Transcript ATOMIC ABSORPTION SPECTROSCOPY

Modern Analytical Chemistry
ATOMIC ABSORPTION
SPECTROSCOPY
z1
State the use of Atomic Absorption (AA)
spectroscopy.
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Atomic absorption spectroscopy is a quantitative
method of analysis that is applicable to many metals
and a few nonmetals.
A few examples include:
Al in blood serum
Ca in blood serum, plants, soil, water
Cu in alloys
Cr in sea water
Fe in plants
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Only a drop of sample needed
The metals need not be removed from other
components (AA is a highly selective technique)
Sensitive in the ppm range (even ppb with the right
equipment)
z2
Describe the principles of AA.
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When metals are exposed to heat, they absorb
light.
Each metal absorbs light at a characteristic
frequency. For example:
Metal
λ (nm)
Zn
214
Fe
248
Cu
325
Ca
423
Na
589
z3
Describe the principles of AA.
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The metal vapor absorbs energy from an
external light source, and electrons jump from
the ground to the excited states
The ratio of the transmitted to incident light
energy is directly proportional to the
concentration of metal atoms present
A calibration curve can thus be constructed
[Concentration (ppm) vs. Absorbance]
z4
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
• A block diagram of the AA spectrometer appears below.
• The IB does not require the inclusion of the photomultiplier tube
(PMT), but it none the less is an important part of the
instrumentation.
z5
Overview of AA
spectrometer.
Light Source
Sample
Compartment
Detector
z6
Describe the use of each of the following components
of the AA spectrometer: fuel, atomizer, monochromatic
light source, monochromatic detector, read out.
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The source of light is a lamp whose cathode is composed of the
element being measured.
Each analyzed element requires a different lamp.
For example, a hollow cathode lamp for
Aluminum (Al) is shown below
z7
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
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The cathode lamps are
stored in a compartment
inside the AA spectrometer.
The specific lamp needed
for a given metal analysis is
rotated into position for a
specific experiment.
z8
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
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The sample is made up, typically in water
A flame is created, usually using ethyne & oxygen (fuel)
The flame gases flowing into the burner create a suction
that pulls the liquid into the small tube from the sample
container. This liquid is transferred to the flame where
the sample is atomized [mixing the sample with air to
create fine droplets]. The metal atoms then absorb light
from the source (cathode lamp).
z9
Light beam
Sample is
vaporized
in the flame.
Aspirator
tube sucks the
sample into the
flame in the
sample
compartment.
z10
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
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The light passes through a monochromater (a
device used to select a particular wavelength of
light for observation)
The intensity of the light is fairly low, so a
photomultiplier tube (PMT) is used to boost the
signal intensity
A detector (a special type of transducer) is used
to generate voltage from the impingement of
electrons generated by the photomultiplier tube
z11
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
A typical photomultiplier tube
z12
Describe the use of each of the following components of the AA
spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
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The read out specified by
the user is displayed on
the computer screen for
each sample measured.
z13
Describe the use of each of the following components of the
AA spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
The resulting
data can be
presented in
a variety of
ways, but
typically a
print out is
made.
z14
Determine the concentration of a solution from a
calibration curve.
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AA can be used to identify the presence of an
element
(qualitative
analysis),
or
the
concentration of a metal (quantitative analysis)
Quantitative analysis can be achieved by
measuring the absorbance of a series of
solutions of known concentration.
A calibration curve and the equation for the line
can be used to determine an unknown
concentration based on its absorbance.
z15
Determine the concentration of a solution from a
calibration curve.
z16
Sample Problem:
Lead is extracted from a sample of blood and analyzed at 283 nm and gave an
absorbance of 0.340 in an AA spectrometer. Using the data provided, graph a
calibration curve and find the concentration of lead ions in the blood sample.
[Pb+2] (ppm) Absorbance
0.000
0.100
0.200
0.300
0.400
0.500
Calculated Pb (II) concentraions (ppm)
0.357
0.000
0.116
0.216
0.310
0.425
0.520
Absorbance
0.340
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The data provided in
the problem appears
in the upper left hand
corner of this MS
EXCEL worksheet.
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The graph was used
to calculate the best
fit line.
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The equation was
then used to
calculate the
concentration of Pb
(II) ions with an
absorbance of 0.340.
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The result, 0.357
ppm, is displayed
above the graph.
Lead (II) Calibration Curve
0.600
Absorbance
0.500
y = 1.0505x
R2 = 0.9988
0.400
0.300
0.200
0.100
0.000
0.000
0.100
0.200
0.300
[Pb+2] (ppm)
0.400
0.500
0.600
z17