Direct APPI

In direct APPI, the ionization process is activated by the photons emitted by the source and takes place if the ionization energies of the molecules are below 10 eV. This is the case of most analytes however, the ionization energy of most HPLC solvents is higher. This implies that the sample must be vaporized prior to detection. The molecular ion is first generated by an impact with photons ... 

Scheme of an APPI source. The sample in solution is introduced perpendicular to the axis of the analyser. The lamp is located in front of the entrance hole of the analyser. This source has been designed initially for direct APPI (see text). 

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 ... 

This is the direct-APPI approach, promoted by the group of Syage [66], In the observations and experimental setup of the group of Bruins [61], the direct-APPI process is not sufficiently efficient. Therefore, an easily ionizable compound, the dopant D, is added to the mobile phase or to the nebulizing gas to enhance the response. Toluene [61] or anisole [68] are frequently used as dopant. With a dopant, the APPI takes place via a charge-exchange reaction between the dopant molecular ion and the analyte molecule ... 

Whereas in both direct-APPI and dopant-APPI, an analyte molecular ion NT would be expected, rather than a protonated molecule [M+H] is observed, for quite many analytes. This is assumed to be due to interaction with the mobile-phase constituents. In the direct-APPI approach, the protonated molecule is formed due to a reaction of the analyte molecular ion with the solvent S ... 

Lau K T, Bar-Chaim N, Ury I and Yariv A 1983 Direct amplitude modulation of semiconductor GaAs lasers up to X-band frequencies Appi. Phys. Lett. 43 11... 

Barone M E and Graves D B 1995 Molecular dynamics simulations of direct reactive ion etching of silicon by fluorine and chlorine J. Appi. Phys. 78 6604-15... 

This chapter is mainly limited to direct API-MS. For LC-API-MS, see Section 73.3.2. APCI, ESI and APPI complement each other in chemical analyses. API-MS was reviewed repeatedly [124,125], and has been the subject of a recent monograph [126]. 

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... 

APPI is a relatively recent development compared with the other techniques. Here, ionisation is achieved photochemically, either directly or mediated by a dopant such as acetone added to the eluent. Both even- and the less stable odd-electron ions (e.g. M ) may be formed. At the time of writing, the mechanisms involved and scope of the technique are still not fully understood. What is apparent is that it provides a complementary technique to ESI and APCI. 

Since the neutral molecules of steroid hormones and metabolites are not easily ionized under either APCP or ESI+/ modes, LC-MS/MS is less sensitive when used directly in either APCP or ESI+/ modes, with LOQs at ng/mL level as shown in Table 4 [20, 21, 52], It has been observed that estrone, 16a-hydroxyestrone, 2-methoxyestrone, 4-methoxyestrone, and 2-hydroxy-3-methoxyestrone are sensitive to APCI+ mode, while 2-hydroxyestrone and 4-hydroxyestrone are sensitive to APCI- mode, and even more sensitive to ESI mode [29], Estrone, estradiol, estradiol, and estriol are sensitive to ESI" mode, and testosterone is sensitive to ESP mode [76, 77], Similarly, estrone and estradiol are sensitive to APPI" mode, and testosterone is sensitive to APPI+ mode with LOQs in a range of 1.5-10 pg/mL [22], which are comparable with those steroid hormones and metabolites derivatized with hydroxylamine or dansyl chloride, and detected under ESP mode [2, 8],... 

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. 

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. 

The answer may lie in the fact that methionine is very sensitive to oxidation, and apPI which has been oxidized at this centre (a,-PI J is both a much less potent (Kj = 1.5 x 10 M) and slower acting (kassoc = 3.1 x 10 M" s ) inhibitor of HLE [45, 46], This decrease in activity can be rationalized by the fact that oxidation of the methionine sulphur to a sulphoxide increases the size and polarity of the P,-substituent. The increase in size may directly interfere with fitting this residue into the Spsubsite of HLE, and the change in polarity may cause a reduction in the net binding energy gained by the transition from a purely aqueous solution to a bound environment. 

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 ionization (API) techniques are the most commonly used techniques in DM studies. Since the ionization occurs at the atmospheric pressure, API can be characterized as a soft ionization technique. There are three commonly used API sources [59] that can directly couple LC with MS electrospray ionization (ESI) [60], atmosphere pressure chemical ionization (APCI) [61-63], and atmospheric pressure photoionization (APPI) [64,65], The properties of the compound, such as its structure, polarity, and molecular weight, lead to the selection of one of these ionization techniques for sample analysis. 

Electrospray is surely the ionization method most widely employed for the liquid chromatography (LC)-MS coupling (Cappiello, 2007). The possibility of performing ionization at atmospheric pressure [also obtained in the case of atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI), allows the direct analysis of analyte solutions. However, some problems arise from the intrinsically different operative conditions of the two analytical methods. First, there are the high-vacuum conditions that must be present at the mass analyzer level. Second, the mass spectrometers generally exhibit a low tolerance for the nonvolatile mobile-phase components, usually employed in LC conditions to achieve high chromatographic resolution. 

Viscous, amber-colored liquid. Viscosity 69 poises at 25 (about that of 95% glycerol). Viscosity reduced considerably by heating to 120-140"F when it may be sprayed directly. dB 1.59-1.63. n f 1,56-1-57. Insol in water miscible with aliphatic and aromatic hydrocarbon solvents, including deodorized kerosene. Loses its chlorine in presence of alkaline reagents, and should not be formulated with any solvent, carrier, diluent or emulsifier, which has an alkaline reaction. LDM i.p. in male rats 343 mg/kg, R. B. Harbison, Toxicol. Appi. Pharmacol. 32, 443 (1975). 

Jarvis S P, Durig U, Lantz M A, Yamada H and Tokumoto H 1998 Feedback stabilized force-sensors a gateway to the direct measurement of interaction potentials Appi. Phys. A 66 S211... 

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