Chem 321 Lecture 20 - Atomic Absorption Spectrophotometry
11/7/13
Although the basic principles, especially Beer’s law, apply, the instrumentation for
doing atomic absorption spectrophotometry (AAS) differs in some significant ways from
what is used for conventional UV-VIS spectrophotometry. The main differences are the
radiation source and sample holder that are used. As the name suggests, AAS involves
absorption by analytes that are atoms (in the gas phase). These atoms are usually
produced in a flame from solutes in aqueous solution. A schematic diagram of a typical
AAS instrument is shown in Figure 13.1.
Figure 13.1 Schematic diagram of an atomic absorption spectrophotometer
The flame not only produces the atomic species, but also serves as the sample
holder. The aqueous sample is drawn up into a mixing chamber through a capillary
tube where it becomes an aerosol, is mixed with the burner gases and drawn into the
burner. The following sequence, using calcium as the analyte, describes the production
of the light-absorbing analyte atoms in the flame.
CaCl2(aq)
6 CaCl2(s) 6 CaCl2(g) 6 Ca(g) + 2Cl(g)
The formation of Ca(g) is not the only possible fate for this analyte. It could also be
ionized or form a polyatomic species, both of which will absorb differently than the
calcium atoms. Proper choice of the burner gases and adjustment of the gas mixture
composition can optimize the production of analyte atoms in the gas phase. In our
experiment, an acetylene/air mixture is used. One assumption made in this experiment
is that the same fraction of analyte is converted to atoms in the gas phase for all
samples. An “exploded” view of a mixing chamber in an AAS instrument is shown in Figure
13.2. Notice the drain outlet at the rear of the mixing chamber. Most of the aspirated
sample never makes it into the flame and is removed through the drain. Another
assumption made in this experiment is that the sample uptake into the flame is the
same for all samples.
Figure 13.2 An “exploded” view of the mixing chamber in an AAS instrument
During the absorbance measurement, aqueous sample is continuously drawn
into the flame at a rate of several milliliters per minute. A steady-state level of the gasphase analyte atoms is eventually established in the flame. These atoms absorb a
portion of the radiation that is directed through the flame. This process is monitored for
a preset interval (4 seconds in our experiment) and the light intensity reaching the
detector (P) is determined. A blank solution that is run in a similar way serves to
establish P0 (recall A = -log P/P0). Since the light-absorbing species is an atom in the gas phase, radiation
absorption occurs in very narrow bands at discrete wavelengths corresponding to the
electronic excitations in the atom. These absorption bands are much narrower than the
band of radiation that is typically selected by a monochromator. However, Beer’s law
holds only if the band of radiation passing through the sample is narrow compared to
the absorption band. Thus, a different type of radiation source must be employed for
AAS. The radiation source in an atomic absorption spectrophotometer is a hollow
cathode lamp (Fig. 13.3). This source, unlike the tungsten lamp and deuterium lamp,
does not emit all wavelengths in a specific spectral region. Instead, the output is the
atomic emission line spectrum of the element specific to the lamp. In the hollow
cathode lamp, electronically excited atoms of the cathode material are produced in the
gas phase. These atoms then emit photons as they return to the ground state. By
selecting a lamp with a cathode made of the same material as the analyte, the lamp
emissions will be at exactly the wavelengths absorbed by the analyte. The width of
these emission lines is sufficiently narrow compared to the absorption band of the
analyte for Beer’s law to hold. The monochromator in an atomic absorption instrument
is used to isolate only one emission line from the hollow cathode lamp and to reject as
much of the emissions from the flame as possible. Usually a different hollow cathode
lamp is used for each analyte.
Figure 13.3 Diagram of a hollow cathode lamp
It is important to note that AAS is a total element method. That is, you are able
to determine how much of an element is present, but not what chemical form is present.
Because AAS is a rather sensitive technique (it can detect ppm levels for most metals)
and the instrumentation is rather inexpensive ($20,000 and up), this has been a widely
used method for trace element analysis. The disadvantages with flame AAS are that
the samples are usually limited to aqueous solutions or other easily aspirated liquids, it
is not suitable nonmetal analysis and a different lamp is often needed for each element
analyzed. The spectrophotometric determination of iron and the atomic absorption
experiment with magnesium are both done in the linear range for these analytes.
Consequently, the calibration curves (plots of absorbance versus concentration of
standard) are quite linear. If your standard data points do not all fall on or very near the
linear least-squares line fit, consider rejecting one of the data points or remaking one or
more of the standards. The AAS determination of calcium, however, uses analyte
concentrations slightly outside the linear range for calcium. Consequently, the calcium
calibration curve is not quite linear and is better fit with a quadratic least-squares line (a
polynomial equation of order 2). Once again, reject a standard point that deviates
markedly from the line fit, or remake standards if necessary. For all of these
experiments, it is appropriate to include a (0,0) data point when preparing each
calibration curve. The best results are obtained when the standards and samples are
all run at the same time (identical instrument conditions).
Atomic Absorption Spectrophotometry
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