Atomic Spectroscopy

1. Atomic Spectroscopic Methods Covered in Ch 313: • Optical Atomic Spectrometry (Ch 8-10) • Atomic X-ray Spectrometry (Ch 12) • Atomic Mass Spectrometry (Ch 11) is combined later on with Molecular Mass Spectrometry (Ch 20) Atomic Spectroscopy

2. Ch 8 - An Introduction to Optical Atomic Spectroscopy • the elements in a sample are converted to gas-phase atoms or ions • detected based on UV-Vis absorption, emission, or fluorescence • detection limits in the part-per-billion (ppb) range

3. Optical Atomic Spectra Energy Level Diagrams - valence electron transitions Na 1s22s22p63s1 Mg+ 1s22s22p63s1

4. Atomic Emission Spectra - valence electrons excited by a flame, plasma, or electric spark to higher energy levels; emission to ground state produces the emission spectra. Na excitation Na emission

5. Atomic Absorption Spectra - the gas phase atoms and ions can also absorb radiation directly from an outside source Po Po monitor missing wavelength LightSource absorption

6. Atomic Fluorescence Spectra - atoms in a flame made to fluoresce by irradiation with an intense light source, e.g. laser

7. Atomic Line Widths  →  → Atomic Molecular

8. Na Na Linewidth Broadening 1. Doppler Broadening - frequency shift of light due to source motion 2. Pressure Broadening - increased pressure increases the number of atomic collisions collisions can activate or deactivate an excited state collisions shift ground state energy because of electron cloud interactions Atomic

9. The Effect of Temperature on Atomic Spectra Boltzmann Distribution Nj = excited state population No = ground state population Pj and Po = degeneracy terms Ej = excited state energy K = Boltzmann’s constant = 1.38 x 10-23 J/K T = Kelvin temperature For the 3p level of Na – 2500 K 2510 K 4 % change 0.5 % change

10. Ch 9 - Atomic Absorption and Atomic Fluorescence Spectrometry 1. Flame Atomization

11. Flame Backgrounds O2 – H2 O2 – C2H2 N2O – C2H2 Increasing T

12. Processes leading to flame atomization - aspiration Nebulization Desolvation Volatilization Free atoms

13. Flame Structure Further reactions with O2 and N2 in the atmosphere = BACKGROUND Homogenous T and compositionequilibriumfree atomsOBSERVE HERE Non-equilibriumunstable combustion productsNOT USED

14. Temperature Profiles Temperatures in the primary combustion zone are the hottest

15. Flame Absorption Profiles Must choose the height within the flame to do the analysis without creating oxides oxides

16. Flame Atomizers • Laminar Flow Burner • Oxidant (air or O2) nebulizes sample • Aerosol mixed with fuel and past baffles that remove larger drops • Mixture ignited in slotted burner head • Longer path length • Danger - “flashback” explosion

17. Electrothermal Atomization Graphite Furnace Atomizer - all parts made from graphite Liquid sample injected using a syringe Drying or desolvation step – 110 oC, evaporates solvent Ash step – 350-1200 oC, organics converted to CO2 + H2O Atomization step – 200 Amps, 2000-3000 oC, vaporization L’vov Platform 1 x 5 cm

18. Graphite Furnace Atomizer – cont’d Constant inert gas flow (Ar) protects the graphite from oxidation and removes analyte from chamber walls.

19. Atomic Absorption (AAS) Instrumentation Radiation Sources – require a very narrow linewidth because of negative deviations in calibration curve due to polychromatic radiation effect . To solve this problem, the lamp is constructed out of the same metal element being analyzed. As long as the temperature of the lamp is less than the flame temperature, Doppler and collisional broadening will be greater in the flame, and the source wavelength will be narrower in the sample.

20. Hollow Cathode Lamps 500 Volts Cathode consists of metal to be analyzed 500 Volts across electrodes ionizes inert gas Cations migrate towards the negative hollow cathode Collisions with cathode “sputters” metal from surface Metal atoms in excited states emit characteristic wavelengths Metal redeposited on cathode or glass

21. 2. Instrumentation – single beam; dual beam also available

22. 3. Interferences A. Spectral – unresolveable peaks The 308.211 nm line of V interferes with the 308.215 nm line of Al. To resolve them – If  = 1.0 µm then D-1 = 2.0 nm/mm but the slit widthis so narrow that diffraction will cause loss of signal.

23. 3. Interferences B. Chemical (i) Releasing Agents – a cation that preferentially reacts with the interferent and prevents interaction with the sample. e.g. Ca (analyte) in the presence of PO43- 3 Ca2+ + 2 PO43- Ca3(PO4)2 insoluble product that passes through flame without atomizing the Ca. Add La3+ or Sr2+ (both of which form even more insoulbe compounds with PO43-) (ii) Protecting Agents – prevent interference by forming a stable but volatile species with the analyte e.g. EDTA combines with Ca while leaving interferents behind like Al, PO43-, and SO42-

24. 3. Interferences B. Chemical (iii) Ionization in Flames M(g) = M+(g) + e

25. Detection Limits

26. Ch 10 - Atomic Emission Spectrometry Plasma, arc and spark emission spectrometry have advantages over flame and electrothermal atomization techniques: • less interelemental interference (many emission lines to choose from) • simultaneous detection of dozens of elements • good for compounds with high Hvap (difficult to vaporize) • can detect elements that form "refractory compounds" (difficult to thermally decompose) such as the oxides of B, P, W, U, Zr, and Nb • can detect nonmentals such as Cl, Br, I, and S • wide dynamic range • no extensive sample pretreatment

27. The Inductively Coupled Plasma (ICP) Source

28. Plasma Appearance and Spectra

29. Sample Injection

30. Plasma Source Spectrometers

31. Echelle Gratings n = d (sin i sin r ) same side i  r =  n = 2d sin 

32. Scanning Echelle ICP Spectrometers

34. Analytical Applications • No matrix (background) effect from – • Natural substances like dissolved organic materials (e.g. humics) and microorganisms • No spectral interference from other ions