Optical Atomic Spectroscopy

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Optical Atomic Spectroscopy

• Optical Spectrometry
• Absorption
• Emission
• Fluorescence
• Mass Spectrometry
• X-Ray Spectrometry

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Optical Atomic Spectroscopy

• Atomic spectra single external electron

Slightly different in energy
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Atomic spectrum Mg
Spins are paired No split
Spins are unpaired Energy splitting
Singlet ground state
Triplet excited state
Singlet excited state
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Atomic spectroscopy

• Emission
• Absorption
• Fluorescence

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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

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Douglas A. Skoog, et al. Principles of
Instrumental Analysis, Thomson, 2007
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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

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Line Broadening

• Doppler broadening

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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

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Temperature Effects

• Bolzmann equation
• Effects on AAS, AFS, and AES

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Atomic spectroscopy

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

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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

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How to get things to atomize?
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How to get samples into the instruments?
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Sample Introduction

• Pneumatic Nebulizers
• Break the sample solution into small droplets.
• Solvent evaporates from many of the droplets.
• Most (gt99) are collected as waste
• The small fraction that reach the plasma have
  been de-solvated to a great extent.

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What is a nebulizer?
SAMPLE AEROSOL
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Concentric Tube
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Cross-flow
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Fritted-disk
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Babington
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What happens inside the flame?
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FLAMES
Rich in free atoms
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FLAMES
T ? E
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GOOD AND BAD THINGS
oxidation
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Boltzmann Equation Relates Excited State
Population/Ground State Population Ratios to
Energy, Temperature and Degeneracy
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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

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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)

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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.

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Radiation Sources for AAS

• Hollow Cathode Lamp
• Conventional HCL

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Ne or Ar at 1-5 Torr
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• 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

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• Hollow Cathode Lamp (Contd)
• 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 ?

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• 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.

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• Boosted discharge hollow-cathode lamp (BDHCL)

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• 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.

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• Electrodeless Discharge Lamps (EDL)

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• 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.

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Single-beam design
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DOUBLE BEAM FAA SPECTROMETER
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Interferences in AAS and AFS

• Spectral Interferences
• Overlapping
• Broadening absorption for air/fuel mixture
• Scattering or absorption by sample matrix

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Background Correction

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

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Continuum-Source Correction
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Continuum-Source Correction
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(The draw is not to scale)
A
B
0.04 nm
The light from the HCL is absorbed by both the
sample and the background, but the light from the
D2 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 D2 lamp. The lamp emission is
much broader than the sample absorption, and an
averaged absorbance taken over the whole band
pass of the monochromator.
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Zeeman Effect Background Correction
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Source Self-Reversal (Smith Hieftje)
Self-absorption Line broadening
A relative new technique
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Source Self-Reversal (Smith Hieftje)
Absorbed by sample reduced, not complete
eliminate! But the background absorbs the same
portion of light.
Absorbed by sample and background
Vandecasteele and Block, 1997, p126
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Interferences in AAS and AFS

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

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• Chemical Interferences
• Atom ionization
• M ? M e

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• Atomic Fluorescence Spectrometry

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