Page 47 - Inorganic Mass Spectrometry : Fundamentals and Applications

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Glow ~isc~ar~e Spect~o~et~ 37

ionization begin again. Here, a faintly luminous region called the positive C O Z ~ ~ ~
can form, and positive charge carriers are balanced by negative charge carriers
[23,24]. Electrons are accelerated out of the positive column, attaining a velocity
that favors only ionization and not excitation in the anode dark space region [24].
Here, the field strength, negative space charge, and negative current density all
increase. Adjacent to the anode, the cross section for electron impact excitation is
large, and the negative current density is the greatest; a luminous region, the anode
glow, is created.
In most glow discharges only a few of these regions are observable, many of
them disappe~ng as the cathode-to-anode distance is decreased. Generally, in
glow discharges used for analytical applications, only the cathode dark space, the
negative glow, and the Faraday dark space are distinguishable. The only region
absolutely necessary for the existence of the discharge is the cathode dark space;
all other regions serve to maintain the current flow [28]. If the anode is moved into
the cathode dark space region, the discharge is extinguished, a feature often
employed in shielding certain parts of the discharge hardware from the glow.

putter ~to~izatio~

One oft-touted strength of the glow discharge is that the sampling step is separate
from the excitation and ionization steps [29]. This aspect of the glow discharge is
unique, providing atoms that retain little “memory” of the chemical environment
from which they came, thus reducing matrix effects. The process of cathodic
sputtering for creating a representative gas-phase sample population is central to
the analytical utility of the glow discharge. Once sample atoms are liberated,
excitation and ionization occur, the latter process placing the species of interest in
a form suitable for mass spectrometric detection.
The cathodic sputtering process liberates atoms directly from the solid
cathode into the gas phase [30]. Unlike thermal processes (see Chapter 1), cath-
odic sputtering results in the release of atoms on impingement of gaseous species.
In the steady state, positive gas ions are accelerated across the cathode fall region
toward the cathode surface. Before impact, these ions recombine with Auger
electrons released from the surface [3 11. These newly created gas neutrals strike
the cathode and implant themselves into the atomic lattice, transferring their
momentum and kinetic energy to the lattice through a collisional cascade. A
fraction of the energy transferred to the lattice is reflected to the surface. If an atom
absorbs energy greater than its binding energy, the atom may be released into the
gas phase above the surface. Figure 2.3 is an illustration useful in visualizing the
sputtering process [32]. As a result of the sputtering event, not only are individual
atoms released, but clusters of atoms, secondary ions, and electrons are liberated
from the surface. Electrons are accelerated across the cathode fall region into the
negative glow, where they are available to participate in excitation and ionization.