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									   Department of Chemistry
Analytical and Environmental Chemistry
                (CH-204)

        Atomic Spectroscopy

          Michael J. Hynes

                                         1
            Atomic Spectroscopy
•  Lecturer
    – Michael J. Hynes
• Texts
(1) Quantitative Chemical Analysis , by Daniel C. Harris
    Pub. Freeman. Chapter 21 (4th Edition)
(2) Atomic Absorption and Emission Spectroscopy by
   E. Metcalfe & F.E. Prichard, Pub. Wiley (ACOL series).
(3) Most Analytical Chemistry Textbooks.
(4) Handout



                                                        2
    Principle of Atomic Absorption pectroscopy
•   Atomic Absorption (AA) is based on the principle that a
    ground state atom is capable of absorbing light of the
    same characteristic wavelength as it would emit if
    excited to a higher energy level.
•    In flame AA, a cloud of ground state atoms is formed by
    aspirating a solution of the sample into a flame of a
    temperature sufficient to convert the element to its atomic
    state.
•   The degree of absorption of characteristic radiation
    produced by a suitable source will be proportional to the
    population of ground state atoms in the flame, and hence
    to the concentration of the element in the analyte.

                                                            3
    Compound            Heat
                                        Atoms
Spectra of atoms consist of SHARP LINES.
Each element has a characteristic spectrum.
Due to sharpness of lines, there is little overlap between the
spectral lines of different elements.
Therefore, there is little interference.
         Atomic Spectroscopy
                  High
    Sample                                 Vapour
              Temperature
Measure absorbance or emission of the atomic vapour.
Atomic spectroscopy deals with atoms.
Fe2+ and Fe3+ will not be distinguished.
                                                           4
5
                       Sensitivity
•   Atomic spectroscopy is very sensitive for most elements.
•   Concentrations at the ppm level may be routinely
    determined using flame atomisation.
•   Using electrothermal atomisation, concentrations at the
    ppb may be determined.
•   1 ppm = 10-6g/g or 1g/g
•   The density of dilute aqueous solutions is approximately
    1.00 so that:
    1 g/g of aqueous solution = 1g/ml = 1 ppm
    1 ppm Fe = 1 x 10-6 g Fe/ml = 1.79 x 10-5 mol dm-3
    1 ppm = 1 second in 11.6 days
    1 ppb = 1 second in 31.7 years.
                                                           6
Atomic Absorption Spectroscopy
Absorbance = -log(It/I0)
Io = incident radiation (on sample)
It = transmitted radiation.

Atomic Emission Spectroscopy
Absorbance = -log(It/I0)
I0 =intensity of radiation reaching detector
    in the absence of sample.
It = intensity of radiation reaching detector
     when sample is being aspirated.


                                                7
•   Both methods are used to determine the
    concentration of an element in solution.
•   Both methods use a standard curve.
•   Difference between UV and IR spectroscopy
    is that sample must be atomised.
•   Sample may be atomised by:
    (1) A flame
    (2) Electrically heated furnace
    (3) A Plasma


                                                8
                                      x
Abs                   W1/2           x


                        
  Band Width = W1/2 = width of band at half the height



Abs                                          W1/2 =
               25 nm+                        0.003 nm


                                           
      Molecular Absorption Band        Atomic Vapour
                                      Absorption Band
                                                   9
 Atomic Absorption and Emission Lines
                        Resonance Lines
                                     E3(Excited state)

                                       E2(Excited state)

                                       E1(Excited state)
                                  E
                                        Eo(Ground state)
Absorption             Emission
  Most Intense Line

  3 Absorption Lines       E = E1 - E0 = h = hc/
  6 Emission lines
                                                   10
Comparison of the atomic emission spectrum of
silicon compared with the molecular absorption
spectrum of ethanal.
                                                 11
                              Sample (Atoms in flame)

Source    Chopper
          h               h               h Detector    i
 HCL               Flame        Monochromator                       Amplifier
           Io                It
                   Burner                                                  V


                   Cloud                                             Output
   Fuel           Chamber                                            e.g. PC/Printer



Oxidant           Nebuliser
 (Air)                           Waste
                                                    HCL = Hollow-cathode Lamp
                Sample uptake

         Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer



                                                                                   12
13
14
Laminar Flow Burner




                      15
EXAMPLES OF FLAME TEMPERATURES
           Fuel        Oxidant       Temperature
                                        (K)
         Acetylene        Air         2400-2700

