Atomic Absorption Spectrometry py

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					                                                                 Flame Atomic Absorption
                                                                      Spectrometry
                                                                      Spectrometry

      Atomic Absorption                                • What is FAAS and how it is used in
                                                           analytical applications?
         p         y
        Spectrometry
                                                       • Why do we need a line source in FAAS?

                                                       • What are the interferences in FAAS?
  Applied Analytical and Inorganic Chemistry Program
     Department of Chemistry, Faculty of Science       • How can we perform BG correction?
                   Mahidol University
                                                         What               i      i
                                                       • Wh t are your experiences in FAAS?




    Flame Atomic Absorption                                                                    p
                                                                                          Absorption
         Spectrometry
         Spectrometry
                                                                                                     Ej
• What is FAAS and how it is used in                    E1
 analytical applications?

• Why do we need a line source in FAAS?
                                                        E0                                           Ei
• What are the interferences in FAAS?
                                                              Molecular Absorption                        Atomic Absorption
• How can we perform BG correction?

  What               i      i
• Wh t are your experiences in FAAS?                   Emol = Eelectronic + Evibration + Erotation           Eatom = Eelectronic




Absorption Spectrophotometric Instrument                      Atomic Absorption Process
              I0      I



                                           molecule

         I0                  I




                                           atom
                  Process in Flame
                                                   Volatilization &
            Nebulization           Desolvation     Atomization
                                                                      Free atoms
                                                                          (M0)

Sample solution            Spray               Dry
                                                   l
                                             aerosol
                                                                               Excitation

                                                  I i ti
                                                  Ionization
                                                                      Excited atoms
                               Ions   (M+)                                  (M*)



                              This t    h ld b
                              Thi step should be
                                  prevented!                                                     Alan Walsh and the First Atomic Absorption Spectrometer




           Basic AA Spectrometer                                                                           Absorption Law

                                                                                            • Similar to molecular absorption, Beer’s law is
                                                                                              used.

                                                                                                    g(    )
                                                                                              A = log(P0/P)   = kbN
                                                                                              where    P0     incident radiant power
                                                                                                       P      radiant power
                                                                                                                      p
                       •Radiation source                                                               k      constant (instrumental and chemical factors)
                        Atomization
                       •Atomization cell                                                               b      p       g
                                                                                                              pathlength
                       •Wavelength selector                                                            N      number of atom/unit volume on light path
                       •Detector
                       •Read out




    Characteristic Concentration                                                                          Detection Limits
    Flame Atomic Absorption                           Requirement of Radiation Source
         Spectrometry
         Spectrometry                                             i AAS
                                                                  in

• What is FAAS and how it is used in                 • Give light at the specified wavelength

 analytical applications?                            • Emission bandwidth is narrower than atomic
                                                       absorption line (0.01 nm)
                                                            p          (       )
• Why do we need a line source in FAAS?
                                                     • Sufficient intensity
• What are the interferences in FAAS?

• How can we perform BG correction?

  What               i      i
• Wh t are your experiences in FAAS?




    e S Continuum g t Sou ce
 Line VS Co t uu Light Source                            e S Continuum g t Sou ce
                                                      Line VS Co t uu Light Source
                                   Line source                         absorption line
 Abs                                                                                        I0/I ~ 1

                                                                         continuum source   Abs
                                                                                            Ab ~ 0



                                  Continuum source
                                                                       absorption line
                                                                                            I0/I ~ 2
                                                                                            Abs ~ 0.3
                            concentration                               line source




                 Non-
       Cause for Non-Linearity                         Absorption Line Broadening
   o abso b g ad at o
• Non-absorbing radiation
  – From another line of the same element in the
    radiation source
  – From fill-gas in the hollow cathode
  –SStray radiation
• Halfwidth of radiation source is wider
  than the absorption line of element
• Less sensitive absorbing radiation
  (      g             g    )
• (See Figure 1.6 in Page 12)
          Line Broadening                  Doppler Broadening
    atu a a     dt
• Natural Halfwidth
  (Heisenberg’s Uncertainty)

• Doppler Broadening

• Collisional Broadening




        p
   Absorption Line ~ 0.01 nm             Hollow Cathode Lamp




Hollow Cathode Lamp: Process                 o o cat ode a p
                                            Hollow cathode lamp
                               •   300 V applied between anode (+) and metal cathode (-)
                               •   Ar ions bombard cathode and sputter cathode atoms
                               •   Cathode made of metal of interest (Na, Ca, K, Fe...)
                               •   Hollow cathode to maximize probability of re-deposition
                                   on cathode restricts light direction
    Linewidth Broadening and Self-Reversal
                       g     Self-                              Electrodeless Discharge Lamp




