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Atomic Spectroscopy(2)

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					Atomic Spectroscopy

      Chapter 9
    Review and Comparisons
• Atomic spectroscopy
  – Absorption and emission of UV-VIS light
    • Atoms and monoatomic ions
       – Conceptually similar to absorption and emission of UV-
         VIS light by molecules
    Review and Comparisons
• Atomic spectroscopy
  – Differences from UV-VIS
    • Limited to the elements
       – METALS
          » Most analysis is for metals!
    • Sample preparation
       – Place metals in water solution
           » Metals present as ions in water
           » Must have a means for converting metal ions into
             free gas phase ground state atoms to be measured
           » Called atomization
           » Uses large amount of thermal energy
   Review and Comparisons
• ATOMIC SPECTROSCOPY
 – Differences from UV-VIS
   • SAMPLE CONTAINER
      – SOURCE OF THE THERMAL ENERGY NEEDED FOR
        THE CONVERSION OF IONS IN SOLUTION TO
        ATOMS IN THE GAS PHASE
         » Atomizer
         » Does NOT resemble a cuvette
         » Has a flame container
   Review and Comparisons
• ATOMIC SPECTROSCOPY
 – Differences from UV-VIS
   • Various types of atomizer and instrument designs
      – Based on the same theory
 – Spectral line sources used as light sources
   • Instead of continuums sources in UV-VIS
      – Several require NO light source at all!
 – Limited types of analytes
   • .: Quantitation is well known
      – Using elements that are well characterized
         » Look up spectra in most reference text
    Summary of Techniques and
       Instrument Designs
• Most important aspect = thermal energy
  – Flame atomic absorption (flame AA)
  – Graphite furnace atomic absorption (graphite furnace
    AA)
  – Inductively coupled plasma atomic emission (ICP)
     • Less important
         – Flame emission and atomic fluorescence
     • Two that do not require thermal energy/minimal thermal
       energy
         – Cold vapor mercury system
         – Metal hydride generation
     • One that requires electrical energy
         – Arc and spark emission
   Summary of Techniques and
      Instrument Designs
• Flame AA
  – Large flame as the atomizer
    • Sample-solution-drawn into the flame by a vacuum
      mechanism
    • Atomization occurs immediately
    • Light beam for the absorption measurements
      directed through width of the flame
    Summary of Techniques and
       Instrument Designs
• Graphite furnace
  – Actually a small graphite tube
    • Quickly electrically heated to a very high
      temperature
       – Small volume of sample solution placed in tube
           » Manually with a micropipette
           » Drawn with a vacuum
       – Electrically brought to high temperature to atomize
         sample
       – Light beam directed through the tube and measured
           » There is a cloud of atoms
    Summary of Techniques and
       Instrument Designs
• ICP
  – Emission technique
        • Does not use a light source
        • Light measured is light emitted by the atoms/monoatomic ions in the
          atomizer
  – Atomizer
        • Extremely hot plasma
            – High-temperature ionized gas composed of electrons and positive ions
                » Confined by a magnetic field
        • Extremely high temperatures
            – Atoms and monoatomic ions undergo sufficient excitation
            – Relatively intense emission spectra result
  – Sample drawn with vacuum
  – Intensity of an emission line is measured and related to
    concentration
     Flame Atomic Absorption
• Flames and Flame Processes
  – After metal ions introduced into flame, several
    processes occur in rapid order (fig. 9.4, pg. 248)
     • Solvent evaporates
         – Leaves behind formula units
     • Dissociation of salt into atoms
         – Metal ions atomize/transformed into atoms
     • Atoms raised to excited states by thermal energy of the flame
         – A resonance process occurs
             » Atoms resonate back and forth between ground state and
               excited statte
     Flame Atomic Absorption
• Flames and Flame Processes
  – Only small % of atoms- <0.1%- found in excited state
    at any moment
     • Atoms drop back to ground state
     • Emission spectrum emitted
     • Atoms in the excited state
        – Emit light in the visible region of spectrum
        – Entire flame in element takes on color characteristic of the
          element that is in the flame
            » Each element has a characteristic color
            » It an atomic fingerprint
            » Possible to quantitate these elements using flame
               emission
 Flames and Flame Processes
• Flames and Flame Processes
  – Unexcited atoms in the flame- ≈99.9%
    • Available to be excited by a light beam
    • Light source used
    • Light beam directed through the flame
       – It is a Beer’s law experiment
       – Width of the flame being the pathlength
            » Flame temperature important both for the
               atomization and excitation process
 Flames and Flame Processes
• Flame Atomic Absorption
  – Flame requirements
    • Fuel and an oxidant
       – Natural gas and air
          » Max temperature ≈1800K
          » Does not sufficiently atomize most metal ions
          » Does not excite a sufficient # of atoms for
            quantitation
          » .: need something hotter!
