Page 59 - Inorganic Mass Spectrometry : Fundamentals and Applications

Glow ~is~harge Mass Spe~tro~et~ 49

Ions, Atoms, Photons
~ara~ay Dark Space

~atho~e Dark Space

Schematic diagram of a planar cathode glow discharge source.

sources need to be operated at currents up 15 d. Planar cathodes are typically
to
5 m to 5 cm in diameter and are between 1 and 5 m in thickness. Cathodes are
formed in the same manner as the coaxial geometry, by machining or pressing.
Because these sources can accommodate larger samples, however, machining
the
sample is often not necessary. Instead, a portion of the sample is simply masked
with a nonconducting sleeve to localize sputtering. Planar cathodes have also
found wide application in trace elemental analysis; one area in which they are
especially useful is in the depth profiling analysis of thin films.
A variation of the planar cathode is shown in Fig. 2.9, the Grimm source.
Named for W. Grim [74], this source is an obstructed discharge in which the
anode extends into the cathode dark space, defining the area of the sample
sputtered. Because the sample is located outside the cell, easy sample interchange
is facilitated. Like other planar cathode sources, the Grimm source is particularly
useful for the analysis of large metal sheets and thick disks. These discharges
typically operate at between 500 and 1000 V, with 25 to 100 d applied current
and a pressure of between l and S torr, These sources are used extensively in
atomic emission spectrome~, and several have been interfaced with mass spec-
trometric detection [75].

Hollow cathode discharges are perhaps the most comon glow discharges used in
analytical chemistry. Most spectroscopists are familiar with these devices as hol-
low cathode lamps used for atomic abso~tion spectroscopy. Figure 2.10 contains