Photoionization atmospheric pressure

Photoionization has been considered, from the beginning of analytical MS, to be highly attractive it exhibits some theoretical advantages with respect to electron ionization, but also has some severe limitations (Morrison, 1986). 

In general, the energy transfer involved in ionization of atoms and molecules must be enough to excite one electron from a bond to an unquantized orbital. The ionization energy is just the lowest energy value required for the occurrence of this phenomenon. 

However, note that photoionization has been used since 1976 as a detection method in GC, proving that, when the sample density is high enough, good sensitivity can be achieved, together with the specificity related to the wavelength employed. 

In an APPI source, a series of different processes can be activated by photon irradiation. Calling ABC the analyte molecule, S the solvent, and G other gaseous species present in the source (N2,02, and H20 at trace level), the first step can be considered their photoexcitation  

At this stage, inside the source, a collection of excited and nonexcited species is present and a series of further processes can occur, as  

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 

In the DA-APPl mode, a large molar concentration of a dopant is infused into the ionization region. The dopant radical cations are first formed by absorption of photons  

The second mechanistic pathway is the solvent-mediated ionization, in which the dopant radical cation first ionizes the solvent molecule by a proton transfer reaction to produce an SH+ ion [reaction (2.29)], which in turn ionizes the analyte molecule by a second proton transfer reaction [reaction (2.30)]  

This alternative pathway will operate only when the PA of the solvent is greater than that of the dopant and the PA of the analyte is greater than that of the solvent. Toluene, being less toxic, is the dopant of choice for many applications. Its IE value is 8.83 eV. Acetone and anisole have also been used as dopants. Direct photoionization of conunon LC solvents is avoided when the 10-eV UV photon lamp is used as a photon source thus, the solvent background, common to many other modes of ionization, is absent, which helps improve detection limits. 

APPl can also be performed in the negative-ion mode [74]. In that case, phenol is used as a dopaut. In this mode, compounds with high gas-phase acidity are ionized as deprotonated ions by proton transfer reactions with O2 or [S — Hj ions, whereas compounds of positive electron affinity form negative molecnlar ions either by electron capture or by charge exchange with the phenoxide ion. 

More recently, atmospheric pressure photoionization (APPI) has been introduced [181] as a complement or alternative to APCI [176,182,183]. In APPI, aUV light source replaces the corona discharge-powered plasma, while the pneumatic sprayer and the heated vaporizer remain almost unaffected (Fig. 12.43) [184-186]. The most relevant modification is the use of a quartz tube to guide the hot vapor toward the sampling orifice. Quartz is required to transmit the UV light emitted by a noble gas discharge lamp. 

Note Modem instmments with atmospheric pressure ion sources are all con-stmcted as to permit easy exchange of spray heads for rapid switching between ESI, APCI, and APPI [184]. There is no interruption of instrument vacuum and mounting of spray heads takes just a minute, but thae is a 10-15-min delay for APCI or APPI vaporizers to fully heat up for operation or to sufficiently cool down before removal is possible without risk of injury ftomhot parts. 

HPLC-APCI analysis of a mixture of glucoconjugated compounds related to DIMBOA. Spectrum from peak 4 does not display the deprotonated molecular species. The molecular mass (3 87 Da) is deduced from the adducts. The sample is obtained from extracts of maize plants. Reproduced (modified) from Cambier V., Hance T., and de Hoffmann E., Phy-tochem. Anal., 10, 119-126,1999. 

Ionization energy of molecules frequently present in APPI sources (solvents, doping compounds, air components). 

However, the direct ionization of the analyte is generally characterized by a weak efficiency. This can be partially explained by the solvent property to absorb photons producing photoexcitation without ionization. This reduces the number of photons available for the direct ionization of the sample, thus reducing the ionization efficiency. Consequently, ionization using doping molecules has also been described. It has indeed been shown that dopant at relatively high concentrations in comparison with the sample allows generally an increase in the efficiency of ionization from 10 to 100 times. This indicates that the process is initiated by the photoionization of the dopant. The dopant must be photoionizable and able to act as intermediates to ionize the sample molecules. The most commonly used dopants are toluene and acetone. Thus, two distinct APPI sources have been described direct APPI and dopant APPI. 

