Atomic spectroscopy

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Atomic spectroscopy

• Elemental composition

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Atoms have a number of excited energy levels
accessible by visible-UV optical methods

• Must have atoms (break up molecules)
• Optically transparent sample of neutral atoms
  (flames, electrical discharges, plasmas)
• Metals accessible by UV-Vis, non-metals generally
  less than 200nm where vacuum UV needed)

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Atomic spectra

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

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Atomic Emission Schematic
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Atomic spectroscopy for analysis

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

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Flame Sources - remove solvent, free atoms,
excite atoms

• Nebulizer or direct injection
• Dry solvent, form and dissociate salt
• T 1700-3200 C gives some neutral atoms
• Thermal or light induced excitation
• Neutrals can react (refractory cpd)
• Molecular emission from gas give broad emission
  interferences)

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General issues with flames

• Turbulence / stability / reproducibility
• Fuel rich mixtures more reducing to prevent
  refractory formation
• High temperature reduces oxide interferences but
  decreases ground state population of neutrals
  (fluctuations are critical)

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Chemical interferences - FES

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

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High energy sources

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

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Atomic emission from spark or arc
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Electrical ARC - sustained discharge between 2
electrodes

• T4000-6000C
• Poor precision due to wander
• Metal or graphite electrodes can be formed
• Different materials volatilized at different
  rates so quantitization difficult

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Electrical SPARK (AC)

• More reproducible as there are multiple discrete
  electrical breakdowns in gas
• T up to 40,000K
• High precision but limited sensitivity (0.01
  level)
• Lots of electrical noise
• Must integrate emissions over time

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Multielement analysis

• Simultaneous emission of many lines requires very
  high resolution
• Gratings have capability to resolve if distances
  are great and overlapping orders are addressed

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Measuring emission lines

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

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Issues and tradeoffs

• Molecular interferences
• Relative vs absolute sensitivity
• Resolution vs S/N or limit of detection
• Standard addition vs calibration curve
• Emission vs AA or fluorescence

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DC coupled plasma emission
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Inductively Coupled Plasma
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Inductively Coupled Plasma
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AA Instrument Schematic
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Atomic Absorption
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AA instrumentation

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

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Hollow Cathode LampEmission is from elements in
cathode that have been sputtered off into gas
phase
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Light Source

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

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Atomizers

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

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AA - Flame atomization

• Use liquids and nebulizer
• Slot burners to get large optical path
• Flame temperatures varied by gas composition
• Molecular emission background (correction devices
  )

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Sources of error

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

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Atomizers

• Flame Atomizers - rate at which sample is
  introduced into flame and where the sample is
  introduced is important
• 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

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Electrothermal atomization

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

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Graphite Furnace AA
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Closeup of graphite furnace
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Controls for graphite furnace
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Detector

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

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Background corrections

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

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Problems with Technique

• Precision and accuracy are highly dependent on
  the atomization step
• Light source
• molecules, atoms, and ions are all in heated
  medium thus producing three different atomic
  emission spectra

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Problems continued

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

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Interferences

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

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Interferences continued
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Gas laser
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Dye laser
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Diode laser