Atmospheric pressure chemical principles

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

Figure 14.3 Principle of atmospheric pressure chemical ionization. The dissolved analyte is sprayed through a capillary. Evaporation of the solvent is supported by a heated gas stream. Within the source, a plasma is formed by a Corona discharge needle, which creates the charged reagent gas (here HgO+j. The ionization of the analyte (M) is performed by the transfer of the charge (proton) via ion-molecule reactions.

Three popular ionization techniques are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption (MALDI). Electrospray is the most widely used ionization technique when performing LC-MS, and has proved to be a most versatile tool for soft ionization [72] of a large variety of analytes such as them described in paper I. Figure 6 shows the principle of the ESI. 

Atmospheric pressure chemical ionization (Bruins, 1991) was developed starting from the assumption that the yield of a gas-phase reaction depends not only on the partial pressure of the two reactants, but also on the total pressure of the reaction environment. For this reason, the passage from the operative pressure of 0.1-1 Torr, present inside a classical Cl source, to atmospheric pressure would, in principle, lead to a relevant increase in ion production, which consequently leads to a relevant sensitivity increase. Furthermore, the presence of air at atmospheric pressure can play a positive role in promoting ionization processes. 

Finally, the Appendix shows schematic representations of the principle of operation of some of the ionization processes presented. " Among these are the theory of ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Also shown are schematics of the ionizers and typical experimental conditions for the APCI and APPI sources as well as that of an ESI-APCl mixed source. It should be noted that these schematics are for sources that are interfaced with a mass spectrometer but are similar to IMS interfaces. 

The technique of atmospheric-pressure chemical ionization (APCl) also serves to analyze LC effluents by mass spectrometry. It is applicable to relatively less polar and thermally stable compounds with an upper mass range of 1500 Da. The principle of ionization in APCI is identical to that described for conventional Cl, with the difference that APCI is performed at attnospheric pressure, at which many more ion-molecule collisions can occur between the sample molecules and reagent ions. Therefore, the ionization efficiency and detection sensitive are improved significantly. 

Figure 2.15. Block diagram of atmospheric pressure chemical ionization source. (Reproduced from C. Dass, Principles and Practice of Biological Mass Spectrometry, Wiley-Inter-science, 2001.)...

In principle however, this method should be portable to other mass spectrometer systems capable of chemical ionization and tandem mass spectrometry. An [M+54] ion was reported using atmospheric pressure chemical ionization (APCI) with a predominantly acetonitrile solvent while analyzing extremely long-chain polyunsaturated fatty acids (16). Such an observation is promising for the use of this method for the analysis of low- or nonvolatile lipids, such as triglycerides and phospholipids. 

Moini, M. in Gross, M. L. and Caprioli, R. M. (eds) (2007) The Encyclopedia of Mass Spectrometry, Atmospheric Pressure Chemical Ionization Principles, Instrumentation, and Applications,... 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

The CAD detection principle has some commonality with atmospheric-pressure chemical ionization (APCI) in mass spectrometry. However, CAD... 

Le Chatelier s principle also predicts that the yield of ammonia is greater at higher pressures. High-pressure plants are expensive to huild and maintain, however. In fact, the first industrial plant that manufactured ammonia had its reaction vessel blow up. A German chemical engineer, Carl Bosch, solved this problem by designing a double-walled steel vessel that could operate at several hundred times atmospheric pressure. Modern plants operate at pressures in the range of 20 200 kPa to 30 400 kPa. 

The mathematical formulation of these principles gives a closed system of equations governing the model. Without going into detail we have to mention that, in accordance of our foregoing discussion, small scale processes also have to be included in the model. In the governing equations the following parameters can be found horizontal and vertical components of the wind (also the random turbulent components after parameterization), the atmospheric pressure, temperature and density as well as the mass fraction of different chemical substances. 

Munson and Field reported in 1966 on a technique of ionizing molecules by gas phase ion-molecule reactions, which they called chemical ionization (Cl). In this way, break-up of the molecules can be greatly reduced or even avoided. Thus, measured ion currents can be correlated with the densities of the respective parent neutral compounds, allowing for on-line monitoring of rather complex gas mixtures. The fundamental principles of gas phase ion chemistry on which Cl is based, as well as the instrumentation for Cl, have been reviewed in great detail by Harrison." The wide variety of Cl methods that has been developed includes Medium Pressure Mass Spectrometry, Fourier Transform Mass Spectrometry, Quadrupole Ion Trap Mass Spectrometry, Pulsed Positive Ion-Negative Ion Chemical Ionization, and Atmospheric Pressure Ionization Mass Spectrometry (API-MS). Of these, API-MS has developed into a very reliable and widely used technique for analysis of VOCs in flavor release studies and human breath. A variety of API-MS applications in these fields of research has been described in a recent volume by Roberts and Taylor. ... 

The reactor in which the chemical coating processes actually take place is the essential part of the system and must be designed according to the specific physical and chemical process parameters. To coat glass by atmospheric-pressure CVD, generally low temperature reactors are used which can be classified according to their gas flow characteristics and operation principles into four basic types [69] ... 

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