atomic absorption 2
TRANSCRIPT
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HL Chemistry - Option A: Modern Analytical Chemistry
ATOMIC ABSORPTION SPECTROSCOPY
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A.6.1 State the use of Atomic Absorption (AA) spectroscopy.
• 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
• 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)
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A.6.2 Describe the principles of AA.
• When metals are exposed to heat, they absorb light.
• Each metal absorbs light at a characteristic frequency. For example:
Metal Zn Fe Cu Ca Naλ
(nm)214 248 325 423 589
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A.6.2 Describe the principles of AA.
• 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]
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A.6.3 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.
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Overview of AA spectrometer.
Light Source Detector
SampleCompartment
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A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
• 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
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A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
• 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.
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A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
• 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).
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Sample is vaporized
in the flame.
Aspirator tube sucks thesample into the
flame in thesample
compartment.
Light beam
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A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light
source, monochromatic detector, read out.
• 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
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A.6.3 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
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A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,
monochromatic detector, read out.
• The read out specified by the user is displayed on the computer screen for each sample measured.
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A.6.3 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.
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A.6.4 Determine the concentration of a solution from a calibration curve.
• 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.
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A.6.4 Determine the concentration of a solution from a calibration curve.
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Sample Problem: pg. 312, #3Lead 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 Calculated Pb (II) concentraions (ppm) Absorbance
0.000 0.000 0.357 0.3400.100 0.1160.200 0.2160.300 0.3100.400 0.4250.500 0.520
Lead (II) Calibration Curve
y = 1.0505x
R2 = 0.9988
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.000 0.100 0.200 0.300 0.400 0.500 0.600
[Pb+2] (ppm)
Ab
so
rba
nc
e
• The data provided in the problem appears in the upper left hand corner of this MS EXCEL worksheet.
• The graph was used to calculate the best fit line.
• The equation was then used to calculate the concentration of Pb (II) ions with an absorbance of 0.340.
• The result, 0.357 ppm, is displayed above the graph.