Page 56 - 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

780 Atomic Spectrometry

Hence, all backgrounds, including any atomic emission andcomplexingagentssuchasEDTA(ethylenediaminete-
spectral interferences, are subtracted out automatically. traacetic acid). Releasing agents increase the magnitude
The monochromator and detection system: The mono- of the signal in real samples. They can also change the
chromator for AAS does not need to have the high reso- magnitude of the signal from the standard solutions.
lution necessary for AES, because the selectivity of AAS Spectral interferences: The elegance of atomic absorp-
depends on the line width of the light source as discussed tion lies in the very high selectivity of the technique. The
above. The role of the monochromator is to reject the ma- HCL only emits light that is characteristic of the elements
jority of the background radiation as well as selecting only in the cathode. While in the atom cell there may be many
the pertinent HCL emission line for AAS measurements, elements present, only one element that corresponds to the
thus monochromators for AAS have a moderate resolution cathode material absorbs the light, because only the light
of 0.02–2.0 nm. The detection system is based on a pho- characteristic of that element is emitted by the light source
tomultiplier tube, and the readout electronics are similar and reaches the detector. The HCL acts like a probe to de-
to single-element AES instruments. termine the concentration of only one metal. This means
that virtually no spectral line interferences affect AAS.
The technique is often much simpler to use than AES, and
2. Interference Effects
the instrumentation can be relatively inexpensive while
Physico-chemical interference effects: The flame atom- providing high sensitivity and selectivity.
ization process is affected by chemical interferences that
prevent facile formation of neutral metal species of M. C. Graphite Furnace Atomic
For example, oxides of many metals are likely to form in Absorption Spectrometry
the flame environment. Oxygen is abundant in any flame
as a consequence of the oxidant (air, oxygen, or nitrous Graphite furnace AAS, also known as electrothermal
oxide) used in the combustion process. The main con- atomic absorption spectrometry (ETV–AAS), is one of the
cern is whether or not there is enough energy in the flame most sensitive techniques available for routine elemental
to break the bond between the metal and the oxygen to analysis. This technique is capable of the determination
release the metal. If the flame does not have enough en- of picogram (10 −12 g) amounts in a few microliters of
ergy, most of the metal oxides remain intact and very little sample.
free metal is available. Phosphates also cause chemical
interference due to formation of a stable compound in the
1. Instrumentation
air-acetylene flame that breaks down slowly to release the
metal atom. This reduces the expected signal size signif- The arrangement for this technique is similar to that for
icantly. In striving to discriminate against chemical inter- flame AAS, except that a graphite furnace replaces the
ferences, the main aim is to obtain some assurance that the flame. The atomizer consists of a graphite tube about 3 cm
signalmeasuredbythedetectionsystemdoesindeedquan- long, with a 6-mm internal diameter and wall thickness of
titatively represent the concentration of the analyte in any about1mm.Twodifferentgraphitetubedesignsareshown
particular matrix. We try to calibrate the instrument with in Fig. 13. The tube is fixed between two electrodes and
a known concentration of the analyte in a deionized wa- is subjected to a low-voltage (up to 12 V), high electrical
ter matrix and then compare directly the signal from a real current. The power supply can be programmed to heat the
sample with the calibration curve. The real sample compo- furnace to several pre-selected temperatures. The furnace
nents in the matrix, such as phosphate in a biological ma- has an inert gas passing through and around it to min-
trix during the determination of calcium, cause the signal imize the ingress of oxygen from the air, which would
to be smaller (a depression) or sometimes larger than the lead to rapid oxidative degradation of the furnace tube.
signal obtained from the same concentration of analyte in Five to 100 µl of the sample solution are placed in the
◦
the calibration solution. One way to circumvent chemical furnace and dried at 100 C. The sample is then pyrolyzed
interferences is to use releasing agents. Lanthanum chlo- to break down or volatilize the matrix component. The
ride, which is usually added (1% weight/volume) to all pyrolysis temperature is set high enough without uninten-
sample solutions, is a good example of a releasing agent. tionally vaporizing the analyte (about 400–1200 C). After
◦
The lanthanum oxide and phosphate compounds are often pyrolysis, the furnace is rapidly heated to the atomization
more stable than the same compounds of other metals. In temperature to volatilize the analyte into the atom cell. The
the flame, the lanthanum oxide or phosphate forms in pref- atomization temperature can be anywhere in the range of
◦
erence to the analyte oxide or phosphate and thus releases a few hundred to about 2700 C. The analyte then absorbs
the analyte atom. Other releasing agents that have been light from the hollow cathode lamp to give the atomic ab-
found to work in many situations are strontium chloride sorption. This signal is a transient (Fig. 13E), because the