Glow Discharge Spectroscopies

Driven by advances in materials research, government regulations, and interdisciplinary collaborations, analytical chemistry has been one of the most active areas of chemical research over the last three decades. Fundamental advances in the fields of environmental modeling, medicine, and materials science are evidence that the discipline has met many of the challenges that have been posed. In developing methods, analytical chemists have become more adept at interpreting and implementing the findings of biologists and applied physicists. The role that computers have had in revolutionizing chemical instrumentation and techniques cannot be underestimated. Advances in analytical chemistry have shown up in the vernacular of the trade; units of quantitation have switched from weight percent to parts per billion (and trillion). Simple functional-group analysis has been replaced by methods of extraordinary selectivity and specificity. Each step toward better sensitivity or specificity in analytical measurements can, in many cases, cause more difficulties than provide answers. For example, an analytical method can be too sensitive for many uses and environments. This volume addresses some of the advances that have taken place in the area of elemental spectro-chemical analysis.

When a sufficiently high voltage is applied across two electrodes immersed in a gaseous medium, atoms and molecules of that medium will break down electrically, forming electron-ion pairs and permitting current to flow. The phenomenon of current flowing through a gaseous medium is termed a “discharge,” also known as a plasma. The breakdown of the medium is characterized by the transition of the gas from a poor electrical conductor with resistivity (resistance × area/separation) of some 10¹⁴ohms • m to a good conductor with resistivity (dependent on the particular conditions) of ~10³ ohms • m. The potential difference between electrodes at which this transition occurs is called the breakdown potential, V _b, which depends on the identity and density of the gas, the electrode material and interelectrode separation, and the degree of preexisting ionization.

The analysis of alloys, powders, and dried solutions by atomic absorption and fluorescence spectrometry (AAS and AFS) using sputtering cells for atomization is receiving wider notice in the literature as the advantages of these methods come to be appreciated. For example, compared with emission spectrometry, these methods have higher spectral resolution and fewer spectral interferences. Rapid multielement AAS analyses are now practical because of recent technological advances. With recent developments, sensitivities approach and in some cases exceed flame and graphite furnace methods, and improvements are continuing.

Optical emission spectrometry is one of the oldest physical methods of analysis, enabling multielement determinations at the level of major elements, minor elements, as well as trace elements. Its historical development is closely related to the progress in basic understanding of the nature of atomic spectra and the structure of matter. The optical emission spectra of atoms and ions (for a discussion see Refs. 1, 2) stem from transitions between the outer electron shells of the chemical elements which give rise to line spectra where the wavelength of the lines relates to the energy difference of the levels concerned according to:

Glow discharge mass spectrometry (GDMS) is associated with two closely related techniques for the characterization of ion populations in glow discharge plasmas. The materials scientist is familiar with GDMS as a tool for plasma processing diagnostics, whereas the analytical chemist is familiar with GDMS as a method for direct solids analysis. The present chapter will focus on GDMS as a maturing technique in elemental analysis. The reader interested in GDMS as a plasma diagnostic tool is referred to reviews of the topic by Aita⁽¹⁾ and Coburn.⁽²⁾

Pioneering observations on the luminescent phenomena generated in evacuated tubes belong to the history of spectroscopy and date as far back as the mid-1800s.^(1,2) Within this field of research, the first description of a hollow cathode discharge (HCD) published in the scientific literature can be found in the early years of this century, when a German physicist, Friedrich Paschen, reported on the quite unique features of this radiation source.⁽³⁾ At the time he was mainly engaged in the investigation of the spectral series of H₂ in the IR region and in the distribution of energy in the spectra emitted by glowing gases. Together with Back in 1913 he discovered the effect (named for both scientists) that gives rise to the splitting of emission lines when the source is subjected to a very strong magnetic field. This phenomenon is actually a modification of the Zeeman effect that requires less intense fields.

New developments in the field of specialized materials continually present new challenges to the analytical chemist. The improved performance of novel glasses and advanced ceramics demands improved analytical techniques because trace contaminants in such materials critically affect their performance. Advances in the material sciences are thus dependent on improved analytical chemistry and more sensitive methods for the analysis of nonconducting materials. Broekaert et al. have reviewed state-of-the-art techniques for the analysis of advanced ceramics, comparing and contrasting the performance of each.⁽¹⁾

Grimm (1968) was the first to demonstrate the principle of using a glow discharge lamp for the analysis of flat samples.⁽¹⁾ Since the Grimm lamp appeared, low-pressure gas discharges have found many applications. Several authors have investigated the potentialities of such a discharge.^(2–9) Principal applications are in the bulk analysis of metals or of nonconducting materials pressed into pellets with a conducting binder. Several other configurations have been described for bulk analysis, mostly to improve the sensitivity, e.g., hollow cathodes,^(10,11) boosted lamps,^(12,13) and magnetic field-enhanced glow discharges.⁽¹⁴⁾

In recent years, the concept of using electrical discharges within graphite furnace atomizers as atomization/excitation sources for atomic emission spectrometry has been established and evaluated.^(1–31) The excitation in these sources is principally dependent on the discharge and is not a direct result of the electrothermal heating of the furnace. Consequently, this unique concept has been designated furnace atomization nonthermal excitation spectrometry (FANES). This can lead to some confusion since “nonthermal” also implies that the line radiance and profile associated with the discharge do not conform to Maxwell-Boltzmann statistics.⁽³²⁾ While it is certain that the discharge is necessary for excitation, there is considerable uncertainty whether all the discharges described in this chapter can be generally categorized as nonthermal. We shall nevertheless continue to use FANES to describe the concept, bearing in mind the above ambiguity.

The development of cost-effective laser systems has generated a host of laser hyphenated techniques that have been introduced into the analytical laboratory. These hyphenated techniques take advantage of the laser’s ability to deliver a high photon flux, high photon energies, and a narrow, tunable photon wavelength range to optimize specific atomization/excitation/ioniza-tion processes in an analytical procedure. A laser system coupled to a glow discharge is one such hybrid technique that offers unique opportunities for both diagnostic and analytical investigations. This chapter will serve as an introduction and overview of several reported methodologies that have advantageously combined laser systems and glow discharges.

There should be little doubt to readers of this volume that reactive plasma processing applied to the development of electronic devices and novel materials has considerable significance for current and future technology. Furthermore, it is fairly apparent that the design and control of plasma processes has been empirical in nature over the years, as is the case in the analytical glow discharge plasmas discussed in the previous chapters. However, we have now perhaps reached a point where it has become necessary to have a more comprehensive diagnostic and theoretical interpretation of discharge reactor systems in order to advance the promise of this technology.⁽¹⁾