Atmospheric Pressure Photoionization (APPI)
Photoionization at atmospheric pressure (APPI) has been introduced as a detection method for gas chromatography (GC). Technically the APPI source is considered APCI by changing the corona discharge by a photon lamp . It consists in using a discharge lamp that generates photons having the wavelength in the ultraviolet range. If photons are absorbed by molecules that have ionization energy (IE), which is less than photon energy, ionization of molecules can occur (equation 1).
M + hν → M+. + e– (equation 1)
Another reaction path can take place, if a carrier gas, such as nitrogen, is used, which strongly absorbs the far UV radiation (equation 2, 3):
N2 + hv → N2* (equation 2)
N2* + M → N2 + M+. + e– (equation 3)
Instead of nitrogen, other molecules that are more effective for ionizing the analyte can be used, these molecules are usually called dopants (equation 4, 5).
D + hv → D+. + e– (equation 4)
D+. + M → D + M+. (equation 5)
In comparison with electrospray, APPI is much more complicated in terms of ion production, the radical ions can be produced directly or via an intermediate using a dopant. But on the APPI spectra, non-free radical ions are mainly observed, these ions are produced by the capture of a proton from the solvent (equation 6).
M+. + S → [M+H]+ + (S-H)• (equation 6)
This reaction explains why protonated molecular ions are often predominant in APPI spectra. It is worthy noted that the abundance of the protonated ions is dependent on the proton affinity (PA) of the analyte vis-à-vis the one of the solvent. The negative ions can be formed in the APPI source, in this case the analyte or another molecule solvent captures a free electron, then proton exchanges can take place according to several mechanisms. The ions formed by these mechanisms generate a current that flows through an electrode and forms the signal in the chromatogram. The lamp is usually selected in such a way that the energy of the photons is less than the ionization energy (IE) of the carrier gas and above the analyte IE. It should be noted that the IE of a compound depends on the size and structure of the compound. Large molecules and compounds that have a high degree of conjugation generally have lower IEs than small molecules. Thus, the analytes usually have IEs in the range of 7-10 eV, while the gases have higher IEs (Table 1). As a result, the analytes can be selectively ionized without interference with gas molecules. Usually, a krypton discharge lamp that emits photons with energy of 10 eV is used.
Table 1. Ionization energies
|Compound||(Ionisation Energie) IE (eV)|
|Le dioxyde de carbone||13,78|
The Photoionization has also been used as a detection method for liquid chromatography   . The ionization of the analytes can be achieved because of the relatively high IE of the solvents (H2O, ACN, Methanol) used for LC (Table 1). The first step of the coupling of the APPI with the LC is the evaporation of the elution solvent of the LC column because the recombination rate of the ions is much higher in the liquid phase than in the gas phase. After evaporation, the analytes are ionized in a manner similar to GC-APPI. The APPI ion source has also been used as a detector for ion mobility  . Revel’skii et al. are the first to introduce the APPI source to mass spectrometry, they have replaced a nebulizer heated by a photoionization lamp  . The gas mixture containing analytes is introduced at the same time with the nebulizer gas without separation. This results in an extended dynamic range and better sensitivity for their analytes. A few years later Bruins et al., Built their own APPI ion source and applied it for LC-MS analysis . They used the APPI source with a Sciex PE triple quadrupole mass spectrometer. The system includes a heated nebulizer and housing, which are identical to those used in the Sciex APCI source. Nitrogen is used as nebulizer gas and lamp gas. Oxygen is used as an auxiliary gas. The source includes a dopant that is added to the auxiliary gas line and vaporized together with the heated solvent in the nebulizer. The dopant flow rate is about 1/10 of the solvent flow rate, which is typically 100 to 300 μl / min. The vapor is swept by a stream of nitrogen gas in the photoionization zone mounted directly at the end of the heated nebulizer probe. The lamp used is a commercially available krypton discharge lamp, the photons emitted by this lamp have an energy of 10 eV. An electrical potential of 1.2 to 1.5 kV is applied to the mounting bracket of the discharge lamp. The APPI source, described in more detail by Bruins et al., Is shown in Figure 1.
Figure 1. Schematic of the APPI source designed by Bruins et al.
The APPI source designed by Bruins et al. was developed in order to expand the number of compounds that can be analyzed by atmospheric pressure ionization (API) techniques for less polar molecules, which are difficult to analyze by ESI or APCI. They tested a group of compounds with different polarities, and compared the results with those obtained with the APCI source. APPI proved to be more sensitive to all test compounds, although it gives a higher signal for high-AP compounds that formed protonated molecules, than for compounds with weak non-polar PAs that form molecular ions. In negative mode, the deprotonated molecules were also observed. Bruins et al. Have used a dopant to increase the ionization efficiency of the analytes, because at atmospheric pressure, the photons easily lose their energy by collisions with instrument surfaces and particles in the gas phase, which decreases the efficiency ionization. By using a large amount of easily ionizable substance that has lower ion energy than the photon energy, the charge can be efficiently transferred to analytes and the ionization efficiency can be improved  . ]. The idea of using a dopant to increase the ionization efficiency was previously implemented in the framework of PI-IMS, where acetone, benzene, toluene and xylene  were applied. successfully as doping agents. The use of benzene has also been reported as a dopant for APCI, where it has improved the signaling of molecular ions formed from weak PA analytes by charge exchange. Another source of APPI ions has been described by Syage et al , which uses the same principle of operation as that of Bruins (Figure 2). The source described by Syage et al. is orthogonal . It is similar to the source of Agilent Technologies APCI, except that the corona discharge needle is replaced by a krypton discharge lamp emitting 10 eV photons. Unlike the Bruins et al APPI source, the Syagen source can achieve significant ionization without the presence of dopants, which is probably due to greater photon emission . Due to a light bulb, direct photoionization of the analytes can be obtained. Of course, the sensitivity is greatly improved when a dopant is used in the Syage source.
Figure 2. Diagram of the APPI source designed by Syage et al. .
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