Optical Atomic Spectroscopy

1. Optical Atomic Spectroscopy • Optical Spectrometry • Absorption • Emission • Fluorescence • Mass Spectrometry • X-Ray Spectrometry

2. Optical Atomic Spectroscopy • Atomic spectra: single external electron Slightly different in energy

4. Atomic spectrum Mg Spins are paired No split Spins are unpaired Energy splitting Singlet ground state Singlet excited state Triplet excited state

5. Atomic spectroscopy • Emission • Absorption • Fluorescence

6. Line Broadening • Uncertainty Effects • Heisenberg uncertainty principle: The nature of the matter places limits on the precision with which certain pairs of physical measurements can be made. One of the important forms Heisenberg uncertainty principle: t ≥ 1 p156 To determine with negligibly small uncertainty, a huge measurement time is required. • Natural line width

7. n Should be  Superposition of tw sinusoidal wave of different frequencies but identical amplitudes. Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

8. Line Broadening • Doppler broadening • Doppler shift: The wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer, and increases if the motion is receding from the transducer. In flame, Doppler broadening is much larger than natural line width

9. Line Broadening • Doppler broadening

10. Line Broadening • Pressure broadening Caused by collisions of the emitting or absorbing species with other ions or atoms High pressure Hg and xenon lamps, continuum spectra

11. Temperature Effects • Bolzmann equation • Effects on AAS, AFS, and AES

12. Atomic spectroscopy • Interaction of an atom in the gas phase with EMR • Samples are solids, liquids and gases but usually not ATOMS!

13. Atomic Spectroscopy Sample Introduction Flame Furnace ICP Sources for Atomic Absorption/Fluorescence Hollow Cathode Lams Sources for Atomic Emission Flames Plasmas Wavelength Separators + Slits +Detectors

14. How to get things to atomize?

15. How to get samples into the instruments?

16. Sample Introduction • Pneumatic Nebulizers • Break the sample solution into small droplets. • Solvent evaporates from many of the droplets. • Most (>99%) are collected as waste • The small fraction that reach the plasma have been de-solvated to a great extent.

17. What is a nebulizer? SAMPLE AEROSOL

18. Concentric Tube

19. Cross-flow

20. Fritted-disk

21. Babington

25. What happens inside the flame?

26. FLAMES Rich in free atoms

27. FLAMES T  E

28. GOOD AND BAD THINGS oxidation

29. Boltzmann Equation: Relates Excited State Population/Ground State Population Ratios to Energy, Temperature and Degeneracy

30. Flame AAS/AES Spray Chamber/Burner Configurations • Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillary • Only about 5% reach the flame • Larger droplets are collected • Some of the solvent evaporates • Flow spoilers • Cheaper, somewhat more rugged • Impact beads • Generally greater sensitivity

32. ElectroThermal AAS (ETAAS or GFAAS) • The sample is contained in a heated, graphite furnace. • The furnace is heated by passing an electrical current through it (thus, it is electro thermal). • To prevent oxidation of the furnace, it is sheathed in gas (Ar usually) • There is no nebulziation, etc. The sample is introduced as a drop (usually 5-20 uL), slurry or solid particle (rare)

33. ElectroThermal AAS (ETAAS or GFAAS) • The furnace goes through several steps… • Drying (usually just above 110 deg. C.) • Ashing (up to 1000 deg. C) • Atomization (Up to 2000-3000 C) • Cleanout (quick ramp up to 3500 C or so). Waste is blown out with a blast of Ar. • The light from the source (HCL) passes through the furnace and absorption during the atomization step is recorded over several seconds. This makes ETAAS more sensitive than FAAS for most elements.

38. Radiation Sources for AAS Hollow Cathode Lamp Conventional HCL

39. Ne or Ar at 1-5 Torr

41. Hollow Cathode Lamp (Cont’d) • a tungsten anode and a cylindrical cathode • neon or argon at a pressure of 1 to 5 torr • The cathode is constructed of the metal whose spectrum is desired or served to support a layer of that metal • Ionize the inert gas at a potential of ~ 300 V • Generate a current of ~ 5 to 15 mA as ions and electrons migrate to the electrodes. • The gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud. • A portion of sputtered metal atoms is in excited states and thus emits their characteristic radiation as they return to the ground sate • Eventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and are re-deposited

42. Hollow Cathode Lamp (Cont’d) • High potential, and thus high currents lead to greater intensities • Doppler broadening of the emission lines from the lamp • Self-absorption: the greater currents produce an increased number of unexcited atoms in the cloud. The unexcited atoms, in turn, are capable of absorbing the radiation emitted by the excited ones. This self-absorption leads to lowered intensities, particular at the center of the emission band Doppler broadening ?

43. Improvement……. • Most direct method of obtaining improved lamps for the emission of more intense atomic resonance lines is to separate the two functions involving the production and excitation of atomic vapor • Boosted discharge hollow-cathode lamp (BDHCL) is introduced as an AFS excitation source by Sullivan and Walsh. • It has received a great deal of attention and a number of modifications to this type of source have been conducted.

44. Boosted discharge hollow-cathode lamp (BDHCL)

45. Operation principle of BDHCL • A secondary discharge (boost) is struck between an efficient electron emitter and the anode, passing through the primary atom cloud. • The second discharge does not produce too much extra atom vapor by sputtering the walls of the hollow cathode, but does increase significantly the efficiency in the excitation of sputtered atom vapor. • This greatly reduces the self-absorption resulting from simply increasing the operating potential (increase Doppler broadening and self-absorption) to the primary anode and cylindrical cathode.

46. Electrodeless Discharge Lamps (EDL)

47. Electrodeless discharge lamps (EDL) • Constructed from a sealed quartz tube containing a few torr of an inert gas such as argon and a small quantity of the metal of interest (or its salt). • The lamp does not contain an electrode but instead is energized by an intense field of radio-frequency or microwave radiation. • Radiant intensities usually one or two orders of magnitude greater than the normal HCLs. • The main drawbacks: their performance does not appear to be as reliable as that of the HCL lamps (signal instability with time) and they are only commercially available for some elements.

49. Single-beam design

50. Note: the Ref bean does not pass through the flame thus does not correct for the interferences from the flame! synchronized DOUBLE BEAM FAA SPECTROMETER