APCI or APPI

API techniques provide efficient ionisation for a variety of molecules including polar, labile and high mass molecules. They are soft ionisation techniques useful 

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Liquid chromatography, coupled to the different ionization sources, is generally the technique most used to characterize the phenolic profile in fruit and vegetable products. With regard to the source ionization, it seems that ESI is used more frequently than other sources, such as APCI or APPI. Another important aspect of this technique is the ionization of phenolic compounds. Negative ionization seems to be more suitable... 

It is acknowledged that ESI is more susceptible to matrix effects compared with APCI or APPI, and that ionization in the negative mode is more selective than the positive mode [55],... 

The type of ionization method is determined by the method of sample introduction. The most useful sources with GC are El and Cl, and with LC, ESI (or another API method, e.g., APCI or APPI). MALDI is a stand-alone ionization method that is best suited to peptides and proteins. Because MALDI is an off-line technique, samples can be investigated multiple times as opposed to LC, where peaks cannot be revisited without reinjecting the sample. Although it is highly desirable to have multiple ionization sources, the budgets for dedicated instruments may not allow such luxuries. 

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. 

Coupling of liquid chromatography to mass spectrometry has not only led to a wide variety of interfaces, it also fostered the development of new ionization methods (Chap. 12) [8,9,12-15,84]. ESI, APCI, or APPI are suitable for LC-MS, then-selection depending on sample properties like molecular mass and polarity (Chap. 12.7). 

Sterols caimot be analyzed by ESI-MS without derivatization as they are not readily ionized [115]. Sandhoff et al. have used chemical sulfatation to achieve high-sensitivity detection of cholesterol [116], Cholesterol has also been deriva-tized with dimethylglycine, MDMABS [117], or ferrocenecarbamate [118], Notably, derivatization can be avoided by using APCI or APPI, which have been applied for the analysis of cholesterol and other sterols [119-122] or oxidized cholesterol [123]. 

Electrospray ionization [21] is one of the most widely utilized ionization techniques employed today for the analysis of thermally fragile molecules. As such, it has assumed an important role in the analysis of biologically important molecules. ESI is a desorption ionization technique. This means that ions are formed before or during the transition from the liquid phase and need not be volatilized in advance of the ionization event (as is the case for El, Cl, etc.). Like APCI and APPI, ESI occurs at atmospheric pressure outside the vacuum chamber of the mass spectrometer (Fig. 11.5). A solution of the analyte passes through... 

The matrix compound is typically mixed together in an aqueous/ organic solution with the analyte, such that the relative concentration of matrix to analyte is on the order of 5000 or 10,000 to 1. The solution is applied to a surface that will be irradiated by the laser beam and the solvent is allowed to evaporate, leaving a solid, crystalline deposit of matrix and analyte. Many of the original applications described instruments in which the surface containing the dried matrix/ analyte sample was introduced into the vacuum housing of a mass spectrometer source housing for irradiation. However, recently it has been demonstrated that MALDI can be successfully carried out at atmospheric pressure (outside the vacuum chamber of the mass spectrometer), much in the same way as the ESI, APCI and APPI techniques. 

In atmospheric pressure ionization sources (API) the ions are first formed at atmospheric pressure and then transferred into the vacuum. In addition, some API sources are capable of ionizing neutral molecules in solution or in the gas phase prior to ion transfer to the mass spectrometer. Because no liquid is introduced into the mass spectrometer these sources are particularly attractive for the coupling of liquid chromatography with mass spectrometry. Pneumatically assisted electrospray (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) are the most widely used techniques. 

Figure 2.4 Comparison of (a) sensitivity, (b) variability, (c) selectivity, and (d) pricing between various chemical and immunological analyses for the presence of PPCPs in the environment. FID = flame ionization detector and EC = electrochemical detection. Note that GC-MS-MS can have mass detectors such as triple quadrupole and ion trap with ionization from El = electron ionization or Cl = chemical ionization, whereas LC-MS-MS with ionization from ESI = electrospray ionization, APCI = atmospheric pressure chemical ionization, or APPI = atmospheric pressure photoionization. (Adapted from Ingerslev and HaUing-Sprensen, 2003.)...

Thus, besides the direct photoionization, the analytes in positive APPI mode are ionized either by charge exchange or by proton transfer. The direct ionization and the charge exchange processes allow the ionization of non-polar compounds. This is not possible either with APCI or ESI. 

Compared with APCI, APPI is more sensitive to the experimental conditions. Properties of solvents, additives, dopants or buffer components can strongly influence the selectivity or sensitivity of the detection of analytes. Nevertheless, this technique allows the ionization of compounds not detectable in APCI or ESI, mainly non-polar compounds. For these last compounds, APPI is a valuable alternative. Thus, APPI is a complementary technique to APCI and ESI. However, for a given substance it remains difficult to predict which ionization source (APPI, APCI or ESI) will give the best results. Only preliminary tests will allow the choice of the best ionization source. APPI appears to be efficient for some compound classes such as flavonoids, steroids, drugs and their metabolites, pesticides, polyaromatic hydrocarbons, etc. [85],... 

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. 

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). 

The absorption of a photon by the molecule and the ejection of an electron forms a radical cation. Better sensitivities have been reported with the addition of dopants such as toluene or acetone. The mechanism of ionization is not fully understood, but two different mechanisms can occur (i) dopant radical cations react with the analyte by charge transfer or (ii) the dopant radical cation can ionize the solvent molecules by proton transfer, which can then ionize the analyte. APPI can also be performed in the negative mode. As APCI, APPI can handle a large range of analytes. The performanee of APPI is dependant on the flow rate, and better sensitivities, compared to APCI, have been reported at lower flow rates. Because APCI and APPI are gas ionization processes, it appears that compared to ESI they are less or differently affected by matrix effects (Robb and Blades, 2008). APPI is attractive for a large variety of neutral analytes such as steroids (Cai et ah, 2005). 

Coupling an API source (APCI, ESI or APPI) to a mass spectrometer, so as to maximize the analyte signal while disposing of the large amount of vaporized LC mobile phase, is an essential and important component of any API-MS system. The following discussion is based upon two excellent early reviews (Bruins 1991 Niessen 1995)... 

Another ionization technique, which seems closely related to TSI and ESI, is atmospheric-pressure chemical ionization (APCI). In APCI, the solvent stream, e.g., the effluent from an LC column, is pneumatically nebuhzed into a heated vaporizer zone, where (almost) complete evaporation of the aerosol droplets is achieved [71-73]. Analyte ionization is initiated by electrons from a downstream corona discharge needle. The electrons act as primary source of ionization of the solvent or mobile-phase constituents, which in turn by gas-phase ion-molecule reactions in the API source ionize the analyte molecules, mostly by proton-transfer reactions, i.e., formation of [M-hH]+ in positive-ion and [M-H] in negative-ion mode. There are also some results, indicating the Na -cationization can take place under APCI conditions. Atmospheric-pressure photoionization (APPI) is an ionization technique closely related to APCI. In APPI, the analyte ionization is initiated by light from a vacuum-ultraviolet lamp, e.g., a Kr-lamp, instead of by means of a corona discharge. Next to direct photoionization of the analytes, gas-phase ion-molecular reactions greatly contribute to the ionization in APPI [74,75]. 

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