Page 56 - Inorganic Mass Spectrometry : Fundamentals and Applications

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homogenized; the resulting powder is then mixed with an equivalent weight of
silver to assure sufficient material for preparing compacted pin cathodes of the
type described. Linear calibration curves (analyte signal intensity versus cathode
concentration) have been demonstrated, although relative sensitivity factors agree
only marginally well with those obtained for solid samples [7 l].

There are several ways of~categorizing glow discharge ion sources. The one most
often employed considers the arrangement of the cathode with respect to the
of
anode. Another is to consider the operating characteristics (pressure and power)
the various sources, and another is to separate them on the basis of whether they
operate with a direct current (dc) or radiofrequency (rf) supply, or with a supply
that operates in a constant or pulsed voltage mode. Because all of the discharges
under consideration can be operated in either the dc or rf mode, a brief review of
each type of operation is presented next; then cathode and anode arrangements
are considered, with operating characteristics such as pressure and power high-
lighted. Finally, there exist several unique sources that do not fit conveniently into
any one category but deserve mention.

Direct Current and Radio Frequency Glow Discharge Operation

a
Analytical glow discharges have conventionally operated with constant negative
dc potential applied to the cathode. There is no reason, however, that they can’t
be operated through the application of a pulsed potential, an applied rf potential,
or a positive potential applied to the cathode. Many variations have been tried
alone and in combination with one another. Perhaps the most interesting among
these (because of the unique capabilities that it provides) is the radio-frequency-
powered discharge. The analysis of nonconductors is covered extensively in a
later chapter, but a brief overview is in order here,
Because current cannot flow through an insulating substrate, the application
of a potential to a ceramic, glass, soil, or other insulating sample produces an ef-
fect analogous to a charging capacitor. On the application of a negative dc
potential to the surface of the material, negative charge exists. Unlike the charge
on a conducting surface, however, this charge decays rapidly to a more positive
potential as a result of charge neutr~ization reactions with ions at the surface [72].
The result is a short-lived discharge that exists until a minimum threshold voltage
is reached; at that point the plasma is extinguished [73]. If one applies an
alternating current (ac) potential to the cathode, the surface is bombarded by an
alternating source of electrons and positive ions. The greater mobility of the
electrons results in the accumulation of more electrons than ions during the
respective half-cycles, resulting in a time-averaged negative dc bias. This phe-