Analytical Glow Discharges: Fundamentals, Applications, and New Developments

Abstract

AbstractOptical (atomic absorption spectroscopy, AAS; atomic emission spectroscopy, AES) and mass spectrometric (magnetic sector and time‐of‐flight, TOF) instrumentation are well suited for coupling to the glow discharge (GD).The GD source is a relatively simple device. A potential gradient (500–1500 V) is applied between an anode and cathode. In most cases, the sample is also the cathode. A noble gas (e.g. Ar, Ne, Kr, and Xe) is introduced into the discharge region before power initiation. When a potential is applied, electrons are accelerated toward the anode. As these electrons accelerate, they collide with gas atoms. A fraction of these collisions is of sufficient energy to remove an electron from a support gas atom, forming an ion. These ions are, in turn, accelerated toward the cathode. These ions impinge on the surface of the cathode, sputtering sample atoms from the surface. Sputtered atoms that do not redeposit on the surface diffuse into the excitation/ionization regions of the plasma where they can undergo excitation and/or ionization via a number of collisional processes.GD sources offer a number of distinct advantages that make them well suited for specific types of analyses. These sources provide analytically useful gas‐phase species from solid samples and can be interfaced with a variety of spectroscopic and spectrometric instruments for both quantitative and qualitative analyses. GD sources afford direct analysis of solid samples, thus minimizing the sample preparation required for analysis. According to the nature of this plasma, the atomization and excitation processes are separated in space and time that helps to minimize the matrix effects that plague so many other elemental techniques. This low influence of the matrix is also supported by the fact that the plasma consists mainly of the discharge gas and the sample is only present in low concentration. Unfortunately, the GD source functions optimally in a dry environment, making analysis of solutions more difficult. The GD sources are working optimally with conductive samples and in low‐pressure environment. It makes adaptation of GD sources for analysis of nonconductive samples and samples with high vapor pressure difficult and challenging.In this article, first the principles of operation of the GD plasma are reviewed, with an emphasis on how those principles relate to optical spectroscopy and mass spectrometry (MS). Basic applications of the GD techniques are considered next. These include bulk analysis, surface analysis, and the analysis of solutions and gaseous samples. The requirements necessary to obtain optical information are addressed following the analytical applications. This section focuses on the instrumentation needed to make optical measurements using the GD as an atomization/excitation source. Finally, mass spectrometric instrumentation is addressed as it pertains to the use of a GD plasma as an ion source.