -500.12,16 An analyte with a larger number of chlorine atoms in a molecule had a longer retention time, as has been previously reported.17 In GC-FID, the (16) Matsunaga, K.; Kondo, M.; Yamabe, S.; Mori, T. Chemosphere 1993, 27, 2317-2324. (17) Mullin, M. D.; Pochlni, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Environ. Sci. Technol. 1984, 18, 468-476.

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Figure 3. (a) Gas chromatogram of KC-300 measured by GC-FID (bar graph of Figure 2). (b)-(e) Mass chromatograms of KC-300 measured by GC/MPI-MS. Monitoring ion: (b) di-, tri-, and tetrachlorobiphenyls; (c) dichlorobiphenyls; (d) trichlorobiphenyls; (e) tetrachlorobiphenyls. The time scale for (a) is slightly changed from the original one, to adjust for those used in the mass chromatograms (b)-(e).

signal intensity is proportional to the concentration and the number of carbon atoms in a molecule. As a result, the sensitivity does not change significantly for PCBs. As reported in ref 12, several compounds are not separated by the capillary column, due to their similar physical properties. For example, the second highest peak observed at ∼12 min in the chromatogram corresponds to 2,2′,4- and 2,2′,5-trichlorobiphenyl and 4,4′-dichlorobiphenyl, as will be discussed in the following section. It would be more difficult to determine trace PCBs in the environment, since the concentration ratio could be significantly different from each other. This is in contrast to the KC-300 used here as an authentic sample of PCBs. This suggests that better selectivity is essential for a practical trace analysis of PCBs since the toxicity equivalent factor is strongly dependent on the number and position of chlorine atoms attached to the phenyl rings. GC/MPI-MS. Figure 3 shows the mass chromatograms for KC-300. Figure 3a is a bar graph chromatograph reproduced from the GC-FID data shown in Figure 2 for comparison with the chromatograms shown in Figure 3b-e. Figure 3b shows the mass chromatogram (GC/MPI-MS) obtained by monitoring PCBs containing two to four chlorine atoms, i.e., dichlorinated, trichlorinated, and tetrachlorinated congeners. In contrast to the data obtained by GC-FID, the signal intensity is not necessarily proportional to the concentration of each compound, since the efficiency of ionization is not identical for all PCB congeners. In fact, only two major peaks are observed in Figure 3b. The former peak, denoted as R, can be assigned to chemical species of 4,4′dichlorobiphenyl, 2,2′,4-trichlorobiphenyl, and/or 2,2′,5-trichlorobiphenyl, based on the previously reported retention times of these 5536