         Acetylene   Nitrous Oxide    2900-3100

         Acetylene     Oxygen         3300-3400

         Hydrogen         Air         2300-2400



Acetylene flow rate: 1000 ml/minute
Air flow rate:        5000 ml/minute
Sample uptake:          2 ml/minute
 80% of sample goes to waste.
Therefore, dilution factor =  15,000
Highly inefficient!
                                                   16
 Effect of temperature in atomic spectroscopy

     Boltzman Distribution
                      E1 (g = 2)                    (Excited state)
       E1
                      E0 (g = 1)                     (Ground state)
                     E
          N1  g1  kT1                              g is the multiplicity
              e
          N o  go 
               

Na      E1 = 3.37 x 10-19 J/atom
At 2600 K
                               -3.37 x 10-19
                    2
              N1
              No    e   1.381 x 10- 23 )(2600)
                                                     1.67 x 104
                    1

 At 2610 K N1/No = 1.74 x 10-4
                                                                             17
Effect of Temperature on Sodium Atoms
   Temp          % Ground % Excited
     K             State    State
    2600           99.9833           0.0167

    2610           99.9826           0.0174



The effect of a 10 K temperature rise on the
ground state population is negligible (0.02 %).

In the excited state the fractional change is
             0.0174 - 0.0167100  4 %
                     0.0167
  So!                                              18
          Effect of Temperature
• Small changes in flame temperature
    (10 K) have little effect in atomic
    absorption but have significant effects in
    atomic emission spectroscopy.
•   Must have good flame control in atomic
    emission spectroscopy.



                                            19
                                             Evaporation
                      + -           + -
                    M X           M X                           MX
                   Solution       Mist                          Solid
                                                                   Vapourisation

                     Thermal     M (gas)         Dissociation
            *
           M (g)                                                MX
                                 X (gas)
                    excitation                                  Gas
   Flame
           emission                   Absorption of radiant             Measure for atomic
                                       energy h
      h                                                                absorption spectroscopy
 Measure for
 flame emission                              *
 spectroscopy                              M (g)
                                                                         o
                                           Re-emit radiation at 90           Measure for atomic
                                           (Atomic fluorescence)             fluorescence spectroscopy

Process by which gaseous atoms are produced in flames


                                                                                                   20
                              Sample (Atoms in flame)

Source    Chopper
          h               h               h Detector    i
 HCL               Flame        Monochromator                       Amplifier
           Io                It
                   Burner                                                  V


                   Cloud                                             Output
   Fuel           Chamber                                            e.g. PC/Printer



Oxidant           Nebuliser
 (Air)                           Waste
                                                    HCL = Hollow-cathode Lamp
                Sample uptake

         Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer



                                                                                   21
                  Radiation Source
•   Beer’s Law only applies to monochromatic radiation.
•   In practice, monochromatic implies that the linewidth of
    the radiation being measured is less than the bandwidth
    of the absorbing species.
•   Atomic absorption lines very sharp with an inherent
    linewidth of 0.0001 nm.
•   Due to Doppler effect and pressure broadening, linewidths
    of atoms in a flame are typically 0.001 - 0.01 nm.
•   Therefore, we require a source having a linewidth of less
    than 0.01 nm.
•   Typical monochromator has a bandwidth of 1 nm i.e. x100
    greater than the linewidth of the atom in a flame.

                                                         22
          Hollow Cathode Lamp

•   Used because of the requirement for a source
    of narrow lines of the correct frequency.
•   Hollow Cathode lamp
     – filled with argon or neon at a pressure of 130
       - 170 Pa (1 - 5 torr)




                                                  23
Monochromator bandwidth
(~100x greater than atomic lines




                                   24
Hollow Cathode Lamp




                      25
    Reactions in the Hollow Cathode Lamp
•  Apply sufficiently high voltage between the cathode and
   the anode:
(1) Ionization of the filler gas:
        Ne + e- = Ne+ + 2e-
(2) Sputtering of the cathode element (M):
        M(s) + Ne+ = M(g) + Ne
(3) Excitation of the cathode element (M)
        M(g) + Ne+ = M*(g) + Ne
(4) Emission of radiation
        M*(g)  (M(g) + h

                                                        26
               Hollow Cathode Lamp
•   The cathode of the hollow cathode lamp (HCL)
    contains the element being analysed.
•   Therefore the atomic radiation emitted by the HCL
    has the same frequency as that absorbed by the
    analyte atoms in the flame or furnace.
•   The linewidth from the HCL is relatively narrow
    (compared to linewidths of atoms in the flame or
    furnace) because of low pressure in lamp and lower
    temperature in lamp (less Doppler broadening).
•   Thus the linewidth from the HCL is nearly
    “monochromatic” (vs sample).
•   Different lamp required for each element although
    some are mulit-element.
                                                         27
28
Hollow Cathode Lamp - The Filler Gas




