2. Atomic spectroscopy Elemental composition

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

4. Atomic spectra ä Outer shell electrons excited to higher energy levels ä Many lines per atom (50 for small metals over 5000 for larger metals) ä Lines very sharp (inherent linewidth of 0.00001 nm) ä Collisional and Doppler broadening (0.003 nm) ä Strong characteristic transitions

5. Atomic Emission Schematic

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

7. Flame Sources - remove solvent, free atoms, excite atoms ä Nebulizer or direct injection ä Dry solvent, form and dissociate salt ä T= 1700-3200 *C gives some neutral atoms ä Thermal or light induced excitation ä Neutrals can react (refractory cpd) ä Molecular emission from gas give broad emission interferences)

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

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

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

11. Atomic emission from spark or arc

12. Electrical ARC - sustained discharge between 2 electrodes ä T=4000-6000*C ä Poor precision due to wander ä Metal or graphite electrodes can be formed ä Different materials volatilized at different rates so quantitization difficult

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

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

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

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

17. DC coupled plasma emission

18. Inductively Coupled Plasma

20. AA Instrument Schematic

21. Atomic Absorption

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

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

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

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

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

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

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

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

30. Graphite Furnace AA

31. Closeup of graphite furnace

32. Controls for graphite furnace

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

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

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

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

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

38. Interferences continued

40. Gas laser

41. Dye laser

42. Diode laser