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compounds using GC-FID.12 Trichlorobiphenyls are reported to be present as major components in this sample by the manufacturer, and thus, this peak can be assigned to 2,2′,4-trichlorobiphenyl, 2,2′,5-trichlorobiphenyl, or both. However, this peak is observed in Figure 3c obtained by monitoring dichlorobiphenyls and is significantly reduced in Figure 3d obtained by monitoring trichlorobiphenyls. Thus, peak R in Figure 3b can be assigned to 4,4′-dichlorobiphenyl. This suggests that 4,4′-dichlorobiphenyl is efficiently ionized, in comparison with 2,2′,4-trichlorobiphenyl and 2,2′,5-trichlorobiphenyl. This findings also suggest that 4,4′dichlorobiphenyl has a high ionization yield among the dichlorobiphenyls since 4,4′-dichlorobiphenyl is reported to be present only as a minor component in KC-300.12 In ref 12, the ratio of the signal intensities for 2,6/2,2′-, 2,4′/2,3-, and 4,4′-dichlorobiphenyl is ∼1:4:0.7. This ratio is obtained for a mixture sample of KC-300, -400, -500, and -600 (not KC-300 only). However, dichlorobiphenyls are contained mainly in KC-300. Therefore, this value may not be significantly changed for KC-300. On the other hand, the ratio calculated from the data shown in Figure 3c is ∼1:5:86. This suggests that 4,4′-dichlorobiphenyl is ionized 2 orders of magnitude more efficiently than the other dichlorobiphenyls. Similarly, the largest peak observed in Figure 3b (denoted as β) can be assigned to 3,4,4′-trichlorobiphenyl rather than 2,2′,3,4′-tetrachlorobiphenyl, since this peak is clearly observed as a major component in the mass chromatogram obtained by monitoring trichlorobiphenyls, as shown in Figure 3d, and is very weak in the mass chromatogram monitoring tetrachlorobiphenyls, as shown in Figure 3e. Similarly, the peak denoted as γ in Figure 3b can be assigned to 3,3′,4,4′-tetrachlorobiphenyl as shown in Figure 3e. Figure 4 shows the mass spectra for KC-300 measured at retention times of 10, 14, and 19.5 min, at which three peaks, i.e., R, β, and γ, appear in the chromatogram in Figure 3b. The data in Figure 4a confirm that 4,4′-dichlorobiphenyl is a major component and 2,2′,4- and 2,2′,5-trichlorobiphenyls are minor components. It should be noted that a few peaks are observed at smaller mass numbers. These peaks may originate from fragments of 4,4′-dichlorobiphenyl, since monochlorobiphenyls are eluted earlier in the chromatogram. Such fragmentation can be observed under intense laser fields. To reduce this undesirable effect, decreasing the laser pulse width is recommended.18,19 In fact, the intensity of the molecular ion increased significantly, and the formation of fragment ions was suppressed by reducing the laser pulse width from 5 ns to 100 ps for PCBs in a previously reported study.8 For efficient ionization of the compound having a short excited-state lifetime, utilization of a narrow-band transform-limited picosecond laser whose pulse width is identical to the lifetime of the compound is suggested.20,21 Similarly, 3,4,4′-trichlorobiphenyl is observed as a major component in Figure 4b. The peak at M ) 237 is suspected to be due to dichlorodibenzofurans present as a contaminant in the syringe or the septum in the GC, since this molecule was investigated in our previous experiments. The other peaks observed at smaller mass numbers can be assigned to (18) Matsumoto, J.; Lin, C. H.; Imasaka, T. Anal. Chem. 1997, 69, 4524-4529. (19) Miyazawa, K.; Imasaka, T. Laser Ionization Mass Spectrometer and Mass Spectrometric Method. Japanese Patent 8-230866, submitted on August 30, 1996. (20) Matsumoto, J.; Imasaka, T. Anal. Chem. 1999, 71, 3763-3768. (21) Yoshida, N.; Hirakawa, Y.; Imasaka, T. Anal. Chem. 2001, 73, 4417-4421.

Figure 4. Mass spectra of KC-300. Retention time: (a) 10, (b) 14, and (c) 19.5 min, which correspond to the peaks denoted as R, β, and γ, respectively, in Figure 3b.

fragments. It should be noted that the peak corresponding to 2,2′,3,4′-tetrachlorobiphenyl, a possible candidate for a peak in the chromatogram obtained by GC-FID (Figure 3a), is negligibly small in Figure 4b; i.e., no appreciable peak is observed at m/z ) 292. On the other hand, the molecular ion of 3,3′,4,4′-tetrachlorobiphenyl is clearly observed as one of the major components in the mass spectrum, as shown in Figure 4c. A mass chromatogram was also measured by monitoring pentachlorobiphenyls. However, no peak was observed, although a few small peaks arising from, for example, 2′,3,4,4′,5-pentachlorobiphenyl, were observed in the chromatogram obtained by GC-FID (Figure 2). This can be attributed to the lower concentrations of pentachlorobiphenyls in KC-300 and a larger number of chlorine atoms that induce a spinorbit interaction and reduce the efficiency of ionization by intersystem crossing. The results obtained in this study suggest that the efficient ionization of PCBs having chlorine atoms at the 4 and 4′ positions can be achieved. In other words, a coplanar PCB having chlorine atoms at the “para” positions, e.g., 4,4′-dichlorobiphenyl or 3,4,4′trichlorobiphenyl, can be efficiently ionized at 266 nm. On the other hand, PCBs having chlorine atoms at the “ortho” positions, e.g., 2,2′,4-and 2,2′,5-trichlorobiphenyls, are less efficiently ionized, and as a result, the signals are strongly suppressed in MPI-MS. This suggests that toxic coplanar PCBs can be more selectively ionized and measured using MPI-MS. It is well known that an aromatic molecule can be selectively and efficiently ionized by

