Ionization region

The detection technique can also have an effect upon the angle- and velocity-dependent intensities. Cross sections refer to fluxes of molecules into a given range of velocities and angles. The connnonly employed teclmique of mass spectrometric detection provides a measure of the density in the ionization region. Since density and flux are related by the velocity, we must include a factor of 1/v hr making the transfonnation indicated in equation (B2.3.10) from the CM cross sections to tire measured laboratory intensities. 

A flow of liquid, for example from high-performance liquid chromatography (HPLC), is treated in such a way that most of the solvent evaporates to leave solute molecules that pass into an ionization region (ion source). 

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

Only the positively charged species are accelerated out of the ionization region neutral radicals—e.g., CULE in Equation 2, and molecules— e.g.y ME in Equation 3, produced by fragmentation and rearrangement, and un-ionized sample are pumped away. 

As voltage is increased in the ionization region (Region II), there is no appreciable increase in the pulse height. The field strength is more than adequate to ensure collection of all ions produced however, it is insufficient to cause any increase in ion pairs due to gas amplification. This region is called the ionization chamber region. 

When a single gamma ray interacts with the gas in the chamber, it produces a rapidly moving electron which produces secondary electrons. About 10,000 electrons may be formed depending on the gas used in the chamber. The applied voltage can be increased until the amount of recombination is very low. However, further increases do not appreciably increase the number of electrons collected. This region in which all 10,000 electrons are collected is the ionization region. 

The ionization chamber is a detector that operates in the ionization region. 

The experimental data obtained at the same time (Fig. 8.3) show similar electron density distribution, except for the preformed plasma due to the local early gas breakdown induced by the nanosecond ASE pre-pulse (visible in the left part of the ionized region). The investigation was completed with imaging and spectroscopy of the transmitted laser pulse after the propagation in the gas. 

In summary, the distinctiveness of this soft ionization technique is that source optimization can be achieved through an independent adaptation of the ionization region, the electrical field gradient, and the gas flow. It is well suited for aromatic hydrocarbons that are selectively ionized, including nonpolar species, such as PAH, yielding to the lowest detection limits by orders of magnitude, as compared to all other API methods. 

Figure 2.33 Number of ions produced per cm free path length and per mmHg pressure in the ionization region of an electron ionization source as a function of electron energy. The ionization efficiency curves show a plateau between 50 and 80eV. (H. Kienitz (ed.), Massenspektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)...

The interactions in the ionized region of the monolayer were obtained from the variation of k8 with pH. For a given film (constant m)... 

---