Photoionization time-of-flight mass

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

B. Molecular Beam-Photoionization-Time-of-Flight Mass Spectrometry. . 189... 

Cass, G. R M. H. Conklin, J. J. Shah, J. J. Huntzicker, and E. S. Macias, Elemental Carbon Concentrations Estimation of and Historical Data Base, Atmos. Environ., 18, 153-162 (1983). Castaldi, M. J., and S. M. Senkan, Real-Time, Ultrasensitive Monitoring of Air Toxics by Laser Photoionization Time-of-Flight Mass Spectrometry, J. Air Waste Manage. Assoc., 48, 77-81... 

Oser, H., M.J. Coggiola, S.E. Young, D.R. Crosley, V. Hafer, and G. Grist. 2007. Membrane introduction/ laser photoionization time-of-flight mass spectrometry. Chemosphere 67 1701-1708. 

Photoionization time-of-flight mass spectrometry is used almost exclusively in all experiments described in this review. The ionizing laser sources have included excimer lasers for photon energies up to 7.87 eV and tunable ultraviolet sources up to 6.5 eV. In some early studies, multiphoton ionization was used, but it has become quite clear that this usually results in dissociative ionization. Such effects have been observed in many systems, ranging from Si49,5o ( 51 jQ jjjg transition metals. Thus single-photon ionization has... 

Syage JA, Hanning-Lee MA, Hanold KA. A man-portable, photoionization time-of-flight mass spectrometer. Field Anal Chem Toxicol 2000 4 204-15. 

A number of less commonly used analytical techniques are available for determining PAHs. These include synchronous luminescence spectroscopy (SLS), resonant (R)/nonresonant (NR)-synchronous scan luminescence (SSL) spectrometry, room temperature phosphorescence (RTP), ultraviolet-resonance Raman spectroscopy (UV-RRS), x-ray excited optical luminescence spectroscopy (XEOL), laser-induced molecular fluorescence (LIMP), supersonic jet/laser induced fluorescence (SSJ/LIF), low- temperature fluorescence spectroscopy (LTFS), high-resolution low-temperature spectrofluorometry, low-temperature molecular luminescence spectrometry (LT-MLS), and supersonic jet spectroscopy/capillary supercritical fluid chromatography (SJS/SFC) Asher 1984 Garrigues and Ewald 1987 Goates et al. 1989 Jones et al. 1988 Lai et al. 1990 Lamotte et al. 1985 Lin et al. 1991 Popl et al. 1975 Richardson and Ando 1977 Saber et al. 1991 Vo-Dinh et al. 1984 Vo- Dinh and Abbott 1984 Vo-Dinh 1981 Woo et al. 1980). More recent methods for the determination of PAHs in environmental samples include GC-MS with stable isotope dilution calibration (Bushby et al. 1993), capillary electrophoresis with UV-laser excited fluorescence detection (Nie et al. 1993), and laser desorption laser photoionization time-of-flight mass spectrometry of direct determination of PAH in solid waste matrices (Dale et al. 1993). 

Fig. 22. Size distribution of carbon clusters obtained by laser vaporization and measurement by photoionization time-of-flight mass spectroscopy. From Ref. 353.

Figure 21.1. Photoionization time-of-flight mass spectrum of neutral silicon clusters. The clusters have been ionized with an excimer laser hv = 7.89 eV). In the inset a schematic drawing of the pulsed laser vaporization cluster source is shown.

Bombarding PH3 with electrons generates electronically excited PH for spectroscopic investigations an electron beam of, e.g., 60-eV [9] and periodic pulses of, e.g., 20-keV electrons were used [10]. PH forms via PH3- PH + H+ + H, where the appearance potential of the additional product H of 20.5 1 eV was determined by photoionization time-of-flight mass spectrometry. The formation of PH via PH3 PH + HJ was not observed an appearance potential of 17.95 eV for HJ was calculated from thermodynamic data [11]. 

Middaugh, J.E. (2014) The Study of Bimolecular Radical Reactions Using a Novel Time-resolved Photoionization Time-of-flight Mass Spectrometry and Laser Absorption Spectrometry Apparatus. PhD Thesis, Massachusetts Institute of Technology. 

Figure 2 Section of mass spectra for Cs/Oj clusters using photoionization time-of-flight mass spectrometry shown on three different mass scales. After Martin TP, Bergmann T, Naher U, Schaber H and Zimmermann U (1994) Nuciear Instruments and Methods B88 1.

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

J. E. Braun and H. J. Neusser. Threshold Photoionization in Time-of-Flight Mass Spectrometry. Mass Spectrom. Rev., 21(2002) 16-36. 

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

Femtosecond photodissociation dynamics of nitroethane and l-nitropropane have been studied in the gas phase and in solution by resonance Raman spectroscopy, with excitation in the absorption band around 200 nm. At such short time-scales it is possible to detect changes in the two N-O bond lengths in the Franck-Condon region, prior to C-N bond cleavage. Photolyses of nitroalkanes at 193 nm have been monitored by photoionization of the fragments and time-of-flight mass spectrometry. Both C-N and N-O bond dissociation pathways are observed and, under the conditions of free jet expansion, primary products such as pentyl and hexyl radicals are stabilized and can be detected. 

The sensitivity of fragmentation to the nature of resonances was shown for tetramethylsilane by using the photoion photoion coincidence technique (Morin et al. 1986). In this technique, fragmentation channels are identified by the coincidences of pairs of ions in the time-of-flight mass spectrometer. For tetramethylsilane, two resonances were observed in the ion pair yield curves. The lower energy resonance, below the ionization edge, had been shown from electron spectra to decay into a one-hole state, but the production... 

A particle-sampling mass spectrometer could provide a useful approach to measuring inhalation exposure to pollutants in a wide variety of environments. One example17 employs an aerodynamic lens that samples very fine particles and creates a beam that can be modulated to give a crude time-of-flight mass distribution. The particles impinge on a hot surface, causing vaporization of constituents that can be ionized (by electron impact or photoionization) and subjected to mass analysis by any of several kinds of mass analyzers. 

Laser-induced multiphoton ionization spectroscopy has been applied to the parent, methyl-, and chloro-substituted pyrazines <85ANC29ll). The ions produced by laser photoionization in the supersonic jet are mass-analyzed using a time of flight mass spectrometer, in which the spectra obtained reflect the absorption of the n-n transition. Mass-resolved excitation spectroscopy by laser ionization in the infrared region has also been applied to the conformational analysis of a-alkyl substituted pyrazines <92JA5269). Dynamic and structural properties of electronically excited states of pyrazine have been elucidated from high resolution laser spectroscopy with MHz resolution <88JST(173)201). 

For the more volatile (alkali) metals it has been possible since the late 1970s to create a beam of cold, naked clusters of 1 < n < 100, where n is the number of atoms per cluster 348, 349). Metal vapor is produced in an oven and then cooled by expansion. The clusters are separated in a time-of-flight mass spectrometer (TOFMS). Later it became possible to handle less volatile (transition) metals through vaporization by a laser beam 347, 350-352). For the mass analysis the neutral clusters are photoionized by a suitable UV laser. 

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