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Atomic Absorption

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					 Atomic Absorption
  October 4, 2006
   Latest Review Article:
 K.W. Jackson& S. Lu, 1998,
Anal.Chem., 70 (12), 363 -384,
For atomic absorbance, fluorescence or emission need to break sample up
     into atom to observe atomic spectra
Basic steps involved in atomization of solution sample
a) nebulization – solution sample, get into fine droplets by spraying thru
     thin nozzle or passing over vibrating crystal.
b) desolvation - heat droplets to evaporate off solvent just leaving analyte
     and other matrix compounds
c) volatilization – convert solid analyte/matrix particles into gas phase
d) dissociation – break-up molecules in gas phase into atoms.
e) ionization – cause the atoms to become charged
f) excitation – with light, heat, etc. for spectra measurement.

          Sample Atomization:
        expose sample to flame or
           high-temperature
  Elemental Analysis - Atomic Spectroscopy
Introduction
      Based on the breakdown of a sample into atoms, followed by the
      measurement of the atom’s absorption or emission of light.
i. deals with absorbance fluorescence or emission (luminescence) of atoms or
      elemental ions rather then molecules
          - atomization: process of converting sample to gaseous atoms or
      elementary ions
ii. Provides information on elemental composition of sample or compound
          - UV/Vis, IR, Raman gives molecular functional group information,
      but no elemental information.
iii. Basic process the same as in UV/Vis, fluorescence etc. for molecules


                             E1
            h
                             Eo
               Absorbance                           Fluorescence
           Atomic Spectroscopy
• Based on the breakdown of a sample into atoms, followed
  by the measurement of the atom’s absorption or emission
  of light.
   – deals with absorbance fluorescence or emission
      (luminescence) of atoms or elemental ions rather then
      molecules
   – atomization: process of converting sample to gaseous
      atoms or elementary ions
• Provides information on elemental composition of sample
  or compound
• Basic process the same as in UV/Vis, fluorescence etc. for
  molecules
   – UV/Vis, IR, Raman gives molecular functional group
      information, but no elemental information
Differences for Molecular Spectroscopy
  •  no vibration levels  much sharper absorbance, fluorescence, emission
     bands
  • position of bands are well-defined and characteristic of a given element
  • qualitative analysis is easy in atomic spectroscopy (not easy in molecular
                        spectroscopy)
Examples:

carbon


oxygen



nitrogen
Energy Level Diagrams
   energy level diagram for the outer electrons of an element describes atomic
   spectroscopy process.
        i. every element has a unique set of atomic orbitals
        ii. p, d, f split by spin-orbit coupling
        iii. Spin (s) and orbital (l) motion create magnetic fields that perturb each other
   (couple)         - parallel  higher energy; antiparallel  lower energy
                                                         • Similar pattern between atoms but
                                                           different spacing

                                                         • Spectrum of ion different to atom

                                                         • Separations measured in
                                                           electronvolts (eV)
                                                               1eV =1.602x10-19 J
                                                               = 96. 484 kJ ×mol-1
                                                        • As number of electrons increases,
                                                          number of levels increases
                                                             Emission spectra become more
               Na                            Mg+             complex
                                                             Li 30 lines, Cs 645 lines, Cr 2277
                                                             lines
                                                 Note slight differences in energy due to
                                                 magnetic fields caused by spin
Desire narrow lines for accurate identification
         Broadened by
               i. uncertainty principle
               ii. pressure broadening
               iii. Doppler effect
               iv. (electric and magnetic fields)

                                        Uncertainty principal:
                                                 Dt . DE ≈ h
                                                   
                                                  Dt . D ≈ λ
                                        Dt – minimum time for measurement
                                        D – minimal detectable frequency difference




Peak line-width is defined as width in wavelength at half the signal intensity
  Doppler effect
  - emitted or absorbed wavelength changes as a result of atom movement relative
  to detector
  - wavelength decrease if motion toward receiver
  - wavelength increases if motion away from receiver