       – Acetylene as fuel and air is the oxidant
          » Max temperature ≈2300K
        Spectral Line Sources
• Light sources emit spectral lines
  – Lines in the line spectrum of the analyte being
    measured
     • Preferred b/c they represent the precise wavelengths needed
       for the absorption in the flame
        – Flame contains this particular analyte
     • Emitted b/c they contain the analyte to be measured
        – When lamp is on
           » Internal atoms are raised to the excited state
           » Emit their line spectrum when they return to the ground
             state
           » This is the light directed through the flame
        Hollow Cathode Lamp
• Hollow Cathode Lamp
  – Most widely used spectral line source
  – Cathode
     • Negative electrode
     • Contains the internal atoms
         – Hollowed cup
         – Internal excitation and emission process occurs inside this cup
           when lamp is on
  – Anode
     • Positive electrode
         – Connected with cathode to a high voltage
         – Light emitted
        Hollow Cathode Lamp
• Hollow Cathode Lamp
  – Sealed glass tube
  – Filled with inert gas at low pressure
     • Neon or argon
  – How it works (fig.9.6, pg 251)
     • Lamp turned on and argon atoms ionize
         – Positively charged argon ions then crash into the negatively
           charged cathode
         – Causes sputtering
             » Transfer of surface atoms in the solid phase to the gas
               phase due to the collisions
     • More collisions of argon ions with metal atoms cause metal
       atoms to be raised to the excited state
         – Light emitted with they drop back to the ground state
      Hollow Cathode Lamp
• Hollow Cathode Lamp
  – Must contain the element being measured
    • Usually have number of different lamps in stock
       – Interchanged in the instrument
  – Some are multi-elemental
    • Several different specific atoms present in the
      lamp
       – Separated by a monochromator after the flame to isolate
         the specific spectral line of the analyte
              Premix Burner
• Premix burner
  – Burner for flame AA
  – All components-fuel, oxidant, and sample
    solution-are premixed
    • Take common path to the flame
    • Fuel and oxidant
       – Originate from pressurized sources
           » Compressed gas cylinders
       – Flow is controlled for optimum rate
              Premix Burner
• Sample solution
  – Aspirated by vacuum
    • Converted to aerosol/fine mist before mixing
    • Accomplished with a nebulizer at head of mixing
      chamber
       – Resembles nozzle to create a water spray (fig. 9.7, pg.
         252)
       – Connects to the sample tube
       – Pulls sample into the mixing chamber
       – Produces aerosol spray
              Premix Burner
• Aerosol spray
  – Emerges from nebulizer
    • Contains variable-sized solution droplets
       – Also mixed with oxidant and fuel
       – Contains impact device
           » Baffles or glass bead near tip of nozzle
           » Separates larger particles (fall to bottom of chamber)
           » ≈ 90% of sample never reaches flame
                    Optical Path
• Arranged in this order
  – Light source, flame (sample container),
    monochromator, and detector
  – Flame
     • Positioned in open area
         – Light can leak from room light and flame
     • Monochromator located between flame and detector
  – Detector
     • Receives alternating light signals
         – Source light and flame emissions
         – Flame emissions only
         – Detector able to eliminate flame emissions by subtraction
                Optical Path
• Either single-beam or double beam
  – Single-beam
    • Fewer problems than in UV-VIS
       – Seldom measure absorption spectra
       – Wavelength seldom changed
          » No need for re-calibration with blank as in UV-VIS
       – Source drift and fluctuations still exist
          » Minimized with improved electronics
                Optical Path
• Either single-beam or double beam
  – Double beam
    • Uses beam splitter
       – Diverts light from source around the flame
       – Two beams joined again before entering the
         monochromator
    • Eliminates problems due to source drift and noise
       – Source warm-up time eliminated since changes in
         intensity compensated fro
       – Rapid changeover of lamps possible
 Practical Matters and Applications
• Slits and Spectral Lines
  – More than one spectral line for an element
     • .:more than one line to choose from for setting the
       monochromator
     • One line gives the optimum absorptivity
         – Pick that one!!!!
