5-1

AA and Atomic Fluorescence Spectroscopy

Chapter 9• Sample Atomization• Atomic Absorption Instrumentation• Interference• Atomic Absorption Techniques• Atomic Fluorescence

• Sample Atomization 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

5-2

Technique

5-3

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

5-4

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

oxidantPrimary combustion zone* Blue luminescence due to C2 and CH* Thermal equilibrium not reached in

primary zone

5-5

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

5-6

Flame Structure

Primary zone

interzonalsecondary

Maximumtemperature

5-7

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

AgDoes not readily oxidizeAtomization over flame area

CrForms 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?

5-8

Absorbance Profile

5-9

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 2000-3000 °C Sample measured above heated surface

• High sensitivity for small samples

5-10

Electrothermal atomizer

Sample concentration

5-11

Graphite Furnace

5-12

Atomization Techniques

• Glow Discharge Sputtering of

samples due to Ar ion acceleration

Mixture of atoms and ions

• Hydride generator Forms volatile

speciesAs, Sb, Sn, Se, Bi, Pb

• Cold Vapor (Hg)

5-13

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

5-14

Atomic Absorption Instrumentation

• Light source Use source for element detection

For Na, use Na vapor lamp

* 3p to 2s transition at 589.6 nm Minimize line broadening

Doppler

Pressure

Temperature Need a separate light source for each element

5-15

Atomic Absorption Instrumentation

• Hollow Cathode Lamp Ionization of inert

gas by potential Gas acceleration to

cathode Atoms on cathode

into gas stateSome excitedDeexcite with photon emission

Need to excite specific elements for measurement

5-16

Atomic Absorption Instrumentation

• Electrodeless Discharge Lamps Inert gas in quartz

tube Excite gas with RF

Similar to cathode expect excitation

5-17

Spectrophotometers

• Single Beam Shutter controls

beam Collect blank

Blank provides 100% transmissionInsert sample and measure absorbance

5-18

Spectrophotometers

• Double Beam Light source split Measure light through

flame and light reference light

Determine %T

Does not consider light absorption in flame

5-19

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

5-20

Interference

• Chemical Interference More common than spectral interference Formation of compounds with low

volatilityAdditives to remove such compounds* EDTA

Dissociation equilibriaReaction of oxide species

Ionization equilibriaFormation of ion species, liberation of electron

5-21

Interference

5-22

Detection Limits

5-23

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