Page 61 - Inorganic Mass Spectrometry : Fundamentals and Applications

Glow isc charge ass Spectro~et~ 51

illustrations of two hollow cathode sources: (a) a conventional hollow cathode
lamp and (b) a slightly different version of the source that can be used for atomic
absorption, atomic emission, or mass spectrometry. These devices operate by
means of the hollow cathode effect [76,77]. As a result of the coalescing of two
discharges from parallel plates, the current density in a hollow cathode can be
several orders of magnitude larger than that obtained by using a single planar
cathode at the same cathode fall potential [78]. This results in atom densities that
As with other GD sources, ionization
far exceed those found with planar cathodes.
is
occurs in the hollow cathode lamp’s negative glow. Ionization, too, greater than
that produced by conventional discharges, and many investigations that take
advantage of it have been conducted; the reader is referred to several excellent
articles [76,79-811. The hollow cathode operates at somewhat lower voltages
than other discharges (200-500 V) but at similar pressures (l -5 torr). Discharge
currents may range up to 200 mA. Hollow cathode lamps may be sampled in
several different ways, including near the anode or alternatively on the reverse
side of the cathode [82,83].
A variation of the hollow cathode discharge that holds potential for glow
as
discharge mass spectrometry is the hollow cathode plume. Developed primarily
an atomic ernission source by Marcus and Harrison [83], hollow cathode plume
a
is formed when the discharge is restricted to a small orifice in the base of a
conventional hollow cathode [83-85]. Samples take the shape of disks, 4.5 mm in
diameter and 2 mm in height with a 1.5-mm diameter orifice. Like more conven-
tional discharges, cathodes can either be machined directly or pressed into the
desired shape. Operating conditions are comparable to those of other discharges
(pressures range from 1 to 10 torr with currents of between 50 and 200 mA and
voltages of up to l000 V). Physically, the hollow cathode plume plasma resembles
a torch protruding from a narrow opening. The plume is believed to arise from
pressure and field gradients caused by its construction [83-85]. Preliminary mass
spectrometric data showed several advantageous characteristics [86] that as yet
have not been exploited fully.

Novel Glow Discharges
A number of variations of the conventional glow discharge have been developed
for specialized applications. Some of these are of particular interest to optical
spectrometrists and have not been used for mass spectrometry beyond proof-of-
principle experiments. An excellent article by Harrison et al. [87] reviews many of
these sources, highlighting applications in atomic emission, atomic absorption,
and atomic fluorescence spectroscopies. Two of these sources deserve mention
here. The jet-enhanced glow discharge is similar to the Grim source but with six
strategically located gas jets that improve the analytical performance. Argon from
these jets helps direct the atoms sputtered from the surface away from the sample
at a much faster rate than ordinary discharges. These devices have been marketed