1. Elemental composition
Atomic spectroscopy
Elemental composition

2. Atoms have a number of excited energy levels accessible by visible-UV optical methods
Must have atoms (break up molecules)
Optically transparent sample of neutral atoms (flames, electrical discharges, plasmas)
Metals accessible by UV-Vis, non-metals generally less than 200nm where vacuum UV needed)

3. Atomic spectra Outer shell electrons excited to higher energy levels
Many lines per atom (50 for small metals over 5000 for larger metals)
Lines very sharp (inherent linewidth of nm)
Collisional and Doppler broadening (0.003 nm)
Strong characteristic transitions

4. Atomic Emission Schematic

5. Atomic spectroscopy for analysis
Flame emission - heated atoms emit characteristic light
Electrical or discharge emission - higher energy sources with more lines
Atomic absorption - light absorbed by neutral atoms
Atomic fluorescence - light used to excite atom then similar to FES

6. Flame Sources - remove solvent, free atoms, excite atoms
Nebulizer or direct injection
Dry solvent, form and dissociate salt
T= *C gives some neutral atoms
Thermal or light induced excitation
Neutrals can react (refractory cpd)
Molecular emission from gas give broad emission interferences)

7. General issues with flames
Turbulence / stability / reproducibility
Fuel rich mixtures more reducing to prevent refractory formation
High temperature reduces oxide interferences but decreases ground state population of neutrals (fluctuations are critical)

8. Chemical interferences - FES
Refractory compounds like oxides and phosphates (depends on matrix)
Reduce refractory formation by higher temp., add releasing agent (La) to complex anion, or complex cation (EDTA)
Ionization (electrons in flame depend on matrix)
Keep electrons high and constant with easily ionizes metal (LiCl)

9. High energy sources Reduce chemical interferences
Simultaneous multielement analysis
Introduction of solids
Electrical arcs and sparks (the first general elemental technique)
Plasma sources eliminate many problems with electrical arcs etc but require solutions

10. Atomic emission from spark or arc

11. Electrical ARC - sustained discharge between 2 electrodes
T= *C
Poor precision due to wander
Metal or graphite electrodes can be formed
Different materials volatilized at different rates so quantitization difficult

12. Electrical SPARK (AC)
More reproducible as there are multiple discrete electrical breakdowns in gas
T= up to 40,000*K
High precision but limited sensitivity (0.01% level)
Lots of electrical noise
Must integrate emissions over time

13. Multielement analysis
Simultaneous emission of many lines requires very high resolution
Gratings have capability to resolve if distances are great and overlapping orders are addressed

14. Measuring emission lines
Photographic (simple and inexpensive)
Sequential (scan through wavelengths with only a few seconds per line S/N) Advantages of being inexpensive & simple, but slow and irreproducible
Simultaneous (direct readout using PM tube at each exit slit) Fast (20-60 elements), precise, but expensive

15. Issues and tradeoffs Molecular interferences
Relative vs absolute sensitivity
Resolution vs S/N or limit of detection
Standard addition vs calibration curve
Emission vs AA or fluorescence

16. DC coupled plasma emission

17. Inductively Coupled Plasma

18. Inductively Coupled Plasma

19. AA Instrument Schematic

20. Atomic Absorption

21. AA instrumentation Radiation source (hollow cathode lamps)
Optics (get light through ground state atoms and into monochromator)
Ground state reservoir (flame or electrothermal)
Monochromator
Detector , signal manipulation and readout device

22. Hollow Cathode Lamp Emission is from elements in cathode that have been sputtered off into gas phase

23. Light Source
Hollow Cathode Lamp - seldom used, expensive, low intensity
Electrodeless Discharge Lamp - most used source, but hard to produce, so its use has declined
Xenon Arc Lamp - used in multielement analysis
Lasers - high intensity, narrow spectral bandwidth, less scatter, can excite down to 220 nm wavelengths, but expensive

24. Atomizers
Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced are important

25. AA - Flame atomization Use liquids and nebulizer
Slot burners to get large optical path
Flame temperatures varied by gas composition
Molecular emission background (correction devices )

26. Sources of error solvent viscosity temperature and solvent evaporation
formation of refractory compounds
chemical (ionization, vaporization)
salts scatter light
molecular absorption
spectral lines overlap
background emission

27. Atomizers
Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced is important
Graphite Furnace Atomizers - used if sample is too small for atomization, provides reducing environment for oxidizing agents - small volume of sample is evaporated at low temperature and then ashed at higher temperature in an electrically heated graphite cup. After ashing, the current is increased and the sample is atomized

28. Electrothermal atomization
Graphite furnace (rod or tube)
Small volumes measured, solvent evaporated, ash, sample flash volatilized into flowing gas
Pyrolitic graphite to reduce memory effect
Hydride generator

29. Graphite Furnace AA

30. Closeup of graphite furnace

31. Controls for graphite furnace

32. Detector Photomultiplier Tube Charge Coupled Device
has an active surface which is capable of absorbing radiation
absorbed energy causes emission of electrons and development of a photocurrent
encased in glass which absorbs light
Charge Coupled Device
made up of semiconductor capacitors on a silicon chip, expensive

33. Background corrections
Two lines (for flame)
Deuterium lamp
Smith-Hieftje (increase current to broaden line)
Zeeman effect (splitting of lines in a strong magnetic field)

34. Problems with Technique
Precision and accuracy are highly dependent on the atomization step
Light source
molecules, atoms, and ions are all in heated medium thus producing three different atomic emission spectra

35. Problems continued
Line broadening occurs due to the uncertainty principle
limit to measurement of exact lifetime and frequency, or exact position and momentum
Temperature
increases the efficiency and the total number of atoms in the vapor
but also increases line broadening since the atomic particles move faster.
increases the total amount of ions in the gas and thus changes the concentration of the unionized atom

36. Interferences
If the matrix emission overlaps or lies too close to the emission of the sample, problems occur (decrease in resolution)
This type of matrix effect is rare in hollow cathode sources since the intensity is so low
Oxides exhibit broad band absorptions and can scatter radiation thus interfering with signal detection
If the sample contains organic solvents, scattering occurs due to the carbonaceous particles left from the organic matrix

37. Interferences continued

39. Gas laser

40. Dye laser

41. Diode laser