      Electrodeless d sc a ge lamp
        ect ode ess discharge a p                                 Atomic Absorption Process
•   1-2 orders of magnitude more intense than HCLs
•   no electrodes
•   inert gas and metal of interest
•   intense rf or microwave accelerate ions that excite atoms




    Atomization Technique                                                 Atomization Cell
                                                                    a spo t sa p e aerosol     a e
                                                                • Transport sample ae oso to flame
                                                                • Desolvation
                                                                • Atomization


                                                                  – Nebulizer burner
                                                                         Process in Flame
                                                                                                                  Volatilization &
                                                              Nebulization                       Desolvation      Atomization
                                                                                                                                     Free atoms
                                                                                                                                         (M0)

                                                Sample solution                     Spray                     Dry
                                                                                                                  l
                                                                                                            aerosol
                                                                                                                                                 Excitation

                                                                                                                 Ionization
                                                                                                                 I i ti
                                                                                                                                     Excited atoms
                                                                                            Ions     (M+)                                  (M*)
                          Process occurring
                          P              i
                          during atomization                                              This t    h ld b
                                                                                          Thi step should be
                                                                                              prevented!




              Sample Processes                           Atomization of Mg in Flame
                                                                                  I0                                                   I
Nebulization:
              MX(solution)   →    MX(aerosol)
Desolvation:
              MX(aq)         →    MX(s)
             (sublimation):
Vaporization (           )
                ( )
              MX(s)           →     (g)
                                  MX(g)
                                                MgCl2(solution)                  → MgCl2(s), MgO(s)                                        Evaporation
Atomization:
              MX( )
              MX(g)           →   M( ) + X( )
                                  M(g) X(g)     MgCl2(s) or MgO(s) → MgCl2(g) or MgO(g)                                                    Volatilization
                                                                                                                                           V l ili i

Excitation:                                     MgCl2(g) or MgO(g) → Mg0 + Cl0 + O0                                                        Atomization

              M(g) + hυ      →    M*(g)




                                                      Flame Pneumatic Nebulizer
                                                                     Adjustment of position of inner capillary




                                                                                                                Typical uptake rate: ~5 mL/min
                                                                                                                Typical delivery efficiency: ~5%
                                                          http://www.chemistry.nmsu.edu/Instrumentation
                                                                               Laminar-
                                                                             A Laminar-Flow Burner




                  Types of Flames                                                    Regions in a flame and
                                                                                      temperature profile
                                                                                      t       t       fil




           Fuel/Oxidant Ratio                                                                                                               y
                                                                                                                                      Analyte distribution
                                                                                                                                      in flame




Oxidizing Flame     Stoichiometric Flame   Reducing Flame

Fuel-lean flame
F ll      fl                               Fuel-rich fl
                                           F l i h flame
                                                            Rann, C. S.; Hambly, A. N. Distribution of atoms in an atomic absorption flame.   Anal. Chem. (1965), 37(7), 879-84.
                                                                   Best Location for
   Flame Absorption Profiles
                                                                Ab   b     M         t
                                                                Absorbance Measurement
                                                        Variation d t th d        f id ti f
                                                      • V i ti due to the degree of oxidation for
                                                        a given element
                                                            –       Mg
                                                                     •    Atomizes then oxidizes as Mg approaches
                                                                          secondary combustion area
                                                                            – Formation of MgO reduces absorbance
                                                            –       Ag
                                                                     •    Does not readily oxidize
                                                                     •    At i ti over flame area
                                                                          Atomization       fl
                                                            –       Cr
                                                                     •                       y                        p
                                                                          Forms oxides readily so that oxide is main species
                                                                          in flame

                                                      • A different portion of the flame should be
                                                        used for the determination of each element




Atomic Absorption Spectrometry:                                                                                                              What
                                                                                                                                            would I
Things to be considered                                                                                                                      see?



