Evaluation
Atomic absorption spectroscopy is ideally
suited for the analy-
sis of trace and ultratrace analytes, particularly when
using electrothermal atomiza- tion. By diluting samples,
atomic absorption also can be applied to minor and major analytes. Most analyses use
macro or meso
samples. The small
volume re- quirement for electrothermal atomization or flame microsampling, however, allows the use of micro,
or even ultramicro samples.
When spectral and chemical interferences are minimized,
accuracies of 0.5–5% are routinely possible. With nonlinear calibration curves, higher
accuracy is obtained by using a pair of standards whose
absorbances closely bracket
the sam- ple’s absorbance and assuming that the change
in absorbance is linear over the lim- ited concentration range. Determinate errors for electrothermal atomization are
frequently greater than that obtained with flame atomization due to more serious matrix interferences.
For absorbances greater than 0.1–0.2,
the relative standard
deviation for atomic absorption is 0.3–1% for
flame atomization, and
1–5% for electrothermal atomization. The
principal limitation is the variation in the concentration of free- analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and at- omization in flame atomizers, and the consistency with which the sample is heated
during electrothermal atomization.
The sensitivity of an atomic absorption analysis with flame atom- ization is influenced strongly by the flame’s
composition and the
position in the flame from which absorption
is monitored. Normally the sensitivity for an analysis is
optimized by aspirating
a standard and adjusting
operating condi- tions, such as the fuel-to-oxidant ratio,
the nebulizer flow rate, and the height
of the burner,
to give the greatest absorbance. With electrothermal atomization,
sensitivity is influenced by the drying
and ashing stages
that precede atomiza- tion. The temperature and
time used for
each stage must
be worked out
for each type of sample.
Sensitivity is also
influenced by the
sample’s matrix. We have already
noted, for example, that
sensitivity can be decreased by chemical interferences. An increase in sensitivity can often be realized by adding a low-molecular-weight alcohol,
ester, or ketone to the solution
or by using an organic
solvent.
Due to the
narrow width of absorption lines,
atomic absorption pro- vides excellent selectivity. Atomic
absorption can be used for the analysis
of over 60 elements at concentrations at or below
the level of parts per million.
The analysis time when using
flame atomization is rapid, with sample throughputs of 250–350 determinations per hour when using a fully automated system. Electrothermal
atomization requires substantially more time
per analysis, with
maximum sample throughputs of 20–30 determinations per hour.
The cost of a new
instrument ranges from
$10,000 to $50,000
for flame atomization and $18,000 to $70,000 for electrothermal atomization. The more ex- pensive instruments in each price range
include double-beam optics
and auto- matic samplers, are computer controlled, and can be programmed for multiele-
mental analysis by allowing the wavelength and hollow cathode lamp to be changed automatically.
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