Experimental atmospheric-pressure chemical

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

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. 

Ionization at atmospheric pressure was for many years considered to be an experimental curiosity despite excellent early work (Homing 1974, 1974a Carroll 1975). However, API techniques nowadays dominate on-line LC-MS work. There are currently three kinds of API sources in common use for trace quantitative analyses, namely, atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI) and electrospray ionization (ESI). In addition, atmospheric pressure MALDI has been investigated but not as an on-line approach to quantitative LC-MS. Since the highly practical discussions of Chapters 9 and 10 will be concerned mainly with LC-MS analyses (since GC-MS is now almost routine), an extended discussion of API methods is appropriate here. 

Gas-phase Kinetics. A better appreciation of the experiments to be discussed later will be obtained after a review of some experimental aspects of the transient method. Here we deal with experiments at atmospheric pressure. A flow sheet for kinetic measurements is given in Fig. 1, a descendant of that first given by Bennett et al. (15). Chemical analysis of the gases during transients is ideally done by a mass spectrometer, although Kobayashi and Kobayashi (4 ) used a number of gas chromatographs in order to get samples sufficiently frequently. 

The experimental study of heat produced or absorbed in chemical reactions is usually called thermochemistry. Such investigations are often best conducted by direct calorimetric measurements, but values for heat quantities and their time and temperature derivatives can also be obtained from other kinds of thermodynamic experiments. A heat quantity, Q, which is lost or gained in a process conducted under constant pressure, p (in chemistry and biology most frequently the atmospheric pressure), is defined as the enthalpy change, AH, accompanying the process. 

The surface properties of various phases corresponding to the same chemical formula can substantially differ from one crystallographic form to another. This is well known, and the information about the crystallographic form of the adsorbents is specified in many publications on adsorption, especially when the compound of interest forms more than one phase showing a sufficient stability at the experimental conditions. Since the present survey is devoted to adsorption studies performed at room temperatme and atmospheric pressure the phases existing only at very high pressures or at very high (or very low) temperatures are not considered. 

In spite of the advantages cited above, ion mobility spectrometers operating at atmospheric pressure have been used infrequently to obtain physical chemical data, kinetic and thermodynamic, in the study of ion/molecule chemistry. In this chapter, an overview is given on the type of information obtainable from ion mobility studies at atmospheric pressure and the variety of experimental methods employed in such studies. The data obtained under weU-defined conditions agree favorably with those from other more frequently used methods, for example (i) pulsed high pressure mass spectrometry (PHPMS), which is operated at well-defined temperatures but at pressures ca 200 times lower than IMS and (ii) FT-ICR and ion trap mass spectrometers, which are operated under vacuum. 

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