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On-Line Concentration by Analyte Adsorption and Subsequent Laser Desorption in Supersonic Jet Spectrometry Tomohiro Uchimura,*,†,‡ Yuji Sakoda,† and Totaro Imasaka†,‡ Department of Applied Chemistry, Graduate School of Engineering, and Division of Translational Research, Center for Future Chemistry, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan A narrow capillary, the tip of which was restricted to form a supersonic jet, was employed for sample introduction in time-of-flight mass spectrometry. The analyte was adsorbed at the tip of the capillary, due to a temperature decrease by the Joule-Thomson effect. Then, the analyte was desorbed using a pulsed laser emitting at 532 nm, and was entrained into a carrier gas. The analyte in the jet was subsequently ionized using a pulsed laser emitting at 266 nm. The duration of the analyte passing through the ionization region was 5.4 mm in length (9 µs in time), and the signal intensity was enhanced 310-fold. This technique can also improve selectivity by controlling the nozzle temperature, since volatile compounds are not trapped at the tip of the capillary and then are not concentrated in the jet. In this approach, the analyte can be injected in a pulsed mode into a vacuum without using a complicated mechanical valve even at a repetition rate of >1 kHz from the nozzle heated at a temperature of >300 °C. Resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOF-MS) has been developed as a sensitive, as well as selective, analytical means for the measurement of a variety of aromatic hydrocarbons.1,2 A well-resolved spectral feature, obtained through a combination with supersonic jet spectrometry (SSJ), allows identification of a chemical species with a closely related chemical structure.3–7 For this reason, SSJ/ REMPI/TOF-MS with superior selectivity has been utilized in online, real-time monitoring of analytes.8,9 * To whom correspondence should be addressed. E-mail: uchimura@ cstf.kyushu-u.ac.jp. † Graduate School of Engineering. ‡ Center for Future Chemistry. (1) Lubman, D. M. Anal. Chem. 1987, 59, 31A–40A. (2) Boesl, U.; Weinkauf, R.; Weickhardt, C.; Schlag, E. W. Int. J. Mass Spectrom. Ion Processes 1994, 131, 87–124. (3) Smalley, R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977, 10, 139– 145. (4) Amirav, A.; Even, U.; Jortner, J. Chem. Phys. 1980, 51, 31–42. (5) Hayes, J. M.; Small, G. J. Anal. Chem. 1983, 55, 565A–574A. (6) Syage, J. A. Anal. Chem. 1990, 62, 505A–509A. (7) Imasaka, T.; Moore, D. S.; Vo-Dinh, T. Pure Appl. Chem. 2003, 75, 975– 998. (8) Oudejans, L.; Touati, A.; Gullet, B. K. Anal. Chem. 2004, 76, 2517–2524. (9) Hafner, K.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Kettrup, A. Anal. Chem. 2001, 73, 4171–4180.
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Several techniques have been reported to date for sample introduction in supersonic jet spectrometry.9–14 The techniques can be classified in two categories: pulsed- and continuousintroduction modes. A pulsed nozzle is already commercially available and can produce a gas pulse as short as 50 µs. The analyte is localized in a narrow sample zone and then can be efficiently ionized using a pulsed laser for detection, e.g., by REMPI/TOF-MS. When an ionization laser is operated at 10 Hz, a duty cycle becomes 2,000 ) 100 ms/50 µs. Thus, the signal intensity can be enhanced significantly, increasing the sensitivity in mass spectrometry. However, there are several disadvantages to this approach. For example, the maximum operation temperature and the maximum repetition rate are practically limited to 300 °C in the measurement of highly chlorinated dibenzo-p-dioxins/ dibenzofurans (PCDDs/PCDFs) with high melting points. Otherwise, when this system is applied to an interface of GC and MS, the signal peak is sometimes broadened or even missing in the mass chromatogram. In this study, we used a pulsed laser for desorption of the analyte adsorbed at the tip of the capillary (Figure 1(b)). Then, the analyte molecule was immediately injected into a vacuum and formed a concentrated sample zone. Another laser was subsequently introduced into the jet for ionization/detection, e.g. by REMPI/TOF-MS (Figure 1(c)). The concentration of the analyte increased, and the ion signal was significantly enhanced. When (17) Matsumoto, M.; Nishimura, K.; Uchimura, T.; Imasaka, T. Anal. Chim. Acta 2003, 484, 163–166. (18) Matsumoto, J.; Kai, K.; Imasaka, T. Anal. Chem. 2003, 75, 346–349.
