Letter pubs.acs.org/ac
Resonance-Enhanced Multiphoton Ionization/Time-of-Flight Mass Spectrometry for Sensitive Analysis of Product Ions Formed by Online Concentration from Analyte Adsorption/Laser Desorption Tetsuya Kuraishi and Tomohiro Uchimura* Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan ABSTRACT: Resonance-enhanced multiphoton ionization/ time-of-flight mass spectrometry (REMPI/TOFMS) was developed for the analysis of product ions formed by online concentration/laser desorption (Online COLD). In the online COLD system, which is one of the sample introduction techniques for REMPI/TOFMS, the analyte molecules are adsorbed at the tip of the capillary column and then are heated and vaporized by introducing a desorption laser. In the present study, the molecules concentrated at the tip reacted to the application of an intense desorption laser. The product species was dependent on the reactant, i.e., chlorodiphenyl ether was formed from p-chlorophenol while dibenzo-p-dioxin was formed from o-chlorophenol. In addition, the transient signal intensities of reactant and product ions were monitored, and the probable reaction processes were discussed. The present technique confers advantages such as high sensitivity/selectivity and rapidity, and therefore, this technique provides a novel method for monitoring the process of molecular reaction.
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or for an online measurement, the container can be preferably connected to an analytical instrument. However, it is difficult to measure unstable intermediates and to discern the initial stage of the reaction. On the other hand, if molecular reactions occur via the Online COLD technique, the initial reaction products as well as the reactant species may be detected, both for sensitivity and selectivity, according to the characteristics allowed by the technique. Therefore, the Online COLD technique allows the potential for monitoring reaction products just after thermal reactions and revealing the kinetics of the molecular reaction. In the present study, the occurrence of molecular reactions was confirmed by introducing an intense laser for desorption in Online COLD. Using p- and o-chlorophenol as samples, the differences in product species between the reactant isomers were demonstrated. Moreover, the time courses of the signal intensities were plotted by changing the oscillation intervals of the desorption and ionization lasers, and conceivable reaction processes were discussed. The advantages of this method for the elucidation of molecular kinetics also were discussed and are described here.
esonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOFMS) is a selective analytical means for the measurement of a variety of aromatic hydrocarbons.1−5 In REMPI/TOFMS, continuous or pulsedsample introduction has been widely applied.5−11 Recently, we developed a new sample introduction technique based on online concentration by analyte adsorption/laser desorption (Online COLD).12−14 This technique uses a capillary column with a narrow processing tip. The tip of the capillary column is cooled by the adiabatic expansion of carrier gas, and analyte molecules are adsorbed at the tip. The tip is then heated by irradiating a pulsed laser for desorption. Consequently, the adsorbed molecules can be introduced as a pulse while the carrier gas continuously flows. The desorbed molecules are subsequently ionized by the other pulsed laser. The nozzles based on Online COLD reportedly offer highly efficient sample use, produce a supersonic jet, have the smallest gas pulse with a duration of typically 1 μs, and can be operated at an ultrahigh repetition rate (∼1 kHz or more if needed).14 In previous studies, the tip of the capillary reportedly was substantially heated by introducing an intense laser pulse for desorption, and therefore, optimization of the laser energy was necessary for an optically selective detection of analyte species.13 By contrast, considerable heating of the tip could produce chemical reactions with the adsorbed molecules. Generally, for studying the thermal reactions of molecules, a certain amount of the reactants should be prepared in containers, e.g., an ampule, a chamber, or a reservoir.15−20 The reaction products can then be transferred to the proper analytical instrument, © 2013 American Chemical Society
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EXPERIMENTAL SECTION The REMPI/TOFMS combined with an Online COLD sample introduction technique used in the present study has been reported in detail elsewhere12−14 and is only briefly described Received: December 20, 2012 Accepted: March 19, 2013 Published: March 19, 2013 3493
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Figure 1. Mass spectra of the species resulting from p-chlorophenol without (a) and with (b−d) use of a desorption laser. Delay times of the ionization laser from the desorption laser: (b) 2.8 μs, (c) 4.8 μs, and (d) 7.8 μs.