        I0                  I




                                                                         •Emission from flame
                                                                         •Emission from atom in flame
                                                                         •Radiation from light source




 Elimination of Emission from Flame                                          Light Modulation
   Problem
 • P bl                                                                                                                                      Chopper
   – Continuous emission from flame
   – Atomic line emission from atom in flame
 • How to solve the problem
   – Place monochromator between flame and
     d t t
     detector                                                                                                                                AC operated
                                                                                                                                             source
   – Modulation of radiation source
     • Chopper
     • AC operated source
                                               Source: Concepts, Instrumentation and Techniques in Atomic Absorption Spectrophotometry, Richard D. Beaty
                                               and Jack D. Kerber, Second Edition, THE PERKIN-ELMER CORPORATION
             Source Modulation
                                                                                  Source Modulation
 • A chopper or a modulated
   power supply is used to                           Transparent
                                                                    I
   modulate the source radiation
   that         th   h th
   th t passes through the
   atomizer (flame).                               Opaque




 • The chopper rotates resulting in an alternating atomic                   Iae + Ime       Iae + Ime + I0   Iae + Ime + I
                                    signal.
   absorption and atomic emission signal
                                                                                                                             Time
         g
 • The signal from emission of radiation will be
   continuous and can be subtracted from the modulated                    Without sample
   AA signal.                           AA (AC)                                   p
                                                                          With sample
                                         AE (DC)




    Flame Atomic Absorption
                                                                                        Interferences
         Spectrometry
         Spectrometry
• What is FAAS and how it is used in                                      • Physical Interference
 analytical applications?                                                 • Chemical Interference

• Why do we need a line source in FAAS?                                     Ionization I t f
                                                                          • I i ti Interference
                                                                          • Spectral Interference
• What are the interferences in FAAS?

• How can we perform BG correction?

  What               i      i
• Wh t are your experiences in FAAS?




       Physical Interferences                                        o e o O ga c So e t         a e
                                                                    Role of Organic Solvent in Flame
                                                                   • Increase nebulization efficiency
                                                                        – Lower surface tension
                                                                        – Finer drop size
                                                                        – More aerosols reach flame




 • Matrix Interference from Viscosity
   Effect
           Physical Interference                                Standard Addition Technique
• Viscosity of sample – causes variation in
  particle size and uptake rate
                                                                                              no interference       spiked sample
                                 organic

                                       aqueous


                                           viscous medium                                                       aqueous standards




                g
• Matrix matching and standard addition method
  can be used to overcome the problem




          Chemical Interference                                                        Chemical Interference
                                                                  Formation of Compounds of Low Volatility
 • Formation of compounds of low volatility
     – Oxide formation                                                                                                  Al + La3+
                                                                                                                             L
                                                                      recovery of Sr




     – Stable compound formation                                                                                        Si + La3+
                                                                             y




 • Shift in dissociation equilibria                                                                                     Al
                                                                     %r




                                                                                                                        Si
 • Shift in ionization equilibria
   (ionization interference)                                                                                       Concentration of Al, Si


                                                            Effect of Al and Si on Sr determination (with and without La3+)




          Chemical Interference                                                        Chemical Interference
                                                               Ca c u
                                                             • Calcium
                                                                 – Protective agent
                                                                   Ca2+ + PO43- + EDTA → Ca(EDTA)

                                                      3-
                                C i th presence of PO43
                                Ca in the        f
                                                                 – Releasing agent
                                                                   Ca3(PO4)2 + Sr2+ → Sr3(PO4)2 + Ca2+
                                                                      ( O      S      S ( O



                                                                               y, g               p
                                                                   Alternatively, higher flame temperature such as
 •   Interference of Phosphate on Calcium Determination            N2O-C2H2 may be used.
      Chemical Interference:
                                                            Ionization Equilibria
        i Di     i ti E ilib i
  Shift in Dissociation Equilibria
                                                  • Ionization of atoms is small and can often be
             VOx ↔ V + Ox                           neglected
             AlOx ↔ Al + Ox                       • At higher temperature flames where oxygen of
             TiOx ↔ Ti + Ox                         nitrous oxide serves as the oxidant, ionization
                                                    becomes important
• In the fuel-rich flame condition, enhancement                                         p p
                                                  • Ionization causes reduction in atom population
    f     di      b     ti  i b        d i th
  of vanadium absorption is observed in the
  presence of Al or Ti
• In the fuel-lean flame condition, Al or Ti
  scarcely changes the concentration of Ox.
  Therefore, the equilibrium is not disturbed
  significantly




                                                   Degree of Ionization of Metals at
               M ↔ M+ + e-
                                                        Fl     T        t
                                                        Flame Temperatures

             K = [M+][e-]/[M]

                 problem,
To overcome this problem ionization
suppressor may be used

                B ↔ B+ + e-
IP of B > IP of M




      Ionization Suppressor                               Ionization Suppressor




                                                   • Effect of Added Potassium on Ionization
       Spectral Interferences                                                Spectral Interference:
                                                                       Atomic absorption line of other elements
              p
• Atomic absorption line of other elements
• Broadband absorption or particulate scattering
  resulted from the presence of combustion                Vanadium line at 308 211 nm i t f
                                                        • V    di   li    t 308.211               i th
                                                                                       interferes in the
  products                                                determination of aluminum based on its
                                                          absorption line at 308.215 nm
• Broadband absorption or particulate scattering
    i i t df      th      l    ti
  originated from the sample matrix
                                                        • Al line at 309.27 nm should be used instead
                                                           f
                                                          of the line at 308.215 nm




