Page 42 - Inorganic Mass Spectrometry : Fundamentals and Applications

32 ~urs~ick

aerosols. The purpose of this chapter is to provide some historical back~ound for
today’s research and development in this field. The fundamental operation of the
discharge will be described, as well as various ins~mental con~gurations and
analytical applications. This discussion serves as an introduction to two other
chapters in the book, where glow discharge mass spectrometry (GDMS) of
nonconductors and elemental analysis using glow discharge ion trap instrumenta-
tion are covered.

Y

Unlike most of the ionization sources found on modern mass spectrometers,
discharge devices actually preceded the mass spectrometer itself. Pioneers like
Thomson, Aston, and Bainbridge used electrical discharges as ionization sources
on some of the earliest mass spectrometers constructed [2]. Many of these investi-
gations centered on elucidating info~ation about the “positive rays” or canal-
stru~Ze~ reported several years earlier by Goldstein [2]. What Thomson and others
found was that the spectra generated from these sources provided information not
only about discharge phenomena, but about the atomic masses and isotopic
abundances of the elements in the support electrodes as well. Thornson’s work
with positive rays emerging from gas discharges led to Aston’s development of
ion sources for elements available only as solids of low volatility (i.e., refractory
metals, metallic oxides, etc.) [3]. These types of ion sources dominated the mass
spectrometry landscape in the 1920s and 1930s. During World War I1 (and for
about 20 years after), discharges were largely ignored because a new ion source
appeared, the vacuum spark [4]. This source, still in use today on many emission
spectrometers, has been shown to provide almost full elemental coverage with
only a few exceptions. Detection limits are on the order of 0.1 ppm [5]. The
technique does, however, suffer from several drawbacks, including the need for a
double-focusing mass spectrometer to overcome the wide energy spread and a
plethora of polyatomic interferences. In addition, long integration times are re-
quired to average out the inherent instability of the ion signal. These instruments
were designed so that all ions could be detected simultaneously with photographic
plates, although some instruments have been fitted with electronic detectors. Most
recently, spark source mass spectrometry has largely been replaced by inductively
coupled plasma mass spectrometers (see Chapter 3) or glow discharge mass
spectrometers.
During the 20-plus years that mass spectrometrists lost interest in glow
discharges, optical spectroscopists were pursuing these devices both as line
sources for atomic absorption spectroscopy and as direct analytical emission
sources [6- 101. Traditionally, inorganic elemental analysis has been do~nated
by atomic spectroscopy. Since an optical spectrum is composed of lines corre-