                                 29
    Electrothermal Atomisation - Graphite Furnace

• Sample holder consists of a graphite tube.
• Tube is heated electrically
• Beam of light passes through the tube.
• Offers greater sensitivity than flames.
• Uses smaller volume of sample
   – typically 5 - 50 l (0.005 - 0.05 ml).
• All of sample is atomised in the graphite tube.
• Atomised sample is confined to the optical path for several
    seconds (residence time in flame is very short).
•   Uses a number of stages as shown on next slide.
                                                         30
31
       Stages in a Graphite Furnace
•   Typical conditions for Fe:
     – Drying stage: 125o for 20 sec
     – Ashing stage 1200o for 60 sec
     – Atomisation 2700o for 10 sec

•   Requires high level of operator skill.
•   Method development difficult.




                                             32
Schematic Diagram of a Graphite Furnace




      h
HCL




                                    33
Advantages and Disadvantages of Flame AAS
• Advantages
  – equipment relatively cheap
  – easy to use (training easy compared to furnace)
  – good precision
  – high sample throughput
  – relatively facile method development
  – cheap to run
• Disadvantages
  – lack of sensitivity (compared to furnace)
  – problems with refractory elements
  – require large sample size
  – sample must be in solution                        34
          Advantages and Disadvantages of
             Electrothermal Atomisation

•   Advantages
     – very sensitive for many elements
     – small sample size
•   Disadvantages
     – poor precision
     – long cycle times means a low sample throughput
     – expensive to purchase and run (argon, tubes)
     – requires background correction
     – method development lengthy and complicated
     – requires a high degree of operator skill (compared to
       flame AAS)
                                                          35
                              Sample (Atoms in flame)

Source    Chopper
          h               h               h Detector    i
 HCL               Flame        Monochromator                       Amplifier
           Io                It
                   Burner                                                  V


                   Cloud                                             Output
   Fuel           Chamber                                            e.g. PC/Printer



Oxidant           Nebuliser
 (Air)                           Waste
                                                    HCL = Hollow-cathode Lamp
                Sample uptake

         Figure 1. Schematic Diagram of an Atomic Absorption Spectrophotometer



                                                                                   36
                 Monochromator
• The operation and sensitivity of the atomic absorption
    spectrometer spectrometer depends on the spectral
    band width of the resonance line emitted by the
    primary radiation source (the hollow cathode lamp).
•   The function of the monochromator is to isolate the
    resonance line from non-absorbing lines close to it in
    the source spectrum and from background continua
    and molecular emissions originating in the flame.
•   Hollow cathode lamps emit a number of lines, in the
    case of multi-element lamps, the number can be
    quite large and it is necessary to isolate the line of
    interest.

                                                         37
           Detector/Measuring System
• The intensity of the line source is measured with a
    photomultiplier, which produces an electrical signal
    proportional to the intensity of the incident light.
•   This signal is amplified and processed electronically to produce
    an output which on older instruments was read on a digital or
    analogue meter in either absorbance or concentration mode.
•   In modern instruments, the output is usually displayed on the
    computer screen of the PC controlling the instrument.
• In order to eliminate the unwanted emissions from the flame, the
    light source is modulated by a chopper which is located
    between the hollow cathode lamp and the flame. The amplifier
    which modifies the signal from the photomultiplier is tuned in to
    the same frequency .
•   (Alternatively, the hollow cathode lamp may be modulated by
    applying an AC voltage at say 50 Hz.).


                                                                    38
A   B




        39
               Interferences
• Interference is any effect that changes
    the signal when analyte concentrations
    remain unchanged.
•   While atomic absorption spectroscopy is
    relatively free from interferences, there
    are a number of interferences which
    must be dealt with.