here. A sample of p- or o-chlorophenol mixed with air was placed in a sampling bag. The sample molecules were introduced into a homemade linear-type TOFMS, which is now commercially available (Hikari-GK, HGK-1, Fukuoka, Japan), through a deactivated fused-silica capillary column (GL Sciences, 60 cm long, 320 μm i.d., 450 μm o.d.). The tip of the capillary was synthesized with an inner diameter of ∼50 μm, and the resultant flow rate was ∼1 mL/min. The tip was irradiated with the second harmonic emission of a Nd:YAG laser (Continuum, Minilite II, 532 nm, 3−10 mJ, 10 Hz) for desorption without focusing. The effective pulse energy of the desorption laser irradiating the tip of the capillary column was estimated at ∼5−10% of the total energy (∼500 μJ). The fourth harmonic emission of a Nd:YAG laser (Rayture Systems, GAIA II, 266 nm, 10 Hz) was used as an ionization laser. Typically, a 20-μJ pulse was focused using a plano-convex lens with a focal length of 200 mm. The distance between the nozzle and the ionization region was 2.5 mm and 2.0 mm in the case of p- and o-chlorophenol, respectively. The timing of both lasers was synchronized with a digital delay/pulse generator (Stanford Research Systems, DG 535). The ions induced by multiphoton ionization were detected by a two-stage microchannel plate detector (Hamamatsu, F4655-11). Data acquisitions were performed by a digital oscilloscope (Tektronix, TDS5104), and the data were processed and analyzed using a homemade program written in LabVIEW (National Instruments).
desorption laser (Figure 1a) and with the desorption laser (Figure 1b−d) where delay times from the desorption laser to the ionization laser were 2.8, 4.8, and 7.8 μs, respectively. Compared to Figure 1a, a few intense peaks were observed and are shown in Figure 1b−d; the peaks at m/z = 128 and 130 arose from p-chlorophenol (35ClC6H4OH and 37ClC6H4OH, respectively), and the peak at m/z = 94 was assumed to be phenol. Compared to Figure 1a, the signal intensity of p-chlorophenol was increased 85-fold by introducing the desorption laser where the delay time was optimized, as shown in Figure 1c. Interestingly, ions with an m/z (220 and 222) larger than the molecular ion were also detected in the figure. Cluster ions are sometimes observed as a larger species in REMPI/TOFMS combined with supersonic jet spectrometry.6 However, by taking into consideration the m/z and the intensity ratio of 3:1 for m/z = 220 and 222, a possible candidate might be chlorodiphenyl ether, e.g., 4-(4-chlorophenoxy)phenol, which have thermally formed via a dimerization reaction and will be discussed in more detail later on. In addition, a peak at m/z = 186 (C12H10O2, e.g., 4-phenoxyphenol or 4,4′biphenol) was also observed and is shown in Figure 1c. These products would be identified as a function of laser ionization wavelengths. Thus, the obtained results demonstrated that a condensation reaction occurred with the introduction of an intense desorption laser, and the products were measurable by REMPI/TOFMS. Transient Signals of Analytes. Figure 2a shows the relationship between the delay time of the ionization laser from the desorption laser and the signal intensities of the peak at m/z = 128 (p-chlorophenol, M+), 94 (phenol, P+), and 220
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RESULTS AND DISCUSSION p-Chlorophenol. Figure 1 shows the mass spectra of the species resulting from p-chlorophenol without introducing the 3494
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In Figure 2b, the values for P+/M+ seemed to decrease with an increase in the delay time between the desorption laser and the ionization laser and was further decreased during a delay time of ∼3 −8 μs, during which the sample molecules were introduced as a pulse, while the behavior of D+/M+ appeared to be roughly consistent with that of D+. These results suggest that while the ratio of phenol generated from the above reaction scheme became significantly high for the front portion of the sample gas pulse, due probably to the presence of a large number of hydrogen radicals just after the introduction of an intense desorption laser, the reaction of the hydroxyphenyl radical and p-chlorophenol efficiently proceeded to generate 4-(4-chlorophenoxy)phenol at an optimal temperature for the corresponding reaction:We have begun further study associated
with the reaction temperature to elucidate the reaction process using this technique. o-Chlorophenol. For comparison, the mass spectrum of the species produced from o-chlorophenol with an intense desorption laser was measured and is shown in Figure 3. A peak
Figure 2. Relationship (a) between the delay time of the ionization laser from the desorption laser and signal intensities for p-chlorophenol (M+), phenol (P+), and dimer (D+). The signal intensity axis of D + is enlarged by a factor of 20. The ratio of the signal intensity for each ion to the one for the molecular ion (P +/M+, M +/M+, and 20 × D+/M +) as a function of the delay time is plotted in part b.