The mass spectra obtained by the APPI source in the positive ion mode are characterized by the presence of two main types of ions of the molecular species that may coexist [82] the radical cation M + and the protonated molecule [M + H]+. In direct APPI, the reaction is the classical photoionization leading to the radical cation of the molecular species  

However, the often dominant presence of the protonated molecule suggests gas-phase ion-molecule reactions after the photoionization. The most likely reaction is the abstraction by the molecular ion of an hydrogen atom from a solvent molecule [83]  

Dopants (D) are photoionized first, followed by charge exchange with the analytes (M). The ionization of analytes can also be achieved by means of proton transfer processes  

Dopants are ionized first, followed by the ion-molecule reaction with solvent (S). Proto-nated solvent ions are generated, and subsequently they transfer protons to the analytes. 

In the beginning of the 1970s (i.e., before the development of ESI and APCI), the repertoire of available ion sources was quite narrow. The analysis of polar and thermal labile analytes by MS was problematic. In fact, at that time, sample vapors were normally generated by application of heat prior to ionization. However, this method was not suitable for thermally labile compounds since the heating rate was too low, which - in many cases - led to thermal decomposition. If the sample vaporization was faster than the decomposition rate, intact molecules could be transferred from the condensed phase to the gas phase 

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LC-MS-MS was also the method of choice for the analysis of UV filters in solid matrices. Both LC and UPLC have been applied in three out of the four methods available for the determination of UV filters in sludge. Separation was performed on C8 and C18 LC-chromatographic columns (Zorbax, Eclipse, Vydac, and Purosphere) using binary gradient elution of mobile phases consisting of water/ methanol or water/acetonitrile. MS-MS detection was performed in SRM with ESI and atmospheric pressure photoionization (APPI) in both positive and negative modes. For each compound, two characteristics transitions were monitored. In addition to MS and MS-MS, diode array detection (DAD) was occasionally applied to the determination of OT. Spectra were recorded between 240 and 360 nm and discrete channels at 310 nm. 

The most important techniques belonging to this class are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and, more recently, atmospheric pressure photoionization (APPI). At present the latter does not have applications in cultural heritage, so it will be not described here. 

Rauha JP, Vuorela H and Kostiainen R. 2001. Effect of eluent on the ionization efficiency of flavonoids by ion spray, atmospheric pressure photoionization mass spectrometry. J Mass Spectrom 36 1269-1280. 

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... 

Atmospheric pressure photoionization APPI Photoionization Nonvolatile molecular ions LC-MS Nonpolar compounds... 

D. R. Robb, T. R. Covey, and A. P. Bruins. Atmospheric Pressure Photoionization An Ionization Method for Liquid Chromatography-Mass Spectrometry. Anal. Chem., 72(2000) 3653-3659. 

T. J. Kauppila, T. Kuuranne, E. C. Meurer, M. N. Eberlin, T. Kotiaho, and R. Kostiainen. Atmospheric Pressure Photoionization Mass Spectrometry Ionization Mechanism and the Effect of Solvent on the Ionization of Naphthalenes. Anal. Chem., 74(2002) 5470-5479. 

A. Raffaelli and A. Saba. Atmospheric Pressure Photoionization Mass Spectrometry. Mass Spectrom. Rev., 22(2003) 318-331. 

Yang, C. Henion, J.D. Atmospheric Pressure Photoionization Liquid Chromatographic-Mass Spectrometric Determination of Idoxifene and Its Metabolites in Human Plasma. J. Chromatogr. A 2002, 970,155-165. 

The two most common LG/MS interfaces used for routine quantitative analyses are APCI and APT electrospray. The principles of these techniques in direct infusion analyses have been described earlier (see Sections 3.3 and 3.4). As APTelectrospray has a broader application profile, its use is more widespread than APCI. Other configurations including El, atmospheric pressure photoionization (APPl), and thermospray interfaces with liquid chromatographs are available but are less commonly used for high throughput or routine analysis. 

Robb, D. B., Covey, T. R., and Bruins, A. P. (2000). Atmospheric pressure photoionization an ionization method for liquid chromatography-mass spectrometry. Anal. Chem. 72, 3653 — 3659. 

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