the tip of the capillary was not restricted, no sample cooling occurred, and no signal enhancement was observed. The repetition rate of the nozzle was determined by the repetition rate of the desorption laser, and the maximum temperature of the nozzle was limited by the melting point of the silica capillary. Experimental Apparatus. Figure 2 shows a schematic diagram of the experimental apparatus used in this study. Details oftheREMPI/TOF-MSinstrumenthavebeenreportedelsewhere19,20 and are only briefly described here. The second harmonic emission of a Nd:YAG laser (Tempest, New Wave Research, 532 nm, 5 ns, 10 Hz, Tokyo) was used as a desorption laser, and the fourth harmonic emission of a Nd:YAG laser (Minilite I, Continuum, 266 nm, 5 ns, 10 Hz, Excel Technology, Tokyo) was used as an ionization laser. A delay/pulse generator (DG535, Stanford Research Systems, CA) was employed to synchronize the timing of the lasers. A sample was directly introduced into a time-offlight mass spectrometer from a capillary (0.32 mm i.d.) using air as a carrier gas. The tip of the capillary was heated with a flame to close down the tip and was then ground back until the nozzle opened at the appropriate dimension.9,11 The inner diameter of the nozzle was adjusted to ca. 50 µm. The tip of the capillary was positioned a few millimeters from the metal support that serves as a repeller for ion acceleration. The desorption laser was aligned in a manner that would introduced it at the tip of the capillary without focusing the beam. The pulse energy of the desorption laser irradiating the capillary was estimated to be one-sixth of the total energy, and the power density was ca. 0.4-6 MW/cm2. On the other hand, the ionization laser, the energy of which was adjusted to be 30 µJ, was focused into a vacuum chamber using a planoconvex lens with a focal length of 300 mm. The ion induced by multiphoton ionization was accelerated by an assembly of electrodes and was drifted in a linear time-of-flight tube. The ion was detected by an assembly of microchannel plates (MCP, F465511, Hamamatsu, Shizuoka). The mass spectrum was recorded by means of a digital oscilloscope (TDS5104, Tektronix Japan, Tokyo). The mass resolution was typically 500 at m/z ) 128 in Figure 3. However, the mass resolution sometimes decreased, mainly due to a vacuum pressure; e.g., that was 3.2 × 10-2 Pa in Figure 3, and 7.7 × 10-2 Pa in Figure 6. The chemicals, i.e., chlorobenzene, 1,2,3,4-tetrachlorobenzene, 2,5-dichlorophoenol (WAKO Pure Chemical Industries, Osaka), o-chlorophenol, mchlorophenol, p-chlorophenol (KANTO Chemical, Tokyo) and 1,4dichlorobenzene (Tokyo Kasei, Tokyo), were used without further purification. All experiments were performed at room temperature without heating of the nozzle. (19) Matsumoto, J.; Nakano, B.; Imasaka, T. Anal. Sci. 2003, 19, 379–382. (20) Uchimura, T. Anal. Sci. 2005, 21, 1395–1400.
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Figure 5. Dependence of the signal intensity on the delay time of the ionization laser from the desorption laser. Sample, p-chlorophenol.
Figure 3. Dependence of the molecular ion signal on the pulse energy of the desorption laser. (a) Mass spectra for p-chlorophenol at different pulse energies specified in the figure. (b) Signal intensity for an isotope of 35ClC6H4OH.
Figure 4. Mass spectra for p-chlorophenol. The pulse energy of the desorption laser: (a) 30 mJ and (b) 13 mJ. The ionization laser was introduced into the jet at time “zero” in the abscissa, and the desorption laser was irradiated 13 µs earlier than the ionization laser. The signals in the circle are induced by ionization of the analyte with a desorption laser.
RESULTS AND DISCUSSION Laser Pulse Energy. Figure 3(a) shows the mass spectra for p-chlorophenol measured at different pulse energies of the desorption laser. The delay time from the desorption laser to the ionization laser was optimized to 13 µs. As shown in Figure 3(a), the isotope peaks of p-chlorophenol are clearly observed. The peak width of the mass spectrum remained unchanged, even when the pulse energy of the desorption laser was changed. This suggests that an increase in the analyte temperature by laser desorption provides no appreciable effect in mass resolution under the present conditions, although an increase in the analyte temperature 3800
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sometimes degrades the mass resolution by an increase in initial velocity distribution of the analyte molecule. Figure 3(b) shows the dependence of the signal intensity on the laser pulse energy. The intensity of the molecular ion increases with an increase in pulse energy to 13 mJ and rapidly decreases above 18 mJ. To investigate this unexpected result, the entire mass spectrum was recorded for p-chlorophenol, as shown in Figure 4. Several irregular peaks are observed at –8 to –3 µs at higher pulse energies (e.g., 30 mJ). Thus, decomposition of the analyte molecule could be the reason the signal decreased above 18 mJ. Although it is difficult to assign the irregular peaks in Figure 4, these signals occur apparently as a result of laser desorption. Then, they should arise from direct ionization of p-chlorophenol or fragments induced by the desorption laser. Analyte Cooling at Capillary Tip. To confirm analyte cooling and laser desorption at the tip of the capillary, the direction of the capillary was reversed; i.e., the capillary was restricted at the inlet side of the capillary and was not restricted at the outlet side in the vacuum. In this configuration, the flow rate of the carrier gas remains unchanged, but provides no cooling effect at the tip of the capillary. In the experiment, no enhancement of the signal was observed by introduction of the desorption laser, suggesting that the analyte was not cooled at the tip of the nonrestricted capillary. Localization of Analyte Molecule in the Jet. Figure 5 shows the dependence of the signal intensity of the molecular ion on the delay time from the desorption laser to the ionization laser. The experiment was performed using p-chlorophenol as a sample and a restricted nozzle at the tip of the capillary. The full width at half-maximum of the signal was 9 µs, suggesting that the analyte molecule was localized in the jet. The length of the sample zone was calculated to be 5.4 mm from the average speed of the molecule in the jet (ca. 600 m/s).21 Although mechanical valves operated at a pulse width of 10 µs are reported elsewhere,22,23 they were used under limited conditions, e.g.,