        Spectral Interference
                                                   Spe al In rfere e
                                                   S ectra nter enc
        pp g         y        p
• Overlapping of analyte absorption line with
  matrices absorption (non-specific absorption,
  molecular absorption)
                  p    )

                         Ba absorption




                              CaOH molecular
                              absorption
                                                                            i    ti i t d th t C    ill i t f     i B
                                                                         It is anticipated that Ca will interfere in Ba
                                                                         determination. The effect is easily eliminated by
                      Ba line source 553.6 nm                                 i hi h t           t   fl      t d
                                                                         using higher temperature flame to decompose CaOHC OH




    Flame Atomic Absorption                                      g                       q
                                                             Background Correction Techniques
         Spectrometry
         Spectrometry
                                                          Use f         lamp (continuous)
                                                        • U of D2 or H2 l    (   ti     )
• What is FAAS and how it is used in                                   – Alignment might be a problem
                                                                       – Require extra radiation source
 analytical applications?
                                                                 Two Line
                                                        • Use of Two-Line Correction Method
• Why do we need a line source in FAAS?
                                                        • High Current (Smith-Hieftje)
• What are the interferences in FAAS?                                  – Alignment is not a problem
                                                                       – No extra radiation source required
• How can we perform BG correction?                                    – Short lift time of radiation source

                                                        • Zeeman Background correction
  What               i      i
• Wh t are your experiences in FAAS?
             g
      D2 Background Correction                                                                  g
                                                                           Continuum Source Background Corrector




                y             y     p
     AA + BG by HCL and BG by D2 lamp
                                                                      Source: Concepts, Instrumentation and Techniques in Atomic Absorption Spectrophotometry, Richard D. Beaty
     So, the difference is AA                                         and Jack D. Kerber, Second Edition, THE PERKIN-ELMER CORPORATION




 Atomic and Background Absorptions                                                Simplified Timing Diagram
     Continuum source
I0

                                                                            Abs
                                                                                                 analyte                                    analyte

                  p
          no absorption               p
                          atomic absorption       g           p
                                              background absorption


                                                                                                   BG                   BG                    BG                    BG
I0 Line source


                                                                                                                                                                          Time


          no absorption   atomic absorption   background absorption




                                                                           Limitations of Continuum Source
Continuum Source BG Correction
                                                                                B k        d Correction
                                                                                Background C       ti
                          0.424)
 • With HCL (absorbance = 0 424)                                           1. Requires additional continuum light source(s) and
      – Abs (BG) = log (I0/I) = log (100/50) = 0.3                            electronics
      – Abs (AA) = log (I0/I) = log (100/75) = 0.124                       2. Requires the intensities of the primary and
                                                                              continuum sources to be similar
 • With D2 (absorbance = 0.300)
                                                                           3. Two continuum sources are required to cover the
      – Abs (BG) = log (I0/I) = log (100/50) = 0.3
                                                                              full wavelength range
      – Abs (AA) = log (I0/I) = log (100/100) = 0                          4. Requires critical alignment of the continuum and
 • Difference                                                                 primary sources for accurate correction
                                                                           5. May be inaccurate for structured background
     0.424 – 0.300 = 0.124
                                                                                   p
                                                                              absorption
                                                                         Self Reversal
 Two-Lines Background Correction
 Two-
                                       BG    AA + BG
                                                                     Background Correction
                                                              Smith-Hieftje Background Correction Method

                                                           • High currents produce large
                                                             concentrations of nonexited
                                                             atoms, which are capable of
                                                             absorbing the radiation produced
                                                             from the excited species
                                                           • High current also broadens the
                                                             emission band of the excited
The reference line may be from an impurity in the hollow     species
cathode or a nonabsorbing line of the analyte




 Self Reversal Background Correction:
                                                               ee a   ac g ou d Co ect o
                                                              Zeeman Background Correction
             S ith-Hieftje
             Smith
             Smith-Hi ftj



                                   Low current:
                                   both AA and BG
                                   absorb
Intensity


                                   High current:
                                   only BG absorbs




        Flame Atomic Absorption
             Spectrometry
             Spectrometry
• What is FAAS and how it is used in
   analytical applications?

• Why do we need a line source in FAAS?

• What are the interferences in FAAS?

• How can we perform BG correction?

  What               i      i
• Wh t are your experiences in FAAS?