Enhancement of Molecular Ions in Mass Spectrometry Using an

Apr 5, 2010 - Department of Applied Chemistry, Graduate School of Engineering, and Division of Translational Research, Center for Future Chemistry, ...
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Anal. Chem. 2010, 82, 3441–3444

Letters to Analytical Chemistry Enhancement of Molecular Ions in Mass Spectrometry Using an Ultrashort Optical Pulse in Multiphoton Ionization Takashi Shimizu,† Yuka Watanabe-Ezoe,† Satoshi Yamaguchi,† Hiroko Tsukatani,‡ Tomoko Imasaka,§ Shin-ichi Zaitsu,† Tomohiro Uchimura,†,| 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, Laboratory of Chemistry, Graduate School of Design, Kyushu University, 4-9-1 Shiobaru, Minami-ku, Fukuoka 815-8540, Japan, and Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaifu, Fukuoka 818-0135, Japan The spectral domain of an ultraviolet femtosecond laser was expanded by stimulated Raman scattering/four-wave Raman mixing, and the resulting laser pulse was compressed using a pair of gratings. The pulse width was then measured using an autocorrelator comprised of a Michelson interferometer equipped with a multiphoton ionization/mass spectrometer which was used as a two-photon detector. A gas chromatograph/mass spectrometer was employed to analyze triacetone triperoxide (TATP), and the molecular ion induced by multiphoton ionization was substantially enhanced by decreasing the laser pulse width. Numeous efforts have been made to detect and measure molecular ions in mass spectrometry. Some of the well-known techniques include electrospray ionization (ESI) and matrixassisted laser-desorption-ionization (MALDI). However, a universal technique for consistently producing a molecular ion has not been reported to date. Single-photon ionization using vacuum ultraviolet (VUV) emission has attracted considerable attention, since it is useful for achieving the soft ionization of organic molecules.1 Even in this case, however, it is sometimes difficult to observe a molecular ion derived from a flexible oxygen-containing molecule, such as triacetone peroxide (TATP).2 Typically, a molecule has many vibrational modes, and one could be a coordinate of molecular dissociation. The shortest time period of vibration is 8 fs for a hydogen molecule having the largest vibrational frequency of 4155 cm-1. Thus, if it were possible to use a 1 fs laser pulse * To whom correspondence should be addressed. E-mail: imasaka@ cstf.kyushu-u.ac.jp. † Department of Applied Chemistry, Graduate School of Engineering, Kyushu University. ‡ Fukuoka Institute of Health and Environmental Sciences. § Laboratory of Chemistry, Graduate School of Design, Kyushu University. | Division of Translational Research, Center for Future Chemistry, Kyushu University. (1) Hanley, L.; Zimmermann, R. Anal. Chem. 2009, 81, 4174–4182. (2) Mullen, C.; Irwin, A.; Pond, B. V.; Huestis, D. L.; Coggiola, M. J.; Oser, H. Anal. Chem. 2006, 78, 3807–3814. 10.1021/ac1003773  2010 American Chemical Society Published on Web 04/05/2010

for multiphoton ionization, a molecule could be ionized before relaxation and subsequent dissociation. In addition, if the excess internal energy in the ion can be properly reduced, e.g., as the kinetic energy of an electron, it would be possible to observe a molecular ion derived from any type of molecule, without any fragmentation occurring. Numerous attempts have been made to generate a 1 fs pulse in the optical region. For example, a nonlinear optical technique has been used to generate a 2.2 fs pulse.3 The focus of our research has been on an approach based on four-wave Raman mixing, referred to as “two-color stimulated Raman scattering” or more simply as “Rainbow Stars”.4 This technique is capable of generating a multifrequency laser emission from the near-infrared (NIR) to the deep-ultraviolet region (DUV).5 As a result, it has a potential for use in generating an “ultimately short” optical pulse breaking the 1 fs barrier, as is recognized from the uncertainty principle. There are several techniques for measuring a laser pulse width. For example, frequency-resolved optical gating (FROG), spectral phase interferometry for direct electric-field reconstruction (SPIDER), and autocorrelation (AC) techniques are currently used for this purpose. Such techniques provide information related to pulse width at the detector with a two-photon response. It is, however, difficult to have accurate knowledge of the pulse width at the sample in analytical spectroscopy. It should be noted that a 1 fs optical pulse is expanded to a 1.8 fs pulse when it travels 1 cm in the air and to a 18 fs pulse when traveling only 50 µm in a solid/liquid sample by dispersion of the medium. For this reason, it is not possible to separate the instrument used to evaluate the laser pulse width and that used for the analysis of the sample. TATP is currently used as an explosive by terrorists. It is, however, difficult to observe a molecular ion for this compound in mass spectrometry.2 It would be possible to measure TATP by monitoring a fragment ion, but acetone and related compounds (3) Weigand, R.; Mendonc¸a, J. T.; Crespo, H. M. Phys. Rev. A 2009, 79, 063838. (4) Katzman, S.; Zubritsky, E. Anal. Chem. 2001, 73, 357A–359A. (5) Kawano, H.; Hirakawa, Y.; Imasaka, T. Appl. Phys. B: Laser Opt. 1997, 65, 1–4.

