Page 62 - Inorganic Mass Spectrometry - Fundamentals and Applications

under the name Atomsource by Analyte Corporation [SS]. Piepmeier et al. have
used these sources extensively for atomic spectroscopy as well as mass spec-
trometry and have demonstrated pe~o~ance better than that of conventional
a
of
Grim-type sources [89,90]. Typical operating conditions approximate those
Grimm source: 30 nu4 current, 900 V, and 2.5 torr argon in the cell, Widespread
use of these sources for mass spec~omet~ has not been observed, perhaps
because the vendor has primarily marketed these sources for atomic absorption
spectrometry, or perhaps because there is little flexibility in the size and shape of
the sample that can be analyzed (i.e., as in the Grimm source, samples are disks a
few centimeters in diameter by a few millimeters thick).
Another novel source is the atmospheric sampling glow discharge [91]. This
that
device is based on establishing a glow discharge in ambient air is drawn into a
region of reduced pressure between two parallel plates. Unlike in conventional
glow discharges, ionization occurs as a result of chemical ionization (CI), pre-
sumably from ion-molecule reactions. The sample is often entrained in the air-
stream flowing through a 0.2-m orifice in the first plate. The discharge is
maintained in the central region at a pressure of -0.5 torr; 300-400 V is applied
the
between the electrodes. Under these conditions, discharge current is 3-10 d.
Air is pulled through the orifice at a rate of -5 &/sec; because of this relatively
high flow rate, analyte response is almost instantaneous. Although this device
holds the potential for analyzing inorganic analytes entrained in air, to date it has
been used almost exclusively for detecting highly volatile organic analytes
[9 1,921. Ease of operation, part-per-trillion (ppt) detection limits, and a wide linear
dynamic range have made it an ideal source for explosive detection. A portable
version is being developed for use in airport security [93].

~econ~a~ Cathodes
Clearly, nonconducting samples pose a special challenge for the analyst, and yet
a sizable fraction of the materials desirable to analyze by GDMS are nonconduct-
ing, Two alternatives that aid in the analysis of these materials have already been
discussed: the use of a radio frequency glow discharge and mixing of the non-
con~ucting sample with a conducting binder. A third, a secondary or s~ogate
cathode in combination with a conventional dc discharge, has also been used.
Such an approach consists simply of “masking” the insulating cathode with a thin
(-0.25 m) high-pu~~ metal (e.g., gold or tantalum). A numb~r of configurations
have been tried, and each has met with a degree of success. The most widely used
(-3-6
approach is to use a solid disk with an orifice in the center m in diameter)
as the secondary cathode [94-961. In most discharges a large ~raction of the
sputtered atoms are deposited back on the conducting cathode and subsequently
sputtered again. Because the conducting mask has a hole in it, a fraction of the
atoms from the mask are redeposited on the insulator beneath it. The sample