Page 51 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

Atomic Spectrometry 775

C. Electrical Discharges for Optical 2. dc Plasma
Emission Spectrometry
The dc plasma (DCP) technique is similar to ICP and less
1. dc and ac Arcs and Sparks like the electrical discharges described above. Figure 9
shows the basic arrangement of the three-electrode DCP
Arcs and sparks were the dominating emission techniques
system. The plasma is formed by the discharge of dc cur-
nearly four decades ago and in most arenas they have been
rent at low voltage across graphite electrodes bathed in
replaced by ICP emission spectrometry. Nonetheless, arcs
argon. The main difference between the DCP and the dc
and sparks have escaped complete extinction due to their
arc is that the sample is introduced in liquid form. The
versatility for analysis of solid samples. These techniques
sample is aspirated and converted into small droplets (us-
are still in use in many foundry-based industries and nu-
ing a nebulizer), in a way similar to the ICP systems. The
clear fuel-fabrication laboratories. These types of elec-
premixed droplets and argon are then allowed to flow into
trical discharge are comprised of two electrodes, across
the DCP observation zone. The sensitivity of the plasma
which an electric current is passed. In the case of the dc
for the determination of metals in samples is inferior to
and ac arcs, a low voltage of 10–50 V is used and a current
that of ICP. There are minimal chemical interferences,
of 1–35 A flows between the electrodes. The ac arc is a
and spectral interferences are as serious as with ICP. DCP
series of separate discharges that occur once during each
is also affected by easily ionized element (EIE) interfer-
half-period of the power cycle. The ac spark is based on the
ence, which cause a 30–80% enhancement of the signals
discharge of a capacitor that has been charged to 1–30 kV.
of some elements. DCP instrumentation is, in principle,
The spark occurs 120–1800 times per second. The tem-
similar to that of the ICP, because multi-element analyses
peratures of these discharges are in the same range as the
are possible by use of a spectrometer that has multiple
ICP; hence, many species can be excited, and quantitative
photomultiplier tubes.
and qualitative analyses can be obtained.
The sample is typically placed in a cup in the bottom
electrode, so the discharge occurs between the anode elec- 3. Glow Discharge Plasma
trode and the sample (cathode). The sample can be a con-
ductive solid or it may be crushed and mixed with a con- A direct current glow discharge (GD) plasma is formed
ducting material such as powdered graphite. in a low-pressure, inert buffer gas (e.g., 0.1–10 torr of
The instrumentation for these discharges is essentially Ar). The sample is composed of conductive material,
the same as those used for all emission experiments. The which becomes a part of the electrical circuit (cath-
detection system after the monochromator (also known as ode). The discharge is typically sustained at several hun-
a spectrograph) can be the multiple photomultiplier ar- dred volts at a current of a few mA. GD plasmas have
rangement, the older photographic plate arrangement, or a high charge density and large electric field gradients
a charge injection device. The intensity of the image of (kV/mm) near the cathode surface. Initially, the argon
each line is proportional to the amount of light emitted buffer gas becomes ionized and is accelerated toward
from the discharge at each wavelength, and the concentra- the cathode due to a net electric field. As the fast mov-
tion can thus be interpolated by use of standards of known ing Ar impinges on the cathode surface (sample), the
concentration. sputtering process removes a few layers of material.
Electrical discharges are affected by a number of se- The sputtered material is subsequently excited and ion-
rious matrix interferences associated with the way that ized through a series of complex gas-phase processes.
∗
−
+
the sample is vaporized into the discharge as well as var- These include electron impact (A + e → A , A , A ),
∗+
ious chemical and physical interactions within the dis- Penning ionization by the metastable species of the
m
charges. Spectral interferences are as serious as for ICP– buffer gas (A + Ar → A +∗ + Ar), radiative recombi-
OES. These interferences can be mitigated, in part, by nation (A + e → A + hν), and radiation trapping
∗
−
+
∗
the use of internal standards and the concentration ratio (A + hν → A ).
method. However, these approaches are not as successful Two types of glow discharge sources are illustrated
as with the ICP because the latter has fewer interferences in Fig. 10. The hollow cathode GD is often used as a
and better precision. Internal standards have to be cho- line source for many spectroscopic applications (such as
sen with great care because they must behave in the same atomic absorption, which is discussed in the next section).
way as the analyte. This is not trivial because the physico- A potential of up to 400 V is placed between the anode
chemical interference problems that occur in these atom and cathode of the lamp to initiate the plasma. During
cells vary greatly from element to element and sample to operation, an electrical current between 4 and 40 mA at
sample. 150–350 V sustains the plasma in a low pressure of an