Atmospheric pressure ionization interfaces

Sakairi, M. and Kato, Y., Multi-atmospheric pressure ionization interface for liquid chromatography-mass spectrometry, ]. Chromatogr. A. 794, 391, 1998. 

Fig. 1.7 Single stage pumping atmospheric pressure ionization interface with curtain gas. The size of the orifice is ca. 100 pm, qO acts as a focusing quadrupole and the nitrogen curtain gas prevents neutrai molecules being introduced into the mass spectrometer. T = Temperature of the cryoshells (in Keivin) p = pressure.

Today, the two most common LC/MS interfaces are atmospheric pressure ionization interfaces, electrospray (ESI) and ion spray (ISI). Electrospray (Fig. 15.8) and its subtype, nanospray, are recommended for use with proteins and highly polar or ionized compounds. They are very soft ionization, concentration-dependent techniques that result in very little fragmentation and often produce multiply charged molecular ions. 

Several LC-MS interfaces have been developed since 1974, when Arpino et al. [90] described the first attempt to couple the LC system to the mass spectrometer. Some of them were commercialized [91], but it was the development of atmospheric pressure ionization interfaces (API) which actually lead to the great expansion of LC-MS applications. 

Matrix effects (ME), caused by co-eluting endogenous and exogenous matrix components, significantly affect the efficiency and reproducibility of the ionization process of target analytes. This phenomenon represents a major concern for LC-MS bioana-lytical method precision, accuracy, sensitivity, and robustness. Amongst the atmospheric pressure ionization interfaces used in LC-MS systems, ESI source is more prone to signal alteration (ion suppression or enhancement) due to matrix. Therefore, careful evaluation and correction for ME must be considered particularly with ESI-MS. 

Figure 3 Schematic drawing showing the immunoaffinity chromatography/reverse-phase chromatography/atmospheric pressure ionization mass spectrometry experiment. System components are represented as follows P, HPLC pump IAC, immunoaffinity column HPLC, analytical column MS, mass spectrometer with atmospheric pressure ionization interface DS, data system and W, waste. Switching valves are represented as .

Atmospheric pressure chemical ionization uses an atmospheric pressure ionization interface. The interface is similar to that used for ESI, but a corona discharge is used to ionize the analyte in an atmospheric pressure region. (Note This is different than the chemical ionization source described in Chapter 20 for GC-MS.) The gas-phase ionization of less polar analytes is more efficient than with ESI, but the mass range is hmited to about 2000 daltons. ESI and APCI are complementary. APCI has largely replaced thermal ionization as an interface in commercial instraments. 

Mohamed R, Hammel YA, LeBreton MH, Tabet JC, Jullien L, Guy PA, Evaluation of atmospheric pressure ionization interfaces for quantitative measurement of sulfonamides in honey using isotope dilution liquid chromatography coupled with tandem mass spectrometry techniques, J. Chromatogr. A 2007 1160(l-2) 194-205. 

Hewlett-Packard, Development of a new Orthogonal Geometry Atmospheric Pressure Ionization Interface for LC-MS, 1998... 

Figure 1 Schematic diagram of an atmospheric-pressure ionization interface.

As has been mentioned, nowadays MS/MS detection is widely used in the analysis of flavonoids, not only for its good sensitivity but also because of the identification and structural characterization capacity that these detectors have [66-69]. In almost all applications, atmospheric pressure ionization interfaces are used, such as APCI (atmospheric pressure chemical ionization) and electrospray ionization (ESI) [48]. 

Mordehai, a. V Hopfgartner, G. Huggins, T. G. Henion, j. D. Atmospheric-pressure ionization interface for a bench-top quadrupole ion trap. Rapid Commun. Mass Spectrom 1992, 6, 508-516. 

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