(dimer, D+), where the signal intensity axis of D+ was enlarged by a factor of 20. As shown in this figure, three analytes were all introduced as a pulsed mode. The full-width at half-maximum (fwhm) of the durations was less than 5 μs, and the delay time at which the maximum signal intensity was achieved was 4.8 μs. To carefully check the transient behavior of the signal intensities, the ratio of the signal intensity for each ion to the one for the molecular ion, i.e., P+/M+, M+/M+ (= 1), and 20 × D+/M+, respectively, as a function of the delay time is plotted in Figure 2b. The maximum value of P+/M+ showed a delay time of 2.8 μs, at which the front portion of the sample gas pulse was ionized. These molecules had higher velocities due to the substantial heat from the irradiation of an intense desorption laser. At this delay time, the value of P+/M+ was further increased when the energy of the desorption laser was increased. Therefore, just after an intense desorption laser was introduced, reductive dechlorination was assumed to have occurred with some portion of the molecules adsorbed at the tip of the capillary:
Figure 3. Mass spectrum of the species produced from o-chlorophenol when using a desorption laser. The delay time of the ionization laser from the desorption laser is 3.5 μs.
arising from the dimerization product at m/z = 220, which was similar to the results obtained with the use of p-chlorophenol, was observed, but an isotope peak at m/z = 222 was not detectable due to a small signal intensity. In addition to this product species, a peak at m/z = 184, rather than at m/z = 186 as observed in Figure 1c, was also detected. Dibenzo-p-dioxin is a possible candidate, which might have formed from an intramolecular dehydrochlorination of the dimer at m/z = 220:Therefore, different products were formed from the
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(5) Hafner, K.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Kettrup, A. Anal. Chem. 2001, 73, 4171−4180. (6) Hayes, J. M. Chem. Rev. 1987, 87, 745−760. (7) Imasaka, T.; Okumura, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2152−2155. (8) Oser, H.; Thanner, R.; Grotheer, H.-H. Combust. Sci. Technol. 1996, 116−117, 567−582. (9) Heger, H. J.; Dorfner, R.; Zimmermann, R.; Rohwer, E. R.; Boesl, U.; Kettrup, A. J. High Resolut. Chromatogr. 1999, 22, 391−394. (10) Uchimura, T.; Yamaguchi, S.; Imasaka, T. Chem. Lett. 2009, 38, 744−745. (11) Okudaira, H.; Uchimura, T.; Imasaka, T. Anal. Sci. 2012, 28, 683−687. (12) Uchimura, T.; Sakoda, Y.; Imasaka, T. Anal. Chem. 2008, 80, 3798−3802. (13) Sakoda, Y.; Uchimura, T.; Imasaka, T. Anal. Chem. 2010, 82, 1283−1287. (14) Uchimura, T.; Nakamura, N.; Imasaka, T. Rev. Sci. Instrum. 2012, 83, 014101. (15) Lindahl, R.; Rappe, C. Chemosphere 1980, 9, 351−361. (16) Weber, R.; Hagenmaier, H. Chemosphere 1999, 38, 529−549. (17) Wehrmeier, A.; Lenoir, D.; Schramm, K.-W.; Zimmermann, R.; Hahn, K.; Henkelmann, B.; Kettrup, A. Chemosphere 1999, 36, 2775− 2801. (18) Nilsson, C.-A.; Andersson, K.; Rappe, C.; Westermark, S.-O. J. Chromatogr., A 1974, 96, 137−147. (19) Imasaka, T.; Hozumi, M.; Ishibashi, N. Anal. Chem. 1992, 64, 2206−2209. (20) Uchimura, T.; Imasaka, T. Anal. Chem. 2000, 72, 2648−2652.
different isomers of chlorophenol in REMPI/TOFMS combined with Online COLD. We have reported that dibenzop-dioxin could be generated from o-chlorophenol heated by a conventional heater.20 It is noteworthy that the reaction and subsequent detection in the present study was performed in the range of several microseconds. Moreover, though the distance between the nozzle and the ionization region was set to a few millimeters in the present study, the distance can be decreased or even zero, i.e., the ionization laser was adjusted to the tip of the capillary. Therefore, the details of transient ion signals derived from the reactants, products, and possibly the intermediates would be obtained by changing the nozzle−ionization region distance. Advantages. The present technique based on Online COLD has several advantages. As described above, reactants, products, and even intermediates formed in a short period of time can be time-dependently measurable, which is helpful to explain the reaction processes. Moreover, though one type of reactant isomer was applied to each measurement in the present study, the reactants and atmospheric gases can be easily changed even during measurement. In addition, the Online COLD sample introduction technique was originally developed for the ultrasensitive detection of the analyte molecules.12,13 Thus, the reactants as well as the products can be detected in a sensitive manner, even when using small samples. Both sensitivity and optical selectivity are superior; a REMPI spectrum obtained by using a tunable laser can lead to the identification of the product species.
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CONCLUSIONS In the present study, we proposed a novel analytical approach to elucidate the reaction process by using a sample introduction technique that was based on Online COLD for REMPI/ TOFMS. The product compounds from the condensation reactions of p- and o-chlorophenol, and their differences, were confirmed. In addition, the transient signals of reactant and product ions were obtained by changing the delay time between the desorption laser and the ionization laser. The present technique has several advantages such as sensitivity, selectivity, and versatility, and therefore, it could be very useful for the investigation of a molecular reaction and its kinetics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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