Usage in measurement of velocity of galaxies, age of universe and big bang theory

Pressure broadening:
Collisions with atoms/molecules transfers small quantities of vibrational
energy (heat) - ill-defined ground state energy

Effect worse at high pressures:
        • For high pressure Xe lamps (>10,000 torr) turns lines into continua!
Effect of Temperature on Atomic Spectra
      - temperature changes number of atoms in ground and excited states
      - need good temperature control

                                       Boltzmann equation


    N1 and No – are the number of atoms in excited and ground states
            k – Boltzmann constant (1.28x10-23 J/K)
             T – temperature
           DE – energy difference between ground and excited states
    P1 and Po – number of states having equal energy at each quantum level


Na atoms at 2500 K, only 0.02 % atoms in first excited state!

Less important in absorption measurements - 99.98 %
atoms in ground state!
Atomic Absorption Spectroscopy (AAS)
   – commonly  used for elemental analysis
    – expose sample to flame or high-temperature
    – characteristics of flame impact use of atomic absorption
   spectroscopy




 Flame AAS:
 • simplest atomization of gas/solution/solid
 • laminar flow burner - stable "sheet" of flame
 • flame atomization best for reproducibility (precision) (<1%)
 • relatively insensitive - incomplete volatilization, short time in
Different mixes and flow rates give different temperature profile
   in flame
    - gives different degrees of excitation of compounds in path of
        light source
Types of Flame/Flame Structure – selection of right region in flame
important for optimal performance
    a) primary combustion zone – blue inner cone (blue due to emission from C2, CH &
       other radicals)
          - not in thermal equilibrium and not used
    b) interconal region
          - region of highest temperature (rich in free atoms)
          - often used in spectroscopy
          - can be narrower in some flames (hydrocarbon) tall in others (acetylene)
    c) outer cone
          - cooler region                                  Temperature varies significantly across flame –
                                                           need to focus on part of the flame
          - rich in O2 (due to surrounding air)
          - gives metal oxide formation



                          Primary region for
                          spectroscopy


                        Not in thermal equilibrium
                        and not used for spectroscopy



 Flame profile: depends on type of fuel and
 oxidant and mixture ration
Most sensitive part of flame for
AAS varies with analyte


         Consequences:
         - Sensitivity varies with element
         - must maximize burner position
         - makes multi-element detection difficult
Basic instrument design (Flame atomizer)


    Single beam




    Double beam
Atomizers:      1) Laminar Flow Burner
        - adjust fuel/oxidant mixture for optimum excitation of desired
                 compounds
        - usually 1:1 fuel/oxidant mix but some metals forming oxides
                 use increase fuel mix
        - different mixes give different temperatures.




                                     2) Laminar – nonturbulent
                                        streamline flow
                                     •   sample, oxidant and fuel are mixed
                                     •   only finest solution droplets reach burner
                                     •   most of sample collects in waste
                                     •   provides quite flame and a long path
                                         length
Electrothermal (L’vov or Graphite furnace)
       - place sample drop on platform inside tube
       - heat tube by applying current, resistance to current creates heat
       - heat volatilizes sample, atomizers, etc. inside tube
       - pass light through to measure absorbance




                      Po




                                                 P
     Place sample droplet on platform
a) Electrothermal (L’vov or Graphite furnace) :
advantages:
        - all sample used
        - longer time of sample in light beam
        - lower limit of detection (LOD)
        - can use less sample (0.5 – 10)
disadvantage:
        - slow (can be several minutes per element or sample)
        - not as precise as flame (5-10% vs. 1%)
        - low dynamic range (< 102, range of detectable signal intensity)
        - use only when there is a need for better limit of detection or have
           less sample than Laminar flow can use
b) Laminar Flow Burner
advantages:
   - good b (5-10 cm)
   - good reproducibility

