HL Chemistry - Option A : Modern Analytical Chemistry

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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 Na
? (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.
Sample Compartment
Light Source
Detector
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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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Light beam
Sample is vaporized in the flame.
Aspirator tube sucks the sample into the flame
in the sample compartment.
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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.

• 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.