                                            40
            Spectral Interference
• This refers to overlap of analyte signals
    with signals originating from other
    elements in the sample or with signals due
    to the flame or furnace.
• Example:
    – Al    308.216 nm
    –V      308.211 nm
•   Solution:
    – Separate elements or use a different line
      (which may be less sensitive).              41
               Chemical Interference
    Formation of Stable or Refractory Compounds
• Elements that form very stable compounds
    are said to be refractory because they are
    not completely atomised at the temperature
    of the flame or furnace.
•   Solution
    – Use a higher flame temperature (nitrous
      oxide/acetylene)
    – Use a release agent
    – Use protective chelation
                                                42
                 Examples
•   Determination of calcium in the presence of sulfate or
    phosphate (e.g. in natural waters)
    3Ca2+ + 2PO43- = Ca3(PO4)2
                   (stable compound)
•   Release agent
    Add 1000 ppm of LaCl3
    2LaCl3 + Ca3(PO4)2 = 3CaCl2 + 2La(PO4)
    CaCl2 readily dissociates
•   Protective chelation
    Ca3(PO4)2+3EDTA = 3Ca(EDTA) + 2PO43-
    Ca(EDTA) dissociates readily.
                                                             43
             Ionisation Interference
•         M(g)  M+(g) + e-
•   A problem in the analysis of alkali metal ions at low flame
    temperatures and other elements at higher temperatures.
•   Because alkali metals have the lowest ionisation
    potentials, they are most extensively ionised in flames.
•   At 2450 K and a pressure of 0.1 Pa, sodium is 5% ionised.
•   Potassium is 33% ionised under the same conditions.
•   Ionised atoms have energy levels which are different to
    the parent atoms
     – therefore the analytical signal is reduced.

                                                          44
                      Solution
  • Add an ionisation suppressor
                  [M  ][e- ]
               K
                    [M]
                     [M  ][e- ]
               [M] 
                        K
Add an easily ionised element such as Cs.
Add 1000 ppm of CsCl when analysing Na or K.

Cs is more readily ionised than either Na or K.
This produces a high concentration of electrons in the
flame.
                                                    45
                    Matrix effects
•   The amount of sample reaching the flame is
    dependent on the physical properties of the solution:
    – viscosity
    – surface tension
    – density
    – solvent vapour pressure.
• To avoid differences in the amount of sample and
    standard reaching the flame, it is necessary that the
    physical properties of both be matched as closely as
    possible.
•   Example:
     – Analysis of blood
                                                        46
             Non-Atomic Absorption
•   Non-atomic absorption is caused by molecular
    absorption or light scattering by solid particles in the
    flame.
•   The absorption measurement obtained with a hollow
    cathode lamp is the sum of the atomic absorption and
    the non-atomic absorption.
•   The interference is corrected for by making a
    simultaneous measurement of the non-atomic
    absorption using a continuum source (usually
    deuterium)
     – this is called background correction

                                                          47
        Operational Parameters
 • Sensitivity
    – the concentration of an element which will
      reduce the transmission by 1%

            It       99 
Abs  - log   - log
           I                0.00436
            0        100 
This corresponds to an absorption of 0.00436



                                                   48
              Calculations

• For an absorbance of 1.0 we require
 1.0/0.00436 = 230 times the sensitivity.
• For Cu, sensitivity = 0.05 ppm
• For an absorbance of 1 we require a
 concentration of 11.5 ppm.
• Using scale expansion of 10 we can usually
 obtain an absorbance of 1 using a 1 ppm
 solution of copper.
                                            49
                 Detection Limit
• The concentration of an element that
    gives a signal equal to three times the
    peak to peak noise level of the baseline.
•   Measure the baseline while aspirating a
    blank solution.
    Absorbance




                           Peak to peak noise level

                         Time

                                                      50
Terms to understand in atomic spectroscopy (1)

 •   Atomic absorption spectroscopy
 •   Atomic emission spectroscopy
 •   Atomisation
 •   Background correction
 •   Boltzman distribution
 •   Chemical interference
 •   Detection limit
 •   Graphite furnace
 •   Hollow-cathode lamp
                                           51
Terms to understand in atomic spectroscopy (2)
 •   Inductively coupled plasma
 •   Ionisation interference
 •   Ionisation suppressor
 •   Matrix
 •   Matrix modifier
 •   Nebulisation
 •   Premix burner
 •   Releasing agent
 •   Spectral interference
                                           52
      Applications of AAS
• Agricultural analysis
  – soils
  – plants
• Clinical and biochemistry
  – whole blood, plasma and serum Ca, Mg,
   Li, Na, K, Cu, Zn, Fe etc.
• Metallurgy
  – ores, metals and alloys


                                            53
      Applications of AAS

• Lubricating oils
  – Ba, Ca, Mg and Zn additives
• Greases
  – Li, Na, Ca



                                  54
      Applications of AAS
• Water and effluents
  – many elements e.g. Ca, Mg, Fe, Si, Al, Ba
• Food
  – wide range of elements
• Animal feedstuffs
  – Mn, Fe, Co, Cu, Zn, Cr, Se
• Medicines
  – range of elements
                                            55

								
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