Atmospheric pressure photon ionization

An ideal interface should not cause extra-column peak broadening. Historical interfaces include the moving belt and the thermospray. Common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Several special interfaces include the particle beam—a pioneering technique that is still used because it is the only one that can provide electron ionization mass spectra. Others are continuous fiow fast atom bombardment (CF-FAB), atmospheric pressure photon ionization (APPI), and matrix-assisted laser desorption ionization (M ALDl). The two most common interfaces, ESI and APCI, were discovered in the late 1980s and involve an atmospheric pressure ionization (API) step. Both are soft ionization techniques that cause little or no fragmentation hence a fingerprint for qualitative identification is usually not apparent. 

Atmospheric pressure laser ionization (APLl) is a complementary technique to existing API methods. It is based on resonant or near-resonant two-photon ionization of aromatic ring systems. APLI utilizes resonantly enhanced multiphotonionization (REMPI) as the primary ion-production mechanism, nevertheless it occurs at atmospheric pressure [80],... 

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. 

The recently introduced atmospheric-pressure laser ionization system (APLI) can be considered as a modification of APPI (Ch. 5.7.3). In APLI, the one-step photoionization of APPI is replaced by a two-photon process in resonantly-enhanced multi-photon ionization [148]. Enhanced response for polycyclic aromatic hydrocarbons (relative to APCI) was demonstrated. Molecular ions rather than protonated molecules are generated in APLI (cf. Ch. 6.5). 

Ionization of condensed-phase analytes occurs by mixing a sample in a suitable matrix and bombarding the matrix-analyte mixture with an energetic beam made of either laser photons as in MALDI, high-energy fission particles as in Cf plasma desorption, or high-energy fast atoms or ions (FAB or liquid SIMS). When an analyte is present in a solution, such as an effluent from a separation device, it can be ionized via thermospray ionization, atmospheric-pressure chemical ionization, atmospheric-pressure photoionization, or electrospray ionization. Desorption electrospray ionization and direct analysis in real time are new modes of ionization that are accomplished in ambient air. 

Note Reaction sequences analogous to API do occur whenever ionization is performed in a moist atmosphere. The initiating source of ionization can be delivered by a corona discharge as in atmospheric pressure chemical ionization (APCI, Chap. 12.8), by UV photons as in atmospheric pressure photoionization (APPI, Chap. 12.9), or by a glow-discharge in helium or argon as used in direct analysis in real time (DART, Chap. 13.5). 

Atmospheric pressure matrix-assisted laser desorption/ ionization AP- MALDI Photon induced desorption/ ionization Nonvolatile molecular ions Soft method Large molecules... 

L8. Loeb, L. B., The Threshold for the positive pre-onset burst pulse corona and the production of ionizing photons in air at atmospheric pressure, Phys. Rev. 73, 798 (1948). 

In 1990, Zhu et al. [112] performed resonant two-photon ionization (R2PI) at 266 and 213 nm of substituted nitrobenzenes (NBs). Typically, in REMPI, the molecular ion itself is not observed for large parent molecules. They found that although extensive fragmentation occurs under vacuum, R2PI under atmospheric pressure (1 atm, He) has... 

The APPI source is one of the last arrivals of atmospheric pressure sources [80,81]. The principle is to use photons to ionize gas-phase molecules. The scheme of an APPI source is shown in Figure 1.34. The sample in solution is vaporized by a heated nebulizer similar to the one used in APCI. After vaporization, the analyte interacts with photons emitted by a discharge lamp. These photons induce a series of gas-phase reactions that lead to the ionization of the sample molecules. The APPI source is thus a modified APCI source. The main difference is the use of a discharge lamp emitting photons rather than the corona discharge needle emitting electrons. Several APPI sources have been developed since 2005 and are commercially available. The interest in the photoionization is that it has the potential to ionize compounds that are not ionizable by APCI and ESI, and in particular, compounds that are non-polar. 

A relatively widely-available alternative ionization technique is atmospheric-pressure photoionization (APPI) [102]. In APPI, the ionization process is initiated by photons from a discharge lamp rather than from a corona discharge electrode. APPI is promising in the analysis of relatively non-polar analytes. Commercial systems are available (Photospray from Sciex, photoionization from Syagen Technology and other instrament manufacturers). Ionization under APPI is discussed in Ch. 6.5. 

