Page 276 - Inorganic Mass Spectrometry - Fundamentals and Applications

262 ~~rcu~

into the gas phase for subsequent excitation and ionization [9]. In both cases, an
electrical current must be maintained to sustain the respective plasma discharges.
as an electrode in a basic diode type of assembly. In
In this way, the sample serves
the case of samples that are electrically insulating (nonconductive), this flow of
current cannot be accommodated, so methods of remediation must be imple-
mented. This is most typically achieved by mixing the sample with a powder of a
conducting metal or graphite, and pressing this mixture into the form of a nomi-
nally conductive disk [ lo]. This process is usually referred to as co~~ac~io~. If the
sample is in bulk rather than powder form, it must first be ground or pulverized. Zn
this instance, the primary benefit of using a solid sampling method, the elimina-
tion of a matrix modification step, is voided.
As described in previous chapters, glow discharge mass spectrometry (CD-
MS) has established itself since the mid-1980s as one of the most sensitive and re-
liable methods for performing direct solids elemental analysis of conductive solid
sample types [ 1 1-14]. The ability of the easily controlled low-pressure plasma to
yield a reproducible ion beam affording sensitivities to the sub-parts per billion
(ppb) level is its most outstanding feature. The successive removal of pseudoatomic
is
layers provides additional information content because the technique capable of
performing very-~igh-sensitivity depth profiles for layers ranging from tens of
nanometers to micrometers in thickness. It must be admitted, though, that the
ca-
pability of performing quantitative depth profiles still in an infant stage relative
is
to its sister method, glow discharge atomic emission spectroscopy (CD-AES)
[15-l?].
In terns of the range of sample types amenable to glow discharge mass spec-
trometry (GD-MS) analysis, it must be realized that the glow discharge, like its
spark and arc discharge cousins, is a diode-type electrical device. Electrical cur-
rent must continuously flow [in the case of direct current (dc) powering] to and
from the sample cathode. In the case of electrically insulating samples, the sam-
ples must be modified to become electrically conductive by the methods originally
developed for spark and arc sources [ 18-20]. In doing so, however, one of the ad-
vantages of GD methods, the ability to create depth-resolved elemental profiles, is
lost. ~o~uitously, because glow discharges are low-pressure devices, alternative
of
methods of applying the discharge power that allow sustaining the plasma at the
surface of nonconductors exist. These methods include use of a secondary cath-
the
ode [2l-251 to provide a conductive coating on the sample and the use of radio
frequency (rf) power [26-351 applied to the sample.
This chapter deals exclusively with the methods that have been developed
for the direct solids analysis of nonconductive samples by glow discharge mass
spectrometry. The basic approaches to operation and sample preparation for the
three primary methodologies of compaction, secondary cathode, and radio fre-
quency powering are described. Examples source performance and practical ap-
of
plications of each are taken from the analytical literature. Whereas this chapter de-