Analytes multiphoton ionization

Lubman, D.M. (1988) Analytical multiphoton ionization mass spectrometry. Part I. Theory and instrumentation. Mass Spectrometry Reviews, 7, 535-554. 

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

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. 

Recently, laser multiphoton ionization of solutes has been used. Defining the threshold of ionization, iJth, can be a problem in some of these methods. A recent multiphoton technique, utilizing femtosecond laser pulses, appears to give quite accurate thresholds [56]. In this work, a conductivity spectrum is measured at visible wavelengths and a sharp drop in current occurs as the mechanism changes from -photon excitation to (n+ l)-photon excitation, where n is typically 3 to 4. The threshold is defined by fitting the current to an analytic function that defines the midpoint of this transition. Eth is then n times the energy at which the midpoint occurs. The thresholds are sensitive to Vg and could be used for determination of this quantity. 

A pulsed sample introduction interface for LC-MS in a TOF instrument was described by Wang et al. [131-132]. Analyte ionization is performed by means of laser-induced multiphoton ionization. The interface is based TSP nebuhzation into a heated expansion chamber and a high-temperature pulsed nozzle. Experimental parameters in on-line LC-MS were evaluated [132]. 

Czech Republic). Development and application of semipermeable membrane devices (SPMDs) as environmental dosimeters for PCB contaminants in water, air, sediment, and soil is the subject of ongoing research by Huckins and Petty at Columbia Environmental Research Center in Missouri. Also at the Columbia Environmental Research Center, C. Orazio et al. are developing analytical methods for determining PCBs in environmental matrices. A reliable method for continuous monitoring of PCBs in incinerator stack gas emissions using resonance-enhanced multiphoton ionization spectroscopy in conjunction with time-of-flight mass spectroscopy (REMPI/TOFMS) is the topic of current research by... 

The essentials of the SSEA for numerically compufed energy-normalized N-elecfron wavefunctions were published in 1994 by Mercouris ef al. [54], The firsf application was not only to the multiphoton ionization of H (whose specfrum is known exactly analytically), as a test case, but also to the multiphoton detachment of the four-electron Li negative ion, with two free channels, Li ls 2s S and ls 2p P°. Li (or Be) is the first system of fhe Periodic Table for which the proper description of the zero-order electronic structure requires a multiconfigurational Hartree-Fock (MCHF) description. In the context of the review of the SSEA, we also discuss briefly the formulation of the problem in terms of the full atom-EMF interaction,Vext(f), which is computationally convenient as well as necessary for certain problems involving, say, off-resonance coupling of Rydberg states, for which use of just the electric dipole term is inadequate [55-57]. 

Becker CH and Gillen KT (1984) Surface analysis by non-resonant multiphoton ionization of desorbed or sputtered species. Analytical Chemistry 56 1671-1677. 

Hgure 2 Survey of laser ionization schemes for gas-phase analytes single photon ionization (SPI), multiphoton ionization (MPI) with distinction between resonant ionization (Rl), resonance enhanced multiphoton ionization (REMPI), and nonreso-nant multiphoton ionization (NRMPI). Real and virtual states are denoted by R and S, respectively, while C refers to the ionization continuum. 

Analytes with IP > 8 eV require multiphoton ionization (MPI), i.e., consecutive absorption of two or more photons. A first photon transfers the analyte into a specific, electronically excited state and subsequent uptake of photon(s) is required to overcome the IP. The efficiency of this sequential process depends on the decay rate of the intermediate state relative to the photon flux density that promotes its population. The balance between the two competitive effects can be quantified by a quality factor as in... 

APLl was developed in 2005 [68]. It is a soft ionization method with easy-to-interpret spectra for nonpolar aromatic substances and only minor tendency for fragmentation of the analytes. APLI is based on the resonance-enhanced multiphoton ionization (REMPI), however, at AP. The REMPI method allows the sensitive and selective ionization of numerous compounds. Here, for example, the following approach is used ... 

Another selective ionization approach involves the use of multiphoton ionization of analytes as they elute from the chromatographic column (159,160). Using a tunable laser, analytes can be specifically ionized by choosing the appropriate wavelength. This allows background components to be excluded in some cases if they have different UV absorption frequencies. 

Ions can be collected very efficiently and detected mass-selectively. The method of choice for the production of ions for analytical purposes is the species-selective multiphoton ionization, with lasers in the visible or the UV. When used for identifying the absorbing species the method is implemented via a resonance-enhanced route as discussed below. But using very high laser powers, it is possible to ionize in an indiscriminate (or not resonant) fashion any species that is present in the path of the laser beam. In dynamics, mnltiphoton ionization is used to probe reaction products as noted in Sections 7.1.2.1 and 7.1.2.4 and in specific examples. 

Direct Multiphoton Ionization of Matrix or matrix-Analyte Complexes... 

Some very high resolution or ultrasensitive spectroscopies emerging as of ca 1996 were beam spectroscopy, multiphoton absorption and ionization, and frequency-modulation spectroscopy (8). Most of these were used primarily for laboratory research as of the mid-1990s, but eventual application to analytical or sensor applications is expected. 

The intensities of lasers exceed by many orders of magnitude those of conventional light sources. At high intensity levels, nonlinear interaction of light with atomic and molecular systems becomes pronounced. New analytical techniques involving multistep and multiphoton excitation and ionization, optical saturation, and excitation of forbidden transitions become possible. High radiation intensity also increases the sensitivity of laser analytical techniques,... 

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