1. Atomic Absorption & Emission Spectroscopy
Chapter 5

2. Sub Topics Introduction to Atomic Spectroscopy
Atomic Absorption Spectroscopy AAS
Atomic Emission Spectroscopy
AES
Interferences and Comparisons

3. Atomic Absorption and Emission Spectroscopy
What is it used for?
Study of trace metals in the environment
Examples:
Contamination of water
Food stuffs
Petrol products
Quality control
Qualitative and Quantitative method for >70 elements with detection levels in ppm/ppb range
Fast and convenient with high selectivity

4. Three Types Done in the Gas Phase Why is this significant?
No vibrational or rotational energy levels, only electronic transitions
Therefore spectra made of quantised spectral lines
Spectral ranges of interactions? 180 – 800 nm (UV-Vis)
Where is electromagnetic radiation absorbed / emitted in each spectroscopy?

5. Spectrum
Related to Electronic Configuration(s)

6. Basic Instrumentation
Requirements: Atoms / Ions in gas phase AES: No Radiation Source AAS: As Shown AFS: Radiation Source at 90’ to Detector

7. Atomisation Sample needs to be in gas phase
Atomisation is therefore crucial
Several ways:
AES
Inductively Coupled Plasma (ICP)
AAS
Flames
Electrothermal Atomizers
Two Types of Atomisation
Continuous Sample Continuously Introduced
Discrete Sample introduced by syringe/autosampler

8. Sub Topics Introduction to Atomic Spectroscopy
Atomic Absorption Spectroscopy AAS
Atomic Emission Spectroscopy
AAS
Interferences and Comparisons

9. Process of Atomisation
Nebulization Production of mist / spray Commonly used for solutions. Solids: Electric Spark or Laser
Sample
Spray
Dry-Aerosol
Free Atoms / Molecules / Ions
Nebulization
Desolvation
Volatilization

10. Example of Atomisation Process Flames
Geometry of a Flame
Different Fuels
Different fuels give different temperatures
Normal Flame Alkali/Alkaline Metals ’C
O2 or N2O Heavy Metals ’C

11. Sample Introduction and Atomisation
Pneumatic Nebulizer Continuous
Achieved using Venturi Effect
Jet effect to form mist / spray

12. Interferences
Any effect that changes signal from proper signal Blank Interferences Spectral Analyte (multiplicative) Interferences Physical (matrix) Chemical Ionisation

13. Spectral Interferences
Independent of Analyte concentration From elements other than Analyte E.g. Na ( nm) overlaps Mg (285.21)nm Can subtract blank Na spectrum Results in Line Broadening Spectral lines have finite widths, but determined by spectrometry properties Not needed to know different types

14. What if too concentrated?

15. Sub Topics Introduction to Atomic Spectroscopy
Atomic Absorption Spectroscopy AAS
Atomic Emission Spectroscopy
AES
Interferences and comparisons

16. Inductively Coupled Plasma
Plasma Atomisers Plasma is a conducting gaseous mixture containing cations and anions. Argon typically used for AAS Once Argon ions formed, can absorb enough power from external source to reach high temperatures so further ionisation can occur (T > 10,000 K) Power Sources DC Electric Powerful Radio Frequency (RF) Generators Powerful Microwave Frequency (MW) Generators (ICP) RF / ICP provide best advantage, sensitivity and freedom from interference DC Plasma cheaper and simpler

17. Inductively Coupled Plasma
Three concentric quartz tubes End of torch is a RF induction coil which induces magnetic field Gases (Ar or N2) flow in outer tubes to cool torch. Stabilizes plasma. Torch is seeded with electrons using spark. H accelerates to high energy to ionise gases. White hot fireball formed (8000 – K)

18. Inductively Coupled Plasma
Sample (from nebulizer, + Ar gas) is injected into it via central tube. Advantages Higher Excitation Temperature than any flame Lower detection limits Wide range of metals and non-metals Disadvantages Many Lines Initial and Running Costs Large! Other Points Long, Linear Calibration Curves (4-5 orders magnitude) Useful for both Atomic Spectroscopy and Mass Spectrometry