disadvantages:
   - not sample efficient (90-99% sample loss before flame)
   - small amount of time that sample is in light path (~10-4 s)
   - needs lots of sample
Light source
       - need light source with a narrow bandwidth for light output
                 AA lines are remarkably narrow (0.002 to 0.005 nm)
       - separate light source and filter is used for each element




 problem with using typical UV/Vis continuous light source
       - have right λ, but also lots of others (non-monochromatic light)
        - hard to see decrease in signal when atoms absorb in a small
          bandwidth
        - only small decrease in total signal area
        - with large amount of elements  bad sensitivity
Solution is to use light source that has line emission in
  range of interest
     - laser –but hard to match with element line of interest
      - hollow cathode lamp (HCL) is common choice          Coated with
                                                                  element to be
                                                                  analyzed


 Hollow Cathode Lamp




  Process: use element to detect element
           1. ionizes inert gas to high potential (300V)
                     Ar  Ar+ + e-
           2. Ar+ go to “-” cathode & hit surfaces
           3. As Ar+ ions hit cathode, some of deposited element is excited and
             dislodged into gas phase (sputtering)
           4. excited element relaxes to ground state and emits characteristic radiation
  - advantage: sharp lines specific for element of interest
  - disadvantage: can be expensive, need to use different lamp for each element tested.
Source Modulation (spectral interference due to flame)
          - problem with working with flame in AA is that light from flame and light source
            both reach detector
          - measure small signal from large background
          - need to subtract out flames to get only light source signal (P/Po)

i. done by chopping signal:
                    Flame + P




                  Flame only
                                    P                  Flame + P

ii. or modulating P from lamp:



                                              Flame only
                                                                              time
Corrections For Spectral Interferences Due to Matrix
     - molecular species may be present in flame
     - problem if absorbance spectra overlap since molecular spectrum is much
              broader with a greater net absorbance
     - need way of subtracting these factors out
Methods for Correction
   1) Two-line method
    - monitor absorbance at two l close together
          > one line from sample one from light source
          > second λ from impurity in HCL cathode, Ne or Ar gas in HCL, etc
    - second λ must not be absorbed by analyte
          > absorbed by molecular species, since spectrum much broader
    - A & e are ~ constant if two λ close
    - comparing Al1, Al2 allows correction for absorbance for molecular species

          Al1 (atom&molecule) – Al2 (molecule) = A (atom)


Problem: Difficult to get useful second λ with desired characteristics
 Continuous source method
          - alternatively place light from HCL or a continuous source D2 lamp thru flame
          - HCL  absorbance of atoms + molecules
          - D2  absorbance of molecules




advantage:
     -available in most instruments
     -easy to do
disadvantage:
     -difficult to perfectly match lamps (can give + or – errors)
Zeeman Effect
    - placing gaseous atoms in magnetic field causes non-random orientation of atoms
    - not apparent for molecules
    - splitting of electronic energy levels occurs (~ 0.01 nm)
    - sum of split absorbance lines  original line
    - only absorb light with same orientation
    - can use Zeeman effect to remove background                    Background
                 place flame polarized light through
                 sample in magnetic field get
                 absorbance (atom+molecule) or                     z          z
                 absorbance (molecule) depending
                 on how light is polarized
                                                               *         *         *

                                                             Background+Absorbance




                                                       z * z
Chemical Interference - more common than spectral interference;
Formation of Compounds of Low Volatility
        - Anions + Cations  Salt
                 Ca2+ +SO42-  CaSO4 (s)
        - Decreases the amount of analyte atomized  decreases the absorbance signal
        - Avoid by:
                  increase temperature of flame (increase atom production)
                  add “releasing agents” – other items that bind to interfering ions
                           eg. For Ca2+ detection add Sr2+
                                    Sr2+ + SO42-  SrSO4 (s)
                                    increases Ca atoms and Ca absorbance
                  add “protecting agents” – bind to analyte but are volatile
                           eg. For Ca2+ detection add EDTA4-
                                    Ca2+ + EDTA4-  CaEDTA2-  Ca atoms 2)
Formation of Oxides/Hydroxides
        M + O  MO
                                 non-volatile & intense molecular absorbance
                             A
        M + 2OH  M(OH)2

