Page 43 - Inorganic Mass Spectrometry : Fundamentals and Applications

Glow Discharge Mass Spectromet~ 33

sponding to the accessible transitions of each element, identification and quan-
tification of a sample’s elemental composition are straightforward. For a sample
composed of a single pure element, spectral interpretation is easy. However, when
it is applied to the analysis of multicomponent materials, the spectral complexity
often makes interpretation problematic. Mass spectrometry has an advantage in
this area since there exist far fewer isotopes of a given element than potentially
populated energy levels. Coburn and coworkers recognized the power of mass
spectrometry when they brought the glow discharge source back to the attention of
mass spectrometrists in the analysis of solids using both direct current (dc) and
radio frequency (rf) discharges [ 1 1 - 131. Other milestones occurred when Har-
rison and Magee [14] and later Colby and Evans [15] demonstrated that a hollow
cathode glow discharge coupled to a magnetic sector inst~ment could be used to
provide elemental analysis of the cathode. In 1978, Bruhn et al. [16] reported the
use of a qua~pole-based glow discharge mass spectrometer. This work indicated
that a relatively low-cost mass spectrometer could be dedicated to trace elemental
analysis of solids. Since this time there have been many advances made in
ins~mentation, sample preparation, and techniques for data analysis. Several
good review articles give further insight into the historical developments of the
glow discharge as an ion source for mass spectrometry 117-191. Two achieve-
ments worth noting are the introduction of commercial instrumentation [203 and
the development of a radio frequency glow discharge as an analytical tool for
the direct analysis of nonconducting samples [21]. As more individuals “redis-
cover” the power of the glow discharge for trace elemental analysis of solids, the
future of GDMS is promising.

Y
The purpose of this section is to provide a basic understanding of glow discharge
(GD) processes; it is in no way intended to be a complete treatment of GD theory.
For a complete treatise on fundamental glow discharge plasma processes, the
reader is referred to an excellent chapter by Fang and Marcus in Glow Discharge
Spectroscopies [22].
Before glow discharge atomization and ionization processes can be ex-
plained, it is necessary to establish a vocabulary of the terns used. The glow dis-
charge is a specific example of a gaseous discharge, which is one type of plasma.
A plasm^ is a partially ionized gas consisting of equal numbers of positive and
negative charges and a larger number of neutral molecules [23]. The term gas
refers to the flow of electric current through a gaseous medium [24]. For
~isch~rge
this to occur, a fraction of the gas particles must be ionized. In addition, an electric
field must exist to accelerate the charged particles, thereby allowing current to
flow. In the classical version of the glow discharge, a voltage source develops a