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Figure 1. Experimental apparatus used to generate an ultrashort optical pulse and for the measurement of the mass spectrum.

contained in cosmetics are usually present in the environment and would severely interfere with a trace analysis of TATP. Therefore, it would be desirable to develop an analytical technique capable of producing a molecular ion in mass spectrometry. In this study, we generated an ultrashort optical pulse by stimulated Raman scattering/four-wave Raman mixing. After pulse compression using a pair of gratings, the laser pulse width was measured using an autocorrelator comprised of a Michelson interferometer equipped with a mass spectrometer which served as a two-photon detector. A TATP sample was injected into a gas chromatograph that was interfaced with a mass spectrometer. The analyte was then measured by changing the laser pulse width, and the intensity of the molecular ion was found to be increased significantly by decreasing the laser pulse width. EXPERIMENTAL SECTION Figure 1 shows the experimental apparatus developed in this study. A femtosecond Ti:sapphire laser (Legend, Coherent, 800 nm, 2 mJ, 1 kHz) was introduced into nonlinear optical crystals for frequency-tripling (267 nm, 130 µJ). After beam collimation, this ultraviolet femtosecond beam was focused on a Raman cell filled with hydrogen gas (10 atm) to expand the spectral domain by rotational stimulated Raman scattering and rotational four-wave Raman mixing. The laser pulse chirped by optics such as a Raman cell window was compressed using a pair of gratings. The laser pulse was measured under dispersion-free conditions using an autocorrelator comprised of a Michelson interferometer equipped with an in-house fabricated time-of-flight mass spectrometer.6,7 Nitrogen oxide gas (NO) was allowed to continuously flow into the mass spectrometer, and the NO+ ion arising from the nonresonant two-photon ionization process was monitored, permitting the mass spectrometer to be used as a two-photon detector. One of the retro-reflectors in the interferometer was periodically translated to produce an interferometric autocorrelation trace. The pulse width was calculated by dividing the (6) Onoda, T.; Saito, G.; Imasaka, T. Anal. Chim. Acta 2000, 412, 213–219. (7) Zaitsu, S.; Miyoshi, Y.; Kira, F.; Yamaguchi, S.; Uchimura, T.; Imasaka, T. Opt. Lett. 2007, 32, 1716–1718. (8) Mullen, C.; Huestis, D.; Coggiola, M.; Oser, H. Int. J. Mass Spectrom. 2006, 252, 69–72. (9) Yamaguchi, S.; Uchimura, T.; Imasaka, T. Rapid Commun. Mass Spectrom. 2009, 23, 3101–3106.

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Figure 2. (A) Spectrum of the fundamental beam (- - -) and Raman emission (s) and (B) interferometric autocorrelation trace (black line) and intensity autocorrelation trace (white line).

full width at half-maximum (fwhm) of the intensity autocorrelation trace by a factor of 1.54 by assuming a hyperbolic secant (sech2) function for a temporal profile of the laser pulse. If the TATP analyte were to be introduced into the mass spectrometer, the analyte could be immediately measured under the same conditions. However, it is necessary to reduce the amount of sample as much as possible, to avoid the explosion of TATP during the experiment. After passing the beam through fused silica plates for dispersion compensation, we introduced the laser beam into another mass spectrometer combined with a gas chromatograph (GC/MS) using a heated capillary. A standard sample of TATP dissolved in acetonitrile (100 ng/ mL) was purchased from AccuStandard (New Haven, CT), and only a small aliquot of the sample, typically 10 ng/µL, was injected into the gas chromatograph for safety assurance. After recording two-dimensional data of GC/MS, one-dimensional data at the specified retention time, at which TATP was eluted, was extracted for the construction of a mass spectrum. A conventional GC/quadrupole-MS (GC/QMS) was employed to compare the data with those obtained using electron impact ionization. RESULTS AND DISCUSSION Laser Pulse Width. Figure 2A shows the spectrum of the laser beam passing through the Raman cell. The spectral domain was expanded from 1 to 4 nm; strong rotational Stokes and weak (shoulder) anti-Stokes emissions are generated by stimulated Raman scattering and four-wave Raman mixing, respectively. This bandwidth was sufficient to generate a transform-limited pulse of 20 fs. Figure 2B shows an interferometric autocorrelation trace of the laser pulse. The intensity autocorrelation trace, shown as a white line in the figure, was obtained by averaging the signal for the interferometric autocorrelation trace. The results indicate that

