Resonance enhanced multiphoton spectrometry

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

The vibration spectrum of the first excited state of guanine was measured using laser desorption jet-cooled resonance-enhanced multiphoton ionization (REMPI) spectrometry <1999JA4896>. The millimeter wave spectrum of purine was collected using a free jet spectrometer, and the observed rotational spectrum was assigned to the N(9)-H tautomer <1996CPL189>. 

Photolysis of the dimer, reaction (44), proceeds primarily via generation of Cl + ClOO (Cox and Hayrnan, 1988 Molina et al., 1990). For example, Molina et al. (1990) reported the quantum yield for this channel at 308 nm to be unity, with an uncertainty of 30%. Okumura and co-workers (Moore et al., 1999) and Schindler and co-workers (Schmidt et al., 1998) have reported that the quantum yield is less than 1.0. For example, Schmidt et al. (1998) used resonance-enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry to follow the production of oxygen and chlorine atoms as well as CIO in vibrational levels up to v" = 5 in the photolysis of the dimer. At a photolysis wavelength of 250 nm, the quantum yield for chlorine atom production was measured to be 0.65 + 0.15, but CIO was not observed. Assuming that all of the excited dimer dissociates, this suggests that the production of CIO in vibrational... 

MOS metal oxide sensor, MOSFET metal oxide semiconductor field-effect transistor, IR infrared, CP conducting polymer, QMS quartz crystal microbalance, IMS ion mobility spectrometry, BAW bulk acoustic wave, MS mass spectrometry, SAW siuface acoustic wave, REMPI-TOFMS resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry... 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Resonance-Enhanced Multiphoton Ionisation Time-of-Flight Mass Spectrometry... 

Zimmerman, R., Heger, H.J., Kettrup, H.J., Boesl, U. (1997) A mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometry device for online analysis of aromatic pollutants in waste incinerator flue gases first results. Rapid Commun. Mass Spectrom. 11 1095-1102. 

Recently, new 2D-methods for the analysis of complex mixtures have been developed for time-of-flight mass spectrometry (22), which could also be utilized in external ionization FTMS. Specifically, the combination of IR-laser desorption of nonvolatile neutrals, followed by adiabatic cooling to 2°K in a supersonic jet, and subsequent compound-selective Resonance-Enhanced Multiphoton Ionization (REMPI) could increase the role of FTMS in the analysis of biological mixtures. The coupling of supersonic jets to the external ion source would also be of interest in ion- and neutral cluster experiments. 

Several libraries of heterogeneous catalysts have been screened using IR thermographic imaging (51), resonance-enhanced multiphoton ionization (52), and mass spectrometry (53), among others. Example of these screening methods are given below. 

The overlap of molecular spectra with atomic lines, which occurs in optical flame spectrometry, has been less commonly encountered with LEI. Native flame species such as OH and CH are not observed because of their high ionization potentials. Resonantly-enhanced multiphoton ionization of molecules such as NO 45,46) may cause interferences in some flames. The LEI spectra of oxides of lanthanum, scandium,... 

Vibrational spectra of aniline/cyclohexane and aniline/benzene clusters investigated by infrared depletion spectroscopy with selective resonance—enhanced multiphoton ionization with time-of-flight mass spectrometry (REMPI-TOF mass spectrometry) show186 an... 

Cao, L. Muhlberger, F. Adam, T. et al. Resonance-enhanced multiphoton ionization and VUV-single photon ionization as soft and selective laser ionization methods for on-line time-of-flight mass spectrometry investigation of the pyrolysis of typical organic contaminants in the... 

The combination of pulsed lasers, pulsed molecular beams, and time-of-flight mass spectrometry represents a powerful technique for studying the selective excitation, ionization, and fragmentation of wanted molecules out of a large variety of different molecules or species in a molecular beam [9.93-9.99]. The technique, developed by Boesl et al. [9.93] is illustrated by Fig. 9.29 rotationally and vibrationally cold neutral parent molecules M in a supersonic molecular beam pass through the ion source of a time-of-flight mass spectrometer. A pulsed laser LI forms molecular ions M by resonant enhanced multiphoton ionization. By selecting special intermediate states of M, the molecular ion can often be preferentially prepared in a selected vibrational level. 

Removal of the Iti electron from NH(X S ) leads to the ionic ground state X removal of the 3a electron to the excited ionic states a A B A, and C Only a few experimental data for the first, third, and fourth ionization potentials Ej of gaseous NH are available. Resonance-enhanced multiphoton ionization (REMPI) of NH coupled with photoelectron spectroscopy (PES) yielded the most accurate results so far [1] and confirmed the values for the first E, obtained by electron-impact mass spectrometry (EIMS) [2] and by He I PES of NH [3]. Values for the second and third Ej to be observed in the He I PES of NH were predicted [3] from the optical emission spectra of NH [4]. Adiabatic and vertical Ej s (in eV) are compared in the following table ... 

Since multiphoton excitation in mass spectrometry takes place in the more or less tight laser focus, which can easily be shifted in space and time or be subject to other variations, it can be combined with different ion optical or mechanical arrangements (e.g. sources of neutral molecular systems) without the need for much additional hardware. Thus, by combination with chromatography (particularly gas chromatography), species selection has successfully been realized. Another very promising combination, which has frequently been applied in the recent past for the study of involatile molecules (e.g. polycyclic aromatics, biomolecules), is that of laser desorption of neutral molecules and resonance enhanced multiphoton ionization. All the benefits of multiphoton mass spectrometry, such as soft ionization, selective ionization, controllable fragmentation or secondary excitation for tandem mass spectrometry, may be used in this field. 

Both the high species selectivity and the soft ionization make resonance enhanced multiphoton excitation an excellent ion source for chemical trace analysis. In addition, the use of pulsed lasers combined with TOF mass spectrometry (also a pulsed technique) enables the recording of single mass spectra within a few milliseconds, depending on the repetition frequency of the laser pulses (typically 20 to 50 FIz). The unification of high speed, selectivity and sensitivity indicate that multiphoton mass spec-... 

Grotemeyer J, Boesl U, Walter K and Schlag EW (1986) A general soft ionization method for mass spectrometry resonance-enhanced multiphoton ionization of biomolecules. Organic Mass Spectrometry 21 645-653. 

Zimmermann R, Lenoir D, Kettrup A, Nagel H and Boesl U (1996) On-line emission control of combustion processes by laser-induced resonance-enhanced multiphoton ionization mass spectrometry. Twenty Sixth Symposium (International) on Combustion, pp 2859-2868 Pittsburgh The Combustion Institute. 

---