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AA and Atomic Fluorescence Spectroscopy Chapter 9

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Presentation on theme: "AA and Atomic Fluorescence Spectroscopy Chapter 9"— Presentation transcript:

1 AA and Atomic Fluorescence Spectroscopy Chapter 9
Sample Atomization Atomic Absorption Instrumentation Interference Atomic Absorption Techniques Atomic Fluorescence For techniques samples need to be atomized Techniques are useful for element identification Molecular information destroyed by atomization Flame Atomization Sample nebulized Mixed with fuel Carried to flame for atomization

2 Technique

3 Flame Atomization Evaporation of solvent Produces molecular aerosol
Molecules dissolution leads to atomic gas Atoms ionize to product cations and electrons Property of flame can affect process Fuel Gas Oxygen ºC Air ºC Methane 2810 1957 Ethane 1960 Propane 2820 1980 Butane 1970 Hydrogen 2660 2045 Acetylene 3100 2400

4 Flame ionization Flame temperature in range of 1700 °C to 2400 °C in air From 2500 °C to 3100 °C with oxidant Need to keep flame stable Flame structure Different zones are properties of fuel and oxidant Primary combustion zone Blue luminescence due to C2 and CH Thermal equilibrium not reached in primary zone

5 Flame ionization Interzonal region Central part of flame
High concentration of free atoms Used for spectroscopy Secondary combustion region Convert elements to oxides Disperse sample to air

6 Flame Structure secondary interzonal Primary zone Maximum temperature

7 Best location for absorbance?
Variation 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 Atomization over flame area Cr Forms oxidizes readily so that oxide is main species in flame Need to consider based on flame sample area Does instrument sample entire flame or just small area?

8 Absorbance Profile

9 Electrothermal Atomization
Atomization of entire sample in short period Average sample time in optical path is seconds Evaporation of sample Microliter volume Low temperature Sample ashed at higher temperature Increase current Sample temperature goes to °C Sample measured above heated surface High sensitivity for small samples

10 Electrothermal atomizer
Sample concentration

11 Graphite Furnace

12 Atomization Techniques
Glow Discharge Sputtering of samples due to Ar ion acceleration Mixture of atoms and ions Hydride generator Forms volatile species As, Sb, Sn, Se, Bi, Pb Cold Vapor (Hg)

13 Atomic Absorption Instrumentation
Radiation Source Sample Holder Wavelength selector Detector Radiation sources AA has narrow lines (0.005 nm) Most light sources provide light with greater bandwidths Absorption of source light Need narrow source

14 Atomic Absorption Instrumentation
Light source Use source for element detection For Na, use Na vapor lamp 3p to 2s transition at nm Minimize line broadening Doppler Pressure Temperature Need a separate light source for each element

15 Atomic Absorption Instrumentation
Hollow Cathode Lamp Ionization of inert gas by potential Gas acceleration to cathode Atoms on cathode into gas state Some excited Deexcite with photon emission Need to excite specific elements for measurement

16 Atomic Absorption Instrumentation
Electrodeless Discharge Lamps Inert gas in quartz tube Excite gas with RF Similar to cathode expect excitation

17 Spectrophotometers Single Beam Shutter controls beam Collect blank
Blank provides 100% transmission Insert sample and measure absorbance

18 Spectrophotometers Double Beam Light source split
Measure light through flame and light reference light Determine %T Does not consider light absorption in flame

19 Interference Spectral interference Overlap of sample spectra
Not very common due to narrow line widths If occurs select different transition Scattering Formation of oxides Correct with different methods Two line method Continuum source Zeeman effect Polarize and split light with magnetic field

20 Interference Chemical Interference
More common than spectral interference Formation of compounds with low volatility Additives to remove such compounds EDTA Dissociation equilibria Reaction of oxide species Ionization equilibria Formation of ion species, liberation of electron

21 Interference

22 Detection Limits

23 Atomic Fluorescence Spectroscopy
optical emission from gas-phase atoms that have been excited to higher energy levels Enhancement of sensitivity over AA Examine electronic structure of atoms Light source Hollow Cathode Lamp Laser Detection Similar to AA

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