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Optical Atomic Spectroscopy Optical Spectrometry Optical Spectrometry Absorption Absorption Emission Emission Fluorescence Fluorescence Mass Spectrometry.

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Presentation on theme: "Optical Atomic Spectroscopy Optical Spectrometry Optical Spectrometry Absorption Absorption Emission Emission Fluorescence Fluorescence Mass Spectrometry."— Presentation transcript:

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

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

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

5 Atomic spectroscopy Emission Emission Absorption Absorption Fluorescence Fluorescence

6 Line Broadening Uncertainty Effects Uncertainty Effects Heisenberg uncertainty principle: 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: One of the important forms Heisenberg uncertainty principle: t 1 p156 t 1 p156 To determine with negligibly small uncertainty, a huge measurement time is required. To determine with negligibly small uncertainty, a huge measurement time is required. Natural line width Natural line width

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

8 Line Broadening Doppler broadening Doppler broadening Doppler shift: 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 Doppler broadening

10 Line Broadening Pressure 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 Bolzmann equation Effects on AAS, AFS, and AES Effects on AAS, AFS, and AES

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

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

14 How to get things to atomize?

15 How to get samples into the instruments?

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

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

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32 ElectroThermal AAS (ETAAS or GFAAS) The sample is contained in a heated, graphite furnace. 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). 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) 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) There is no nebulziation, etc. The sample is introduced as a drop (usually 5-20 uL), slurry or solid particle (rare)

33 The furnace goes through several steps… The furnace goes through several steps… Drying (usually just above 110 deg. C.) Drying (usually just above 110 deg. C.) Ashing (up to 1000 deg. C) Ashing (up to 1000 deg. C) Atomization (Up to 2000-3000 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. 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. 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. ElectroThermal AAS (ETAAS or GFAAS)

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38 Hollow Cathode Lamp Conventional HCL Radiation Sources for AAS

39 Ne or Ar at 1-5 Torr

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41 Hollow Cathode Lamp (Contd) 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 (Contd) High potential, and thus high currents lead to greater intensities High potential, and thus high currents lead to greater intensities Doppler broadening of the emission lines from the lamp 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 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 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. 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. 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. 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. 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. 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). 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. 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. 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. 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.

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49 Single-beam design

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

51 Spectral Interferences Spectral Interferences Overlapping Overlapping Broadening absorption for air/fuel mixture Broadening absorption for air/fuel mixture Scattering or absorption by sample matrix Scattering or absorption by sample matrix Interferences in AAS and AFS

52 Background Correction Two-line Correction (like Internal Standard) Two-line Correction (like Internal Standard) Continuum-Source Correction Continuum-Source Correction Zeeman Effect Zeeman Effect Source Self-Reversal (Smith –Hieftje) Source Self-Reversal (Smith –Hieftje)

53 Continuum-Source Correction

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55 0.04 nm A B The light from the HCL is absorbed by both the sample and the background, but the light from the D 2 lamp is absorbed almost entirely by the background A: HCL lamp, the shaded portion shows the light absorbed from the HCL. The emission has a much narrower line width than the absorption line. B: D2 lamp, the shaded portion shows the light absorbed by D 2 lamp. The lamp emission is much broader than the sample absorption, and an averaged absorbance taken over the whole band pass of the monochromator. (The draw is not to scale)

56 Zeeman Effect Background Correction:

57 Source Self-Reversal (Smith –Hieftje) A relative new technique Self-absorption Line broadening

58 Source Self-Reversal (Smith –Hieftje) Vandecasteele and Block, 1997, p126 Absorbed by sample and background Absorbed by sample reduced, not complete eliminate! But the background absorbs the same portion of light.

59 Chemical Interferences Chemical Interferences Formation of compounds of low volatility Formation of compounds of low volatility Calcium analysis in the presence of Sulfate or phosphate Solutions Higher temperature Higher temperature Releasing agents: cations that react preferntially with the interference ions. Releasing agents: cations that react preferntially with the interference ions. Protection agents: form stable but volatile species with the analytes (i.e. EDTA,APDC….) Protection agents: form stable but volatile species with the analytes (i.e. EDTA,APDC….) Interferences in AAS and AFS

60 Chemical Interferences Chemical Interferences Atom ionization Atom ionization M M + + e

61 Atomic Fluorescence Spectrometry Atomic Fluorescence Spectrometry Commercial AFS instruments are on the market! Learn more in CHM 6157


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