Spark Discharge: Application to Multielement Spectrochemical Analysis

John P. Walters

Science, 198, 787 (1977)

The sudden discharge of energy stored at high voltage in a capacitor is commonly called a spark discharge. If one observes such a discharge across a pair of separated electrodes, the phenomenon appears to consist of a single, somewhat irregular event. But such is not the case. Rather, an orderly collection of many interacting events comprises the discharge. The events may be controlled in many ways. Taken collectively, they constitute a process of energy dissipation which may be called spark discharge. One byproduct of spark discharge is emitted light.

A train of spark discharges is considered to be either condensed, or not, depending on the current density and magnetic pinching effects of and around the current-conducting central core of each discharge. All of my work was done with high current density condensed discharge trains.

Spark discharge is intrinsically interesting as a process. It also has demonstrated practical utility, in that the intensity and wavelength distributions of the light emitted during spark discharge are related to the elemental composition of the spark electrodes.

Analytical spectrochemical spark discharge procedures can be used in the quantitative determination of most elements present in solid, alloyed electrodes. The method has widespread applicability as a routine tool for the industrial quality control of metals and alloys. It is particularly rapid; up to 30 elements can be determined simultaneously in less than 60 seconds on an electrode prepared for analysis simply by belt sanding.

Even though this analytical method is in widespread use and is economically competitive with other approaches, its performance is still in need of improvement. The sensitivity needs to be increased by about 100 fold from the current levels of a few parts per million. This improvement would make possible analysis of, for example, minute but purposeful trace additions to an alloy during its molten refining introduced in an effort to adjust grain growth and nucleation during its solidification. In addition, the precision must be improved by about tenfold from its present 1-5 percent of the amount present to allow, for example, analysis of relatively expensive major alloying elements (Ni or W). This advance could provide improved control of melt compositions during alloy refinement at optimum economy, and with closer conformity to chemical specifications.

With improved performance in spectrochemical analysis, better and expanded uses of the chemical data should follow. It is reasonable to expect more use of compositional data in the economic management of raw materials, in organizing factory operations to give optimum use of furnace energy, and, with other controls, to predict the final mechanical properties of an alloy at the pouring stage rather than after it has been made and can no longer be changed. Although these are real and important reasons to try to improve spectrochemical performance, the key to such progress is not instrumental but rather chemical.

With spark sampling and excitation, the questions that need answers are not how to make a more precise spark source, but rather, how light is generated during spark discharge, and, if the responsible phenomena are understandable, how they can then be more precisely controlled at the atomic level. Observation, sorting, and control of the physical and chemical events comprising the process of spark discharge, for the express purpose of improved spectrochemical analysis, form the basis of the all of the research I directed from 1965 until 1982 at the University of Wisconsin, Madison.