                 - M is analyte
        - Avoid by:
                   increase temperature of flame (increase atom production)
                   use less oxidant
           Atomic Absorption
• Measure of the energy absorbed from ground
  stated to excited state for a particular element
• Element reduced then atomized in plasma
• Plasma radiated by CRT of element of choice and
  the measure of the decrease in radiative power
  related to [ ]
• A single wavelength is chosen for each element
  and a Monochromator used
• Drawback – Elements one at a time
Sodium Energy Level Diagram
                   Na Spectrum Lines:
Used to Calculate Effective Nuclear Charge




  Sala, et al, J.Chem. Ed., 1999, 76(9) 1269.
                  Elements & Limits
                          Detection                                  Detection
Element       l, Å        mg/mL        Element                l, Å   mg/mL
Aluminum      3093         1.00        Lead                   2170   0.30
Antimony      2176         0.50        Magnesium              2852   0.01
Arsenic       1937         5.00        Mercury                2536   10.00
Barium        5536         8.00        Nickel                 2320   0.13
Bismuth       2231         1.00        Potassium              7665   0.03
Cadmium       2288         0.03        Silicon                2516   3.00
Calcium       4227         0.08        Silver                 3281   0.10
Chromium      3579         0.05        Sodium                 5890   0.03
Copper        3248         0.10        Tin                    2863   5.00
Iron          2483         0.10        Zinc                   2139   0.03

 Slavin, Atomic Absorption Spectroscopy, 1968, Interscience, p61.
Response Due to Height in Flame




Robinson, Atomic Absorption Spectroscopy, p69, Marcel Dekker, Inc.
      Hollow Cathode Ray Tube &
              Nebulizer




Menzies, Anal. Chem., 1960, 32(8), 900.
Absorption vs [ ]
Absorption of
 Resonance
    Line
           Noble Metal Curves




Menzies, Anal. Chem., 1960, 32(8), 900.
                     1st Spectrums




          Cobalt and Lead
                                      Mercury



Gray, Anal. Chem., 1975, 47(4), 600
BIAF-AAS




           A. Gaspar & H Berndt,
           1999, Anal. Chem, 72(1),
           240-46.
BIAF-AAS con.
              signal height                                   signal area


                         detection limit
                                                                            detection limit (ng/mL)
                         (ng/mL)


                                                  improve
                                                                                                      improvement of
              RSD                                 ment of
      g/mL)              BIFF-AAS          PN                 RSD (%)       BIFF-AAS          PN      power of
              (%)                                 power of
                                                                                                      detection
                                                  detection
Cda   0.1     2.3        0.25              17     69          1.7           0.078             15      193
Hga   1       2.0        67                3600   54          2.1           19                3900    202
Ag    0.1     2.7        1.7               9.6    6           2.2           0.22              9.4     43
As    20      3.7        1000              1400   1.5         2.8           220               1300    6
Au    0.5     2.6        13                110    8           3.1           4.8               170     35
Bi    1       3.1        120               580    5           3.2           24                1100    47
Cu    0.2     2.3        1.4               31     22          2.2           0.44              28      64
In    1       3.8        110               310    3           3.7           30                310     10
K     0.2     2.8        1.7               6.9    4           2.9           0.26              7.9     31
Pb    0.5     2.4        25                660    26          2.4           6.8               690     101
Pd    1       2.8        55                560    10          2.5           14                570     40
Rb    1       4.2        23                83     4           3.9           4.7               100     22
Sb    1       4.0        49                740    15          3.9           12                690     57
Se    20      2.7        110               2000   18          3.3           23                1700    75
Te    1       4.5        47                690    15          4.0           9.6               660     69
Tl    0.5     3.3        6.9               160    23          2.4           2.1               150     71
Zn    0.1     2.2        0.4               9.6    24          1.9           0.13              12      92

				
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