Ionizing discharge, effect

Additional ionization is effected by including radioactive substances or plasma or glow discharges in the evaporation chamber or by electrical charging of the nebulizer. Such techniques are also discussed in Chapters 8 and 11. 

Powder particles may be charged by the contact electrification mechanism, when they acquire their charge on contact with the electrodes, or by the ion absorption mechanism, when air is ionized under the gas discharge effect followed by charge transfer from ions to powder particles. 

The detection limits of GDMS are in the ppb range, sometimes lower [88,126]. Provided that a large amount of sample material is available, the sensitivity can be enhanced by using intense discharges effecting higher sputter rates and ionization efficiencies [129]. It is recommended to clean the cell in between runs in ultratrace analysis due to possible memory effects by deposition of sample material onto the cell walls [141]. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

The use of radioactive ionization sources in areas subject to explosion or fire is undesirable because of the potential for area contamination with radioactive material which could be disseminated in the event of an explosion or fire. With proper precautions, however, electrical ionizing systems can be safely and effectively utilized while processing electrostatically sensitive energetic materials. Ions are generated electrically by corona discharge... 

In region III, the discharge is maintained only by ionization in the gas phase without electron injection from the cathode. Because of the inertial effect of ions and electrons, only small part of charged particles in the gas phase can arrive on the electrode. Therefore, polymerization may be induced principally by diffused free radicals and/or ion-electron pairs ... 

A plasma is a hot, partially-ionized gas that effectively excites and ionizes atoms [366, 534, 535]. A glow discharge is low-pressure plasma maintained between two electrodes. It is particularly effective at sputtering and ionizing material from solid surfaces. 

In IC-MS systems, the core of the equipment is the interface. In fact, inside the interface evaporation of the liquid, ionization of neutral species to charged species and removal of a huge amount of mobile phase to keep the vacuum conditions required from the mass analyzer take place. Two main interfaces are used coupled to IC, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). In the ESI mode, ions are produced by evaporation of charged droplets obtained through spraying and an electrical field, whilst in the APCI mode the spray created by a pneumatic nebulizer is directed towards a heated region (400°C-550°C) in which desolvation and vaporization take place. The eluent vapors are ionized by the corona effect (the partial discharge... 

The effective cross section s for ionizing collisions depends on the type of gas. According to (2.25), the discharge current i is a function of the number particle density ng, as in a Penning gauge, and it can be used as a measure of the pressure in the range from 10 to 10 mbar. At lower pressures the measurements are not reproducible due to interferences from field emission effects. 

---