Michael Asgill

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Laser-Based Aerosol Diagnostics

• Michael Asgill
• 4/7/09

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Why Laser-Based?

• Fast, real-time response
• Qualitative and Quantitative data
• Little to no preparation
• Non-invasive
• Major , minor, and trace species analysis
• Solids, liquids and gases
• Composition and size measurements

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Uses

• In conjunction with other aerosol diagnostic
  devices to make a measurement (SMPS)
• After use of other devices, like impactor, to
  gain additional information
• Stand-alone system

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Optical Properties?

• Max Planck (1900) - Photon is basic unit of
  light
• h Plancks constant
• ? frequency
• c speed of light
• ? wavelength of light

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What Exactly is Measured?

• Most measure the energy state of atoms/molecules
  (exception elastic light scattering methods)
• Three states
• Electrical
• Orbital level of electrons
• Near IR to UV
• Vibrational
• Oscillatory motion of molecules
• IR
• Rotational
• Microwave

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What is Measured Contd
Electrical, Vibrational and Rotational Energy
States1
Vibrational Energy States1
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What is Measured Contd

• Signal is proportional to atomic/mass
  concentration
• Attain dp with known density
• Signal attained from one of four methods
• Absorption
• Fluorescence
• Emission
• Scattering

Vibrational Energy Modes1
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Calibration

• Must calibrate signal
• Use known concentrations of aerosols to make a
  calibration curve
• Standards to match bulk of sample
• Matrix effects

Example Calibration Curve
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Aerosol Introduction

• Aerosol created or flowed into system
• Dried by co-flow of gas
• Dry aerosol excited/volatized
• Excited aerosol emits light

Aerosol Introduction1
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Absorption

• Incident Radiation is at the sample absorption
  frequency
• Requires knowledge of aerosol
• Breakdown of aerosol by atomizer (flame)
• Loss of aerosol

Absorption of Light by Sample1
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Absorption Contd
Absorption Measurement Setup1

• Requires calibration of signal in relation to
  atomizer
• Record values for flame, blank, and aerosol

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Fluorescence

• Incident radiation usually same frequency as
  aerosol absorption
• Spontaneous emission distributed uniformly in
  space.
• Low signal output because only faction of FL can
  be captured by detector
• Lose aerosol

Fluorescence (Luminescence)1
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Fluorescence Contd

• Four possibilities
• Resonant
• Stokes/Anti-Stokes
• Sensitized
• Multi-photon

Types of Flourescence1
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Emission

• Excitation of aerosol at any wavelength
• Emission along atomic (electrical) lines
• Tells atomic composition of aerosol

Emission Signal
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Infrared Spectroscopy (IR)

• Differentiate between bonds present in aerosol
• Uses differences in vibrational states
• Absorption or Emission
• Must be IR active
• Shift in dipole moment of molecule during
  vibration
• N O bond vs. O O bond
• Signal orders of magnitude less than atomic
  (electronic) emission

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Elastic Scattering

• Elastic outgoing radiation is same wavelength as
  incoming
• Two theoretical models
• Rayleigh Scattering small, non-absorbing,
  spherical particles (altlt1, ma ltlt1)
• Mie Scattering no size or shape limitation,
  absorbing and non-absorbing
• Mie solution converges on Rayleigh solution as
  particle size decreases.

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Elastic Scattering Contd

• Intensity of scattered light dependent on
  scattering angle and particle size
• Optical Particle Counter (OPC)

Rayleigh Solution
Light Scattering Geometry2
i1,i2 f(?)
Mie Solution
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Inelastic Scattering (Raman)

• Molecular spectroscopy method
• Similar to IR spectroscopy but no absorption,
  only scattering.
• Signal proportional to mass density
• Electric field of molecules perturbed at
  frequency of incoming electromagnetic (EM) wave.
• Most of perturbation elastic but chance EM wave
  will impart some of its energy to the molecule

Analogy of Raman Scattering3
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Raman Scattering Contd

• Mathematically, Raman Scattering is Explained by
  the equations below
• Three distinct frequencies, ?0, ?0?vib
  (Anti-Stokes), ?0-?vib (Stokes)
• Requires change in polarizability to be non-zero

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Issues

• Detection Limit minimum concentration that can
  be detected optically
• Signal Saturation incomplete vaporization
• Matrix effects localized vs. bulk plasma
  properties

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Detection Limit
Signal-to-Noise vs. Concentration
Typical Emission Spectrum
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Signal Saturation

• Incomplete vaporization due to heat and mass
  transfer time scales

Signal vs. volume4
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Matrix Effects

• More prominent in multi-component aerosols
• Can have different signal strength of a signal
  for the same concentration if in presence of
  different elements
• Differences in density, heat capacity, thermal
  conductivity, etc. cause different localized
  temperatures
• Rate of dissociation (electron density)
  proportional to temperature

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Instrumentation (LIBS)

• LASER
• Light Amplification by Stimulated Emission of
  Radiation
• Pulsed (ns-fs)
• NdYag (1064 nm)
• Back Scatter of Light
• Fiber Optic
• Spectrometer
• Disperses light
• Charged Coupled Device (CCD)
• Light to electrical signal

Sample Setup
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Conclusion

• Why Laser-Based Diagnosis
• Energy states of a molecule/atom
• Electrical, vibrational, rotational
• Aerosol introduction
• Four different signals can measure
• Absorption, fluorescence, emission, scattering
• Issues with Laser-Based techniques
• Detection limit, saturation, matrix effects
• Instrumentation

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References

• Ingle, J.D. and Crouch, S.R. Spectrochemical
  Analysis, (1988) Prentice Hall, Englewood Cliffs,
  NJ
• Hahn, D.W. Light Scattering Theory, (2004)
  http//plaza.ufl.edu/dwhahn/Rayleigh20and20Mie2
  0Light20Scattering.pdf
• Hahn, D.W. Raman Scattering Theory, (2007)
  http//plaza.ufl.edu/dwhahn/Raman20Scattering20T
  heory.pdf
• Carranza, J.E. and Hahn, D.W., Spectrochimica
  Acta Part B, 57 (2002a) 779-790
• Windom, B. Laser Induced Breakdown Spectroscopy
  (LIBS) for Aerosol Characterization, (2007)