Resonance-enhanced Multiphoton Ionization Mass Spectrometry Using a Two-color Laser Beam Generated by Stimulated Raman Scattering

Abstract

A wide variety of chemical species, such as chlorinated aromatic hydrocarbons, are emitted from an incinerator. Among such compounds, polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD/F) are known to be extremely toxic, and the on-line real-time monitoring of such compounds would be highly desirable. A lengthy pretreatment process followed by high-resolution gas chromatography/mass spectrometry is the current method of choice for the analysis of PCDD/F. 1,2 However, this type of approach is not suitable for the on-line real-time monitoring of PCDD/F emitted from an incinerator.Resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOF-MS) 3,4 constitutes an attractive solution to this problem. Combined with supersonic jet spectrometry (SSJ), 5,6 REMPI/TOF-MS provides superior selectivity than the conventional (room temperature) approach, as the result of sharp spectral features obtained by jet expansion and subsequent molecular cooling. However, there are several problems associated with the measurements of PCDD/F by SSJ/REMPI/TOF-MS. In particular, the ionization potential increases and the 0-0 transition energy decreases with increasing numbers of substituted chlorine atoms, as reported for PCDD. Therefore, the energy of the photon used for excitation at the 0-0 transition becomes insufficient for use in subsequent ionization from the excited state to the ionized levels. For this reason, it is necessary to use a three-photon process, but the efficiency is rather poor. In addition, a highpower laser is needed for three-photon ionization, which leads to non resonant ionization, and makes differentiation of the isomers of PCDD/F difficult. In order to solve this problem, two-color REMPI has been developed. [7][8][9][10] In two-color REMPI, the first photon, with an energy less than the second one, is used for excitation, and the second photon (higher energy than the first) is used for ionization of the molecule, for example, in the case of PCDD/F. A laser source consisting of two sets of laser systems emitting at different wavelengths is currently used for this purpose, although the ionization laser (e.g., the fourth harmonic emission of the Nd:YAG laser) is sometimes constructed utilizing the same Nd:YAG laser as that employed for pumping the tunable laser to simplify the system. 7-10 This approach is complicated because two laser beams (pulses) must be both spatially and temporally adjusted to each other. Moreover, the analytical system becomes more expensive because two lasers are needed. As a result, it is impractical for use in applications to the on-line monitoring of PCDD/F emitted from an incinerator.In this study, SSJ/REMPI/TOF-MS using a two-color laser system was demonstrated. In this technique, a single-color (fundamental) laser beam is focused into molecular hydrogen to generate a Stokes beam by stimulated Raman scattering (SRS). 11 The two beams (pulses) are spatially and temporally superimposed, and can be readily used as a two-color beam for REMPI u...

A Two-Color Three-Photon Ionization Scheme for the Efficient and Selective Ionization of a Chlorinated Aromatic Hydrocarbon

“…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 . However, it is necessary to use a laser that emits at shorter wavelengths (see Figure c,d).…”

“…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 ionization scheme can be used. − As shown in Figure c, 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.…”

Sensitive and Selective Analysis of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans and Their Precursors by Supersonic Jet/Resonance-Enhanced Multiphoton Ionization/Time-of-Flight Mass Spectrometry

“…3. 71 In practice, the two beams are spatially and temporally superimposed in this technique. We found it to be 5 times more efficient than a one-color ionization scheme in the determination of p-DCBz.…”

Data processing technique in gas chromatography/time-of-flight mass spectrometry

“…A pulsed laser is frequently used instead of an electron beam as an ionization source. This technique, based on multiphoton ionization (MPI), has been employed for the measurement of organic compounds [6][7][8][9][10][11][12][13][14][15][16][17][18] and successfully combined with GC (GC/MPI/ TOF-MS) for trace analysis of PeCDFs and their precursors. 19 A femtosecond laser has been successfully employed for this purpose, since the degradation of the ionization efficiency is minimal even for PCDD/DFs having short excited-state lifetimes.…”