Atmospheric Pressure Photoionization APPI

Most early reports of PI detection for LC were simply adaptations of the PI GC detector, i.e., simple detection only, with no complementary MS information. The only exception (Revel skii 1991) involved some experiments that did not explore direct LC-MS coupling, but rather its feasibility in which vaporized samples were transported to an atmospheric pressure photoionization (APPI) source interfaced to a quadrupole MS. Encouraging results were obtained with the exception that, when abundant solvent vapor (water or methanol) was added, a significant decrease in the APPI response for the model analytes was observed. This phenomenon is stiU important for modem LC-APPI, as discussed below. 

The recent activity in development of practical LC-APPI-MS can be attributed to a detailed study (Robb 

XIC of -MRM (2 pairs) tor 474.3/416.3 amu from APCI- infA-injlS-20ul 

This work has initiated a large amount of activity in APPI development and applications in a relatively short 

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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. 

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

ESI operating in the negative ion (Nl) mode has been the interface most widely used for the analysis of anionic PFCs. In addition, ESI has also been optimized for the determination of neutral compounds, such as the sulphonamides PFOSA, Et-PFOSA, and t-Bu-PFOS. The use of atmospheric pressure photoionization (APPI) has been explored by Takino and collaborators [88]. The authors found the main advantage of this technology to be the absence of matrix effects, but the limits of detection were considerably higher than those obtained by LC-ESTMS/MS. 

The real breakthrough in LC/MS development was achieved with the broad introduction in the 1990s of atmospheric pressure ionization (API) techniques, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), which enable the analysis of a wide variety of molecular species. The spectrum of available API techniques has been amended meanwhile by the introduction of sonic spray ionization (SSI) and atmospheric pressure photoionization (APPI). 

Atmospheric pressure photoionization (APPI) was recently introduced in the world of atmospheric pressure ionization techniques to analyze nonpolar molecules that are not efficiently ionized either by ESI or by APCI. Photoionization (PI) was already exploited some 30 years ago as a detection method for GC and LC, but only in recent times it has been used as an ionization method for mass spectrometry [52],... 

APPI Atmospheric pressure photoionization (APPI) is an ionization source similar to APCI but the corona discharge needle is replaced with an irradiation source (e.g., krypton lamp). In comparison to ESI and APCI, APPI can be used to efficiently ionize broad classes of nonpolar compounds. In the bioanalytical tool box, APPI is an important complement to ESI and APCI (Hanold et al., 2004 Syage et al., 2004 Cai et al., 2005 Hsieh, 2005). 

Different methods are used to tackle these problems [10-13], Some of these coupling methods, such as moving-belt coupling or the particle beam (PB) interface, are based on the selective vaporization of the elution solvent before it enters the spectrometer source. Other methods such as direct liquid introduction (DLI) [14] or continuous flow FAB (CF-FAB) rely on reducing the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly pumped into the source. In order to achieve this it must be reduced to one-twentieth of the value calculated above, that is 5 pi min. These flows are obtained from HPLC capillary columns or from a flow split at the outlet of classical HPLC columns. Finally, a series of HPLC/MS coupling methods such as thermospray (TSP), electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) can tolerate flow rates of about 1 ml min 1 without requiring a flow split. Introducing the eluent entirely into the interface increases the detection sensitivity of these methods. ESI can accept flow rates from 10 nl min-1 levels to... 

Earlier methods of ionization applied to carotenoids, including electron impact (El), chemical ionization (Cl), a particle beam interface with El or Cl, and continuous-flow fast atom bombardment (CF-FAB), have been comprehensively reviewed elsewhere (van Breemen, 1996, 1997 Pajkovic and van Breemen, 2005). These techniques have generally been replaced by softer ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), and more recently atmospheric pressure photoionization (APPI). It should be noted that ESI, APCI, and APPI can be used as ionization methods with a direct infusion of an analyte in solution (i.e. not interfaced with an HPLC system), or as the interface between the HPEC and the MS. In contrast, matrix-assisted laser desorption ionization (MALDI) cannot be used directly with HPEC. 

There are other LC/MS interfaces that are less commonly used than ESI and APCI, but are often employed by researchers for analysis of nonpolar or neutral compounds, including particle beam and atmospheric pressure photoionization (APPI). 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

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. 

The commercial softwares, initially developed by instrament manufacturers for open-access operation, were adapted to enable unattended data acquisition and automated data processing for large series of samples from an autosampler supporting the 96-well microtitre plate format, which is the sample format of choice in combinatorial synthesis. Initially, mainly Gilson 215 or 233 XL autosamplers were used, but other systems have become available from other instrument manufacturers. The complete system is under control of the MS data system. It consists of a 96-well-plate autosampler, an LC pumping system, eventually a UV-photodiode-array detector (DAD) in series and/or evaporative hght scattering (ELSD) detector in parallel, and the mass spectrometer eqnipped with ESI, APCI, and/or atmospheric-pressure photoionization (APPI). 

Ranha et al. [5] compared the ionization efficiency for flavonoids with ESI, APCl, and atmospheric-pressure photoionization (APPI) with nine different mobile-phase compositions in both positive-ion and negative-ion mode. The mobile-phase composition can have distinct influence on the response. Best response was achieved with 0.4% formic acid (pH 2.3) for positive-ion ESI and APCl, with 10 mmol/1 ammonium acetate adjusted to pH 4.0 for negative-ion ESI and APCl, and with 5 mmol/1 ammonium acetate in APPI. 

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)). 

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