Page 58 - 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

782 Atomic Spectrometry

appearance of the analyte transient quite early in the heat- centration. Because of the fast heating rates employed in
ing cycle. The main problem in the early appearance of the electrothermal vaporization, the transient from the furnace
atomization profile is with the gas-phase chemistry. The lasts for a very short time and it is necessary to have an
sample is vaporized into a cooler environment, which may instrument with relatively fast (ms time-scale) electronics.
not allow for decomposition of small molecules yielding
a sufficient number of free atoms. But, if the sample is de-
6. Physico-Chemical Interferences
posited on a small graphite platform (inside the graphite
tube), it will be vaporized into the hot environment of The processes that lead to breakdown of the matrix, va-
the furnace later than in the wall atomization case. The porization, and complete release of the analyte as a free
platform heats up later than the tube because of its ther- element vary from analyte to analyte and matrix. These
mal mass and little thermal contact with the tube. As a differing conditions are ultimately responsible for the in-
result, the vaporized sample enters a much hotter envi- terferences experienced by the analyte. These effects are
ronment than in the wall atomization case. The platform well studied and the role of ubiquitous species such as car-
often provides a sufficient delay in vaporization so that bon and oxygen are well documented with regard to the
many samples, upon vaporization, see a constant vapor vaporization process. Even though an attempt is made to
temperature even though each matrix may release the an- maintain an oxygen-free environment, the matrix usually
alyte at slightly different times. One analytical scenario contains oxygen. In addition, oxygen ingress through the
in which the platform vaporization may not be suitable is dosing hole has been observed even during the atomiza-
when refractory elements are to be determined. Refrac- tion stage. Compounds of the analyte, such as oxides or
tory elements have a very high vaporization temperature, halides (formed due to the presence of the matrix) may
and platform atomization will result in premature furnace not break down sufficiently to release all the analyte into
failure due to excessively high temperatures. the source radiation.
In a typical atomization cycle, the sample is vaporized Interference effects can often be minimized by the use
with a heating rate of about 1300–1500 K/s. After reaching of chemicals, which are added to the sample to modify
the desired temperature, the furnace is held at a constant the matrix so that interferences are controlled. A matrix
temperature for 3–4 seconds while the absorption pulse is modifier can both modify the matrix so that the analyte is
measured. vaporized relatively late during the atomization stage and
allow a higher pyrolysis temperature to be used without
3. Pyrolytic Coating of the Graphite Tube premature release of the analyte. Matrix compounds can
then be broken down during the pyrolysis step so that they
Atomic absorption spectrometry furnace tubes are sup-
are less likely to cause interference during the atomization
plied with a hard pyrolytic graphite surface. This helps
stage. Some typical matrix modifiers are ascorbic acid,
to reduce the porosity of the tube, which minimizes ad-
magnesium nitrate, and palladium solutions. Modifiers are
sorption of hot metal vapors and slows down the rate of
added in relatively high amounts (about 250 µg).
oxidation of the graphite and the accompanying mechan-
ical degradation of the furnace tube.
7. Background Correction
4. Inert Gas Requirements Small molecules, such as sodium chloride, that are present
during the atomization stage of a furnace analysis are often
An inert gas is used to flush the furnace during analy-
able to absorb or scatter radiation from the light source and
sis. This prevents oxidation of the furnace. Usually an
3
argon flow of about 300 cm /min. is maintained through cause a spurious absorption signal. This is called spectral
band interference. In addition, smoke and other particles
the furnace during the dry and pyrolysis stages of the anal-
that are present during atomization can cause scatter of the
ysis. The flow is stopped or reduced during the atomiza-
radiation. This again causes an apparent spurious absorp-
tion stage to minimize the gas expulsion from the furnace,
tion signals. These signals can be corrected effectively by
which would reduce the absorbance signal.
the use of one of two different methods of background
correction.
5. The Detection System
Deuterium lamp background correction (two-source
The transient signal is usually measured quantitatively by background correction): Two light sources are used in
evaluating the peak area (integrating) rather than by a mea- a deuterium lamp background corrected instrument. The
surement of peak height. While interference may affect first is the usual hollow cathode lamp, and the second is the
position and height of the absorption peak, the area of the deuterium lamp. The latter emits light over a broad range
transient often does not change for a given analyte con- of wavelengths. The deuterium lamp does not normally