Figure 3. Mass spectrum obtained using an optical pulse of (A) 260 fs and (B) 60 fs; pulse energy, 11 µJ. Table 1. Ratio of the Molecular Ion and the Fragment Ion of C2H3O+ method

ratio (%)

remark

electron impact VUV single photon NIR femtosecond pulse DUV femtosecond pulse DUV femtosecond pulse DUV femtosecond pulse

0.1 3.5 9.1 1.7 4.2 15.0

70 eV (this work)a 108 nm (Oser et al.)2 130 fs, 795 nm (Oser et al.)8 260 fs, 267 nm (this work) 200 fs, 267 nm (Yamaguchi et al.)9 60 fs, 267 nm (this work)

a

GC/QMS was used in the experiment.

a 60 fs pulse is generated from a fwhm value of 92 fs. It should be noted that the chirp remains in the pulse, and the higher-order dispersion is not properly compensated using a pair of gratings. A combination of a grating and a deformable mirror could be used to solve this problem, although this would require a more complicated system for dispersion compensation. Mass Spectrum. Figure 3 shows the mass spectrum obtained using this femtosecond laser. When the pulse width was expanded to 260 fs by evacuating the hydrogen gas from the Raman cell, the peak corresponding to the molecular ion was small. On the other hand, when the pulse width was reduced to 60 fs by filling the Raman cell with hydrogen gas, the intensity of the molecular ion was increased substantially. The ratio of the signal intensities for the molecular ion and the C2H3O+ fragment ion (base peak) is summarized in Table 1, in which data obtained by several other methods are also listed. The conventional technique based on electron impact ionization (70 eV) resulted in the production of a negligible or very small signal for the molecular ion. Photoionization using a VUV laser was also inefficient and provided a small molecular ion peak. It should be noted that no molecular ion was observed even when a tunable VUV emission produced at a

synchrotron facility was utilized.10 In multiphoton ionization, the efficiency is mainly determined by the laser pulse width, and the signal intensity of the molecular ion relative to that of the fragment ion (C2H3O+) was increased from 1.7 to 15% (9-fold) by decreasing the pulse width from 260 to 60 fs. Quantum chemical calculations for a TATP molecule at the B3LYP/ccpVTZ levels using the Gaussian03 program11 suggests stretching vibrational periods of 35 and 28 fs for O-O and C-O bonds, respectively, and these vibrations could be coordinates for molecular dissociation. At the present stage, the laser pulse width was, unfortunately, much longer than the time period for molecular vibration and is not sufficiently short for fragmentfree ionization. The fragmentation would be substantially or completely suppressed if a laser with a pulse width less than 10 fs were to be used, which corresponds to a quarter period of molecular vibration. Such a 10 fs pulse has already been generated in the near-ultraviolet (NUV) region by four-wave Raman mixing after appropriate pulse compression.12 The present approach of “impulsive ionization” provides a useful method for observing a molecular ion in mass spectrometry. Another important issue related to observing a molecular ion would be the suppression of excess vibrational energy in the ion, which has been solved in part by the suppression of the initial vibrational energy of a molecule by supersonic jet expansion into a mass spectrometer.13 A combination of impulsive ionization and supersonic jet techniques has the potential for producing efficient ionization without any fragmentation. CONCLUSIONS We employed an ultrashort optical pulse generated by stimulated Raman scattering/four-wave Raman mixing, for the first time, in spectroscopy/spectrometry. When used as an ionization source for TATP, the intensity of the molecular ion relative to the fragment ion was increased 9-fold by decreasing the laser pulse width from 260 to 60 fs. A period of molecular vibration (O-O and C-O) can be calculated to be ∼30 fs, and an ultrashort optical pulse of less than 10 fs could be successfully used for observing a molecular ion. Since this approach, which is based on four-wave Raman mixing, is capable of generating an ultrashort optical pulse less than 1 fs, it presents a universal means for observing molecular ions exclusively in mass spectrometry. (10) Schramm, E.; Mu˝hlberger, F.; Mitschke, S.; Reichardt, G.; Schulte-Ladbeck, R.; Pu˝tz, M.; Zimmermann, R. Appl. Spectrosc. 2008, 62, 238–247. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M., Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian Inc.: Wallingford, CT, 2003. (12) Kida, Y.; Zaitsu, S.; Imasaka, T. Opt. Express 2008, 16, 13492–13498. (13) Fialkov, A. B.; Amirav, A. Rapid Commun. Mass Spectrom. 2003, 17, 1326– 1338.

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ACKNOWLEDGMENT This research was supported by a Grant-in-Aid for the Global COE program, “Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). This work was also supported by Grants-inAid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and the Creation and Support Program for Start-Ups from the Universities from Japan Science

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and Technology Agency (JST). The quantum chemical calculations were mainly carried out using the computer facilities at the Research Institute for Information Technology, Kyushu University.

Received for review February 9, 2010. Accepted March 30, 2010. AC1003773