Optical Atomic Absorption/Atomic Emission Spectrometry
Portions of Chapter 8-10.
In atomic spectroscopy, samples are vaporized at high temperatures
(2000 – 8000K) and decomposed into atoms. The vapor phase atomic
concentration is measure by absorption (Chapter 9) or emission (Chapter
10) of characteristic wavelengths of UV or visible radiation. These
techniques exhibit high sensitivity and selectivity.
Atoms are produced by aspirating a sample with a nebulizer into a
burner which desolvates, volatilizes, and breaks chemical bonds causing
atomization.
The atoms in the flame (or plasma) are then
detected by absorption of external radiant
energy, or emission from the atoms
themselves.
Atomic fluorescence will not be
discussed, first will be atomic
absorption, then atomic emission.
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Atomic line widths – narrow but not infinitely so as one would surmise
from an atomic energy level diagram. (Important in atomic absorption
instrumentation.)
Sources of atomic line broadening:
Uncertainty Principle – “natural” line widths ~10-5 nm.
Doppler Broadening – most relevant for this discussion.
Δλ/λo = ν/c
Velocity v increases with
Temperature (K) to the 0.5 power.
The hotter the medium, the broader
the greater the Doppler broadening.
Pressure Broadening – important at high pressures (i.e. not
important in this context)
Atomic Absorption (Ch 9)
Atomic absorption (AA) instruments routinely but not exclusively use
flame atomization. A few things about flames which provide the ground
state atoms used for AA.
 Flashback when gas flow rate < burning velocity
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 Higher temperatures required for harder to decompose samples.
 Interzonal region is free atom rich
Where in the interzonal region to perform AA, as well as the preferred
type of flame (fuel “rich” or fuel “lean”) depends on the element. Below
is the flame absorption profile for Ag (hard to form oxides), Cr (forms
very stable oxides), and Mg (intermediate).
The position of the flame is very important
for optimization.
Atomic absorption is almost exclusively used for quantitative analysis of
elements in a sample using Beer’s Law.
Remember:
1. Beer’s Law requires monochromatic radiation to irradiate the
sample. The spectral slit width must be < 10% of the absorption
bandwidth.
2. Atomic absorption bands are extremely narrow.
Since the monochromator
bandwidth >> absorption line
width, a continuous source
cannot be used for quantitative
analysis for AA.
A line source narrower than the
absorption line width is required.
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The most common AA source is a hollow cathode lamp. The cathode is
coated with the
element to be
analyzed.
Ionization of the
inert gas from a
potential (~300
V) between
cathode and
anode causes the
ionized gas to
slam into the
cathode which sputters the cathode coating (same element as analyte)
into the vapor phase. The excited state atoms in the source irradiate light
of the same energy as the analyte. The bandwidth is narrower than the
natural line width because the source is at room temperature and the
atoms are at 2000+K in the flame, so Doppler broadening is much less in
the source.
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AA/quantitative analysis
In theory, a calibration curve will follow Beer’s Law. There are many
cases in which, and many reasons for, non-linearity. Section 9C
discusses many, the data below shows various Sr calibration lines in
different flames and in different matrices.
The details are unimportant to us
here, but linearity cannot be
assumed.
Standard addition calibration and
other methods are used to defeat
matrix effects common in AA.
AA’s place in analytical chemistry:
 Quantitative analysis of ~70 elements (1 at a time, metals and
metalloids only) with sub ppm detection limits
 Sample preparation is the most difficult and time consuming. Most
samples must be converted to homogeneous solutions (digestion
procedures) prior to analysis
 Linear range: 2-3 orders of magnitude
 No information on chemical form of element
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For atomic absorption read chapters 8 & 9 with particular attention to:
 Flame atomization in 9A
 Radiation sources and hollow cathode lamps in 9B
 AA Analytical techniques in 9D
For topics which are important for both AA and atomic emission (next
thing)
 Optical atomic spectra 8A
 Nebulizers (without the details) 8C-1
A related technique with the same information content as AA and used
for the same purposes, is atomic emission. This is discussed in Chapter
10.
The most common method is ICP-AES. (Inductively Coupled Plasma
Atomic Emission Spectrometry). With this method it is possible to
perform simultaneous quantitative analysis of >70 elements with lower
detection limits than AA.
For atomic emission to work, excited
state atoms are required to emit light at
characteristic wavelengths of that
element.
This discussion requires use of the
Boltzmann distribution discussed in
Chapter 8 – the
effect of
temperature on
atomic spectra.
Using our standby Na atom
The energy difference
and 3s atomic orbitals is about
between the 3p
2.1 eV. To do
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emission spectroscopy enough electrons must exist in the 3p excited
state atomic orbital of Na to emit light that can be detected (~589 nm,
equivalent to 2.1 eV).
With, for example, an air/acetylene flame temperature of 2500K, using
the Boltzmann function the fraction of excited state Na atoms can be
calculated at this temperature.
First calculate ΔE, then put that into the Boltzmann equation.
What if you wanted
to do flame emission
of Mg?
In this case the
3p  3s transition
corresponds to light
emitted at 285.2 nm.
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Doing this the same way as before, this light has an energy of 6.94 x 1019
J.
Plugging this into the Boltzmann equation, as before..
Need a hotter flame. Can’t be done with fire (combustion), need a
plasma.
The physics of the ICP is not
important to us, what is important
is its usefulness for AES.
In a 6000K plasma, the fraction of Mg atoms with electrons in a 3p
atomic orbital excited state can be calculated with the Boltzmann
equation.
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So a very hot “flame” or plasma is needed for atomic excitation. A
second absolute requirement is a stable flame temperature.
Remember: both AA and AE are used primarily for quantitative
analysis. But the signal generated and detected from these 2 techniques,
absorption versus emission, is fundamentally different.
You have seen this already:
The signal intensity is directly proportional to the concentration of
excited state analyte.
Earlier the %Na* in a 2500 K flame was found to be 0.0172%.
Now calculate the %Na* in a 2510 K flame.
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A 10K change in flame temperature results in a 4% change in Na*
concentration, which will result in a 4% change in the analytical signal
output for an AE measurement.
Whether it be 2500 or 2510 K however, there is still about 99.98% of Na
in the ground state. Thus the AA measurement is unaffected.
This brings up a second fundamental difference between any absorption
versus emission measurement. All else being equal, an emission
measurement will exhibit much lower (i.e. better) detection limits than
an absorption measurement.
Consider the following scenario analogous to an absorption
measurement where T = P/Po; Abs = -logT
50,000 people in a stadium each with a flashlight on. Po = 50,000
500 people turn off the flashlight. P = 49,500
Consider the competing scenario analogous to an emission measurement
50,000 people in a stadium all with flashlights off.
500 people turn on their flashlight.
A general rule: It is easier to detect a small signal in the absence of
background (emission), than a small change in signal in the presence of
a large background signal (absorption). This is why emission techniques
are more sensitive than absorption techniques.
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The 2nd advantage of ICP-AES over AA is the ability to perform
simultaneous quantitative analysis of many elements using a
multichannel spectrometer since the source is the analyte.
In general, ICP-AES and AA provide the same information. ICP-AES is
superior with respect to detection limits (lower), linear range (larger),
and matrix effects (fewer). ICP-AES also affords the possibility of
simultaneous multi-element analysis. Both methods suffer from sample
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preparation. Not surprisingly, an ICP-AES spectrometer is much more
expensive to buy, but also to maintain.
Questions/problems
Ch. 8: 1,4,6,9
Ch. 9: 1d,f,g,l, 2,9,12,13a,14,20
Ch.10: 1,2,9
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