Docstoc

ATOMIC ABSORPTION SPECTROSCOPY - PowerPoint - PowerPoint

Document Sample
ATOMIC ABSORPTION SPECTROSCOPY - PowerPoint - PowerPoint Powered By Docstoc
					  HL Chemistry - Option A:
 Modern Analytical Chemistry




ATOMIC ABSORPTION
  SPECTROSCOPY
    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)
      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
    λ (nm)   214    248     325    423     589
     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]
  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.
Overview of AA         Sample
spectrometer.        Compartment




      Light Source             Detector
    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
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.
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).
  Light beam

  Sample is
  vaporized
 in the flame.


   Aspirator
 tube sucks the
sample into the
  flame in the
     sample
 compartment.
    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
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
     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.
 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.
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.
A.6.4 Determine the concentration of a solution from a
                 calibration curve.
                                                Sample Problem: pg. 312, #3
         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                   Calculated Pb (II) concentraions (ppm)   Absorbance
           0.000           0.000                         0.357                        0.340
                                                                                                        •   The data provided in
           0.100
           0.200
                           0.116
                           0.216
                                                                                                            the problem appears
           0.300
           0.400
                           0.310
                           0.425
                                                                                                            in the upper left hand
           0.500           0.520
                                                                                                            corner of this MS
                                   Lead (II) Calibration Curve
                                                                                                            EXCEL worksheet.

               0.600
                                                                                                        •   The graph was used
                                                                                                            to calculate the best
               0.500
                                                y = 1.0505x
                                                                                                            fit line.
                                                R2 = 0.9988
               0.400                                                                                    •   The equation was
  Absorbance




                                                                                                            then used to
               0.300
                                                                                                            calculate the
               0.200                                                                                        concentration of Pb
                                                                                                            (II) ions with an
               0.100
                                                                                                            absorbance of 0.340.
                                                                                                        •
               0.000
                   0.000       0.100    0.200        0.300         0.400           0.500        0.600
                                                                                                            The result, 0.357
                                                [Pb+2] (ppm)                                                ppm, is displayed
                                                                                                            above the graph.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:201
posted:5/19/2012
language:Latin
pages:17