Atmospheric pressure photoionization (APPI) is a relatively new technique48-51 but the source design is almost identical to that used for APCI except that the corona discharge needle is replaced by a krypton discharge lamp, which irradiates the hot vaporized plume from the heated nebulizer with photons (10 and 10.6 eV). The mechanism of direct photoionization is quite simple. Where the ionization energy of the molecule is less than the energy of the photon, absorption of a photon is followed by ejection of an electron to form the molecular radical ion M+ (Equation (28)). 

Atmospheric-pressure photoionization (APPI), which was first applied as an LC-MS interface in 2000, is a relatively new ionization technique. The ionization process is initiated by an ultraviolet lamp (krypton discharge lamp), which emits 10.0 and 10.6 eV photons. Any compounds that have ionization energies below 10 or... 

As a special source of particles, low-temperature plasmas are included in this section, which strictly speaking are sources of ionized atoms or molecules as well as photons and electrons that all may interact with the polymer. In addition to well-established low-pressure plasma sources, corona discharge or recently developed atmospheric pressure plasma sources may be used for surface modification. 

Atmospheric-pressure photoionization (APPI) is a relatively new technique suited to analyze LC effluents that contain nonpolar and polar low-mass compounds [70-74]. In this methodology, the liquid effluent is vaporized in a heated nebn-Uzer similar to the one described above for the APCI source to generate a dense clond of gas-phase analytes. Ionization is initiated by a beam of photons emitted... 

The setup for atmospheric pressure photoionization (APPI) (Bos et ah, 2006 Hanold et ah, 2004 Raffaelli and Saba, 2003 Robb et ah, 2000) is very similar to that of APCI (Fig. 8.7). Only the corona discharge is replaced by a gas discharge lamp (krypton,10.0 eV) that generates ultraviolet (UV) photons in vacuum. The liquid phase is also vaporized by a pneumatic nebulizer and different geometries are used. Most analytes have ionization potentials below 10 eV, while high-pressure liquid chromatography (HPLC) solvents have higher ionization potentials (water 12.6 eV, methanol 10.8 eV, and acetonitrile 12.2 eV). 

Table 2.1 List of the ionization energies of common gases, organics, and solvents [47], Energies of photons emitted by different lamps [47, 48]. [Atmospheric Pressure Photoionization Mass Spectrometry. Raffaelli, A., Saba, A. Mass Spectrom. Rev. 22/5. Copyright (2003) John Wiley and Sons]...

When an electron neutralizes a positive ion, the energy released can be dissipated either in photon emission (radiative recombination), or by a third body encounter with the transient excited atom or molecule (three-body recombination) or by the fragmentation of the transient excited molecule (dissociative recombination). Radiative recombination only occurs with a very small probability and three-body recombination only occurs at high pressures or high charge densities, neither of these being appropriate to the atmospheric plasma. It is the dissociative process, exemplified by reactions (5a) and (5b), which is dominant in the ionosphere. In fact, reactions (5a) and (5b) are almost entirely responsible for the loss of ionization in the ionosphere above 85 km altitude (with N2 recombination contributing somewhat) as is readily shown by simple calculations based on laboratory determinations of dissociative recombination coefficients, are, for the dominant molecular ions 02 and NO+. 

As with most proportional counters, the linear PSPC consists of a central anode wire between one or two cathodes. The space between the anode and cathode is filled with a conventional counter gas consisting of an ionizable gas, frequently Ar or Xe, and a quencher, e.g., CH4, typically in the ratio 9 1 and often at pressures of a few atmospheres. A large voltage ( - 1.5 kV) is maintained between the anode and cathode. When an X-ray photon enters the counter, there is a finite probability that it will ionize a gas molecule and the emitted electron then accelerates towards the anode under the influence of the field. This electron may then collide with further gas atoms, resulting in the formation of an electron avalanche near the central wire and thus the formation of a charge pulse on the anode. The total charge in this pulse is proportional to the energy of the incident photon. Thus far, this description is that of a conventional proportional counter. The position sensitivity can be achieved in several ways. 

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