19. Comparison ICP vs DC ICP Advantages Chemically inert environment
Unlike a flame (violent, highly reactive)
Temperature cross-section constant
Thin optical path
Reduces self-absorption
Calibration linear over large orders of concentration
Disadvantages
Does not like organic solvents
Clogging due to carbon build up
DC and other Plasma Sources
Advantages
Less Argon Needed
Can cope with organic solvents
Less expensive
Less sensitive
Good Reproducibility
Disadvantages
Useful for atoms, not ions
Good alignment of optics
Electrodes need to be replaced

20. Recap so far Elemental Analysis Need Light Source
ICP Most common source
Need Light Source
Flame most common source

21. Some more equations
Intensity is donated by 𝐼= 𝐼 0 𝑒 − 𝑘 𝑣 𝑙𝑐 Atomic absorption transmission is given by 𝑇= 𝐼 𝐼 0 = 𝑒 − 𝑘 𝑣 𝑙𝑐 Therefore, with some higgery-pokery Absorbance is given by 𝑨=ε𝒄𝒍

22. Absorbance
𝐴=− log 𝑇 =ε𝑐𝑙 Characteristic Concentration The concentration in mg/L which gives 1% absorbance for a certain element 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝐶ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 = 𝑥 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛𝑜𝑓𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 Why? 1% absorbance = 99% transmission Log(1/0.99) =

23. Requirements for Source
Atomic Absorption Spectroscopy AAS
Narrow resonance line profile from lamp
Stable and reproducible output
Enough intensity for good S/N ratio
Durability
Easy Operation
Hollow Cathode Lamp
Good Choice!

24. Atomization
AAS Flame Air or N2O combined with acetylene Eletrothemal Atomizer Graphite furnace Background Corrections may be required Chemical Interference Continuum Source Corrections Pulsed Hollow Cathode Lamp Corrections Zeeman Effect

25. Electrothermal Atomizers: Graphite Furnace AAS
Drawbacks of AAS/AES/AFS in General Large volumes Poor efficiency nebulization Aqueous solutions required Electrothermal Atomizer: Graphite Furnace (AAS/AES only) Heated with inert purge gas All sample is atomized in confined space! Small volume in furnace with a syringe Programmed heating

26. GF-AAS Procedure Introduce sample Add modifier Drying Pyrolysis/Ashing
5-50 microlitres or solid
Add modifier
Drying
110’C
Pyrolysis/Ashing ’C
Evaporation of matrix?
Atomisation and measurement time resolved ’C
Clean out
Cool down
Whole process takes milliseconds!

27. GF-AAS Interferences
Corrections are needed Matrix Modifier Dual Lamp setup T selective programing Zeeman Correction

28. Zeeman Effect Correction
Strong Magnetic Field splits degenerate spectral lines into several lines according to their Mj values, which have different polarization characteristics Example, Magnesium

29. Zeeman Background Correction

30. Sub Topics Introduction to Atomic Spectroscopy
Atomic Absorption Spectroscopy AAS
Atomic Emission Spectroscopy
AES
Interferences and comparisons

31. Interferences
Ionization Loss of atoms as ions – give completely different spectra Can be supressed by adding ionization suppressor (e.g. Cs) to sample and reference Chemical Key problem for AAS Formation of chemical species which prevent/promote dissociation, giving rise to depressed/enhanced signals Requires releasing/protective agents

32. Calibration
Very Important Encompass range of expected values for samples Blank Triplicate to determine detection limit Standard addition method required for high matrix samples

33. Comparison
What is less prone to spectral interferences? AAS AES Temperature affects signal more in? AES / AFS Because ground state population is largest

34. Comparison
Rank the following in Limit of Detection? AES, ICP-AES, AAS, GF-AAS, AFS AES>AFS>AAS>ICPAES>GF-AAS Rank the following in dynamic working range? ICP-AES>AFS>AES>AAS>GF-AAS

35. Other Comparisons
AAS Ratio method removes systematic errors but sensitivity affected by difference in two large quantities ICP-AES and AFS can be multi element, less so than ASS AAS most cost effective, ICP-AES most versatile AFS good for vapour phase elements Cost of instrumentation ICP-AES>GF-AAS>AAS>AFS>AES