MPI-MS and that the ion signal is strongly enhanced by adjusting the laser wavelength to one of the resonance lines in the MPI spectrum. However, the spectrum for PCBs is congested, since only highly vibronic transitions are observed in the MPI spectrum. This is due to the fact that the torsional angle between the planes of the two phenyl rings is significantly changed by the electronic transition and the 0-0 transition peak becomes negligibly small.7 For this reason, it is difficult to selectively ionize a specific PCB molecule by adjusting the laser wavelength to one of the narrow resonance lines, e.g., the 0-0 transition peak, in a well-resolved MPI spectrum. However, the present approach using MPI at 266 nm may have the potential for use in the selective monitoring of toxic coplanar PCBs. The selective ionization of coplanar PCBs can be explained as follows. Generally, the wavelength of the band origin of the first electronic state is shifted toward longer wavelengths, with an increase in the number of chlorine atoms.1 In fact, the wavelength of the 0-0 transition is located at around 280 nm for biphenyl and 290-310 nm for polychlorinated biphenyls, depending on the number and position of the chlorine atoms substituted on the phenyl rings. It should be noted that the spectral profile of coplanar PCBs is very different from that of noncoplanar PCBs. As reported in ref 7, MPI spectra of PCBs have a unique feature. The torsional angle between the planes of the two phenyl rings is nearly equal to 90° for noncoplanar PCBs in the ground state, because of steric hindrance induced by the chlorine atoms at the 2 and 2′ positions. On the other hand, it has a double minimum potential in the electronic excited state at torsional angles of ∼(45°. This provides a progression band that gradually increases from the band origin and decreases immediately at the potential maximum of the excited state, i.e., at a torsional angle of 90°. Alternatively, a coplanar PCB has a rather flat configuration, e.g., (45° in the ground state and nearly 0° in the excited state, providing a long progression in the MPI spectrum (see ref 7 for details). The present result, i.e., the selective ionization of coplanar PCBs at 266 nm, can be explained by assuming that noncoplanar PCBs have negligible signal intensities at 266 nm as a result of the immediate suppression of the progression band. It should be noted that a wavelength-fixed laser, i.e., the fourth harmonic emission of a Nd:YAG laser, can be successfully utilized to monitor toxic coplanar PCBs by means of GC/MPI-MS. It is known that spectral selectivity is substantially improved, when a supersonic jet technique is used for aromatic hydrocarbons. Although some congeners of PCBs might be identified as a function of laser ionization wavelengths, this technique may not be so successful in applications to PCBs since the signal intensity at the 0-0 transition is very weak and many resonance lines arising from a series of progression transitions appear in the MPI spectrum, resulting in serious spectral congestion and overlap that make selective ionization difficult. For this reason, a combination of a nonjet technique and a wavelength-fixed laser was employed for simplification of the analytical instrument, and a GC separation technique was combined, to enhance selectivity in this study.

CONCLUSIONS The fourth harmonic emission of the Nd:YAG laser was used for the efficient ionization of PCBs. Selectivity in excitation and the subsequent ionization of coplanar PCBs can be improved using Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

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a laser emitting at 266 nm. The present technique based on GC/ MPI-MS provides an isomer-selective analytical approach to the measurement of toxic PCBs. In particular, aliphatic chlorinated hydrocarbons, which may be present in partially decomposed PCBs and ionized by electron impact in conventional MS, do not interfere with the analysis of PCBs by MPI-MS. Therefore, the present technique may have some potential for use in the process monitoring of PCBs, which is urgently required, to solve one of the most important environmental issues in Japan.

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ACKNOWLEDGMENT This work was supported by Grants-in-Aid 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 March 31, 2004. Accepted July 1, 2004. AC049496J