         – Found on the HCL
         – Called the primary line
             » Monochromator usually set at that wavelength
         – Others called the secondary lines
             » May be chosen if 1O is inappropriate
             » If another element is the sample is similar to 1 O
         – Automated equipment usually set to primary line
 Practical Matters and Applications
• Slits and Spectral Lines
  – Slit control
     • Helps correct problem of close lines
         – Wider the slit
         – Greater the bandpass
             » More incidental/close spectral lines allowed to be captured
     • Usually choose between 0.2 and 2.0 nm
         – Value represents the bandpass for both entrance and exits slits
         – If interfering line at the optimum setting
               » Slit is narrowed
               » Or 2O line chosen
               » Both result in less desirable sensitivity
Practical Matters and Applications
• Hollow Cathode Lamp Current
  – Current is adjustable
    • Optimum setting represents most intense light
      without shortening the life of the lamp (VERY
      EXPENSIVE!)
• Lamp Alignment
  – Must have proper alignment for optimum
    intensity through the optical path
    • May need adjustment when changing lamps
 Practical Matters and Applications
• Interferences
  – Causes
     • Chemical sources
     • Spectral sources
 Practical Matters and Applications
• Chemical interferences
  – Result of problems with sample matrix
     • Viscosity/surface
         – May affect aspiration rate
         – Nebulized droplet size
• Standard additions method
  – Certain volume of the sample solution present in
    same proportion in all standard solutions
     • Equivalent to adding standard amounts of analyte to the
       sample solution
         – Solves interference problem
             » Sample matrix always present in same concentration
 Practical Matters and Applications
• Standard additions method
   – Prepare standards in usual way
       • Add a volume of the sample solution to each before diluting to the mark with
         solvent
       • Gives a series of standards which the concentration of analyte added known
   – Standard curve
       • It’s a plot of absorbance vs. concentration added instead of just
         concentration
            – Y-axis is not true position
                 » Offset to the right by the concentration of the zero-added concentration,
                    which is the sample solution
                 » Concentration of THIS solution is the concentration sought
            – To show on graph
                 » Curve extrapolated to intersect with the x-axis (y = 0)
                 » Represents the concentration of the amount added
                 » Precise concentration in the zero-added solution found using the equation of
                    the straight line
                 » Y=mX + b
        Spectral Interferences
• Spectral Interferences
  – Caused by substances in the flame
     • Absorb same wavelength as analyte
     • Causes absorbance measurement to be high
        – Rarely an element
        – If suspected, switch to 2O wavelength
  – More often caused by presence of light-absorbing
    molecules in the flame and light dimming due to small
    particles
     • Called background absorption
        – Fix by background corrections
            » Subtract background interference
            » It’s the purpose of the deuterium lamp
     Safety and Maintenance
• Safety issues with the AA
  – Acetylene, flame, and contamination of lab air
    with combustion products
     • Acetylene
        – Compressed gas cylinders must be secured to
          immovable object-the wall
        – Approved pressure regulators in place
        – Tubing free of leaks
        – Must have independently operated vent hood over flame
            » Removes excess solvent fumes
            » No volatile fumes near the flame!
     Safety and Maintenance
• Safety issues with the AA
  – Flashbacks
     • From improperly mixed fuel and air
        – When flow regulators ore improperly set
        – When air is drawn back through drain line of premix
          burner
  – Cleaning
     • Burner head and nebulizer
        – Ensures minimal noise level from impurities in flame
        – Carbon deposits in slit
           » Scrape with sharp knife or razor blade
                          Summary
• Sensitivity
   – Concentration of an element that will produce and
     absorption of 1%
      • Smallest concentration that can be determined with a
        reasonable degree of precision
• Detection limit
   – Concentration that gives a readout level that is double
     the electrical noise level inherent in the baseline.
      • Qualitative parameter in that it is the minimum concentration
        that can be detected
          –   Not precisely determined
          –   Would tell the analyst that the element is present
          –   Not at a precisely determinable concentration level
          –   Table 9.2 and 9.3, pg. 267

				
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Lingjuan Ma Lingjuan Ma MS
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