From the various sources of excitation mentioned in table 1, the electrical glow discharges produced under low pressure, and in particular the planar cathode and hollow cathode discharges mentioned previously, are less subjected to the processes discussed in the foregoing, and are also practically free of self-absorption. The processes occurring in the excitation source become even more complex when the energy generated by an electrical field is associated with the thermal energy. Self-absorption is also a phenomenon often associated with thermal excitation and is responsible for loss of sensitivity and non-linearity between the emission intensity and actual sample concentration. These interferences are associated to a large extent with those excitation sources in which the production and excitation of particles result from thermal energy, where the thermal characteristics of each chemical species such as melting, boiling, and vaporization temperatures, vapor pressure, dissociation, are specific for every chemical species and play a determining role. Furthermore, interferences due to matrix effects, interelement actions, chemical reactions, and selective energy transfers should be as small as possible.
The excitation source should excite all the chemical species of interest with high sensitivity and stability. The basic conditions required from an excitation source are: capability to be supplied in a controlled manner with the analytical sample in solid, liquid, or gaseous state for both electrically conductive and non-conductivc materials. Although in this case a certain amount of heat is generated as a result of ion bombardment at the cathode, this thermal energy is only incidental to the process and is not necessary for the production and maintenance of the discharge, for the generation of free particles, and for the subsequent excitation processes which occur in these sources. The other extreme, where the production of free particles and their excitation results from the energy generated in electric fields, is illustrated by the low pressure glow discharges, namely the planar and hollow cathode discharge.
An extreme case in this regard is illustrated by the dc arc where the thermal energy is the determining parameter. When electrical discharges are used for the same purpose, the production of free particles and their excitation often result from a combined effect of the energy developed in the electric field and thermal energy. The production of free particles and their excitation through the use of high temperature furnaces or combustion flames is a purely thermal phenomenon. A collection of references to works on low pressure glow discharges, containing 690 entries, concludes this work. The paper will conclude with a discussion of possible future developments of low pressure glow discharges. The use of these discharges will be illustrated with examples taken from the literature and from the measurements performed at NBS. The techniques used for the introduction of various conductive and nonconductive materials into the discharge will be discussed. A description of the hollow cathode developed at the National Bureau of Standards (NBS) will follow. These will he followed by a description of the conventional instrumentation developed for analytical purposes using the hollow cathode and flat discharge.
Some of the fundamental properties of the glow discharge and sputtering phenomena will be discussed, including the relation between the geometry of the discharge, and the nature and pressure of sustaining gas, and current, on the emission characteristics of the discharges. The analytical sample is supplied to the discharge through a sputtering mechanism which provides a stable and non-selective source of particles. Both discharges have similar voltage-current characteristics which are responsible for their radiation stability. The low pressure glow discharges considered in this paper are the hollow cathode (Paschen), and the flat cathode (Grimm).
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