Mass spectrometry laser photoionization

See also Environmental Applications of Eiectronic Spectroscopy Fragmentation in Mass Spectrometry ion Dissociation Kinetics, Mass Spectrometry ion Energetics in Mass Spectrometry Laser Appiications in Eiectronic Spectroscopy Metastabie ions Muitipho-ton Spectroscopy, Appiications Negative ion Mass Spectrometry, Methods Photoeiectron-Photoion Coincidence Methods in Mass Spectrometry (PEPiCO) Photoionization and Photodissociation Methods in Mass Spectrometry Spectroscopy of ions Time of Flight Mass Spectrometers. 

See also Ion Imaging Using Mass Spectrometry Laser Spectroscopy Theory Multiphoton Excitation in Mass Spectrometry Multiphoton Spectroscopy, Applications Photoelectron Spectrometers Photoelectron Spectroscopy Photoelectron-Photoion Coincidence Methods in Mass Spectrometry (PEPICO) ... 

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. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

V. Antonov and V. Letokhov. Laser Multiphoton and Multistep Photoionization of Molecules and Mass Spectrometry. Appl. Phys., 24(1981) 89-106. 

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... 

Ehring, H., Karas, M. and Hillenkamp, R, Role of photoionization and photochemistry in ionization processes of organic molecules and relevance for matrix-assisted laser desorption ionization mass spectrometry. Org. Mass Spec., 27, 472, 1992. 

Femtosecond photoionization mass spectrometry might be useful in the study of the three-dimensional structure of large biomolecules. When a selectively excitable and ionizable chromophore is located on the outer (surface) part of large molecule, one can be detached in the picosecond time scale. However, when the excitable chromophore is located in the inner part of the big molecule, its detachment will require a much longer time, which is needed for spatial rearrangement of the molecule. So, even the simple mass spectrometry of bioorganic molecules with femtosecond laser ionization can reveal some details of their spatial structure. 

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. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Direct experimental measurements of radical decompositions are relatively rare but recently a number of techniques have been applied and are producing new and interesting information. In Section 2.4.2. we shall mainly focus on the technique of laser flash photolysis coupled with photoionization mass spectrometry as a method of monitoring radical decompositions although other techniques will be briefly mentioned. 

Fig. 2.14. Laser flash photolysis/photoionization mass spectrometry apparatus for studying...

IRMPD/PIMS, Infrared multiphoton dissociation/photoionization mass spectrometry. LFP/UV, Excimer laser photolysis, UV absorption detection of hydrocarbon. LFP/PIMS, Excimer laser photolysis, photoionization mass spectrometric monitoring of hydrocarbon. 

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... 

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