Methods to determine particle properties

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Methods to determine particle properties

• Chapter 7

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What ranges do we need to measure
Particle Characterization Light Scattering
Methods
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Principles for different methods

• 1. Visual methods (e.g., optical, electron, and
  scanning electron microscopy combined with image
  analysis)
• 2. Separation methods (e.g., sieving,
  classification, impaction, chromatography)
• 3. Stream scanning methods (e.g., electrical
  resistance zone, and optical sensing zone
  measurements)
• 4. Field scanning methods (e.g., laser
  diffraction, acoustic attenuation, photon
  correlation spectroscopy)
• 5. Sedimentation
• 6. Surface methods (e.g., permeability,
  adsorption)

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Visual methodsMicroscopy

• Benefits
• Simple and intuitive
• Give shape information
• Reasonable amount of sample
• Drawbacks
• Statistic relevance tedious if image analyse
  can not be used
• Risk for bias interpretation
• Difficult for high concentrations
• Sample preparation might be difficult

• Principe of operation
• Optic or electronic measures
• Two dimensional projection
• Projection screen or circles
• Image analysing programs
• Measures
• Feret diameters
• Equal circles
• Size range- 0.001-1000 ?m
• Gives number average,or area average

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Visual methodsEstimations by hand

• Björn B rule of thumb estimate the size of the
  third largest particle
• Compare to a known set of circles and count the
  number of particles in each group.
• Choose a direction and use 0 and 90 degrees feret
  diameters
• Reliability
• Blind your samples
• Count enough particles

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VisualDifferent types of microscope

• Light microscope (1-1000 ?m)
• Fluorescence microscope
• Confocal laser scanning microscopy
• Electron microscope
• SEM (0.05-500 ?m)
• TEM (Å-0.1 ?m)

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Visual methodsImage analysis

• Easy to be fooled
• Difficult to get god contrast and separation
  between particles
• The human eye is much better than any image
  analysing tool in detecting shapes
• Example in Image J

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Separation methods Sieving

• Principe of operation
• stack of sieves that are mechanical vibration for
  pre-decided time and speed
• Air-jet sieving - individual sieves with an under
  pressure and and air stream under the sieve which
  blows away oversize particles
• Measures - Projected perimeter-square, circle
• Size range - 5-125 000 ?m
• Gives weight average

• Benefits
• Simple and intuitive
• Works well for larger particles
• Drawbacks
• Can break up weak agglomerates (granulates)
• Does not give shape information
• Need substantial amount of material
• Needs calibration now and then

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Separation methodsPowder grades according to BP
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Separation methodsChromatography

• Measures
• Hydrodynamic radius
• Principe of operation
• Size exclusion (SEC GPC)
• porous gel beads
• Size range -0.001-0.5 ?m
• Hydrodynamic Chromatography (HDC)
• Flow in narrow space
• Size range capillary -0.02-50 ?m packed column
  0,03-2 ?m

• Benefits
• Short retention times
• Separation of different fractions
• Drawbacks
• Risk for interaction
• Need detector

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Separation methodsFFF Field flow fractionation

• Size range 30nm- 1mm
• Principe of operation
• Flow in a chanel effected by an external field
• Heat
• Sedimentation
• Hydraulic
• Electric

• Benefits
• No material interaction
• High resolution
• Good for large polymers
• Drawbacks
• Few commercial instrument
• Still in development stage

Field
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Separation methods Cascade impactores

• Measure- Aerodynamic volume,
• Principe of operation
• The ability for particles to flow an air flow
• Size range normally 1-10 mm

• Benefits
• Clear relevance for inhalation application
• Can analyse content of particles
• Drawbacks
• Particles can bounce of the impactor or interact
  by neighbouring plates
• Difficult to de-aggregate particles

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Stream Scanning MethodsCoulter counter

• Benefits
• measure both mass and population distributions
  accurately
• Drawbacks
• Risk for blockage by large particles,
• More than one particle in sensing zone
• Particles need to suspended in solution

• Measures - Volume diameter
• Gives number or massavarge
• Size range - 0.1-2000 ?m
• Principe of operation
• Measurement on a suspension that is flowing
  through a tube, when a particle passes through a
  small hole in a saphire crystal and the presence
  of a particle in the hole causes change in
  electric resistance

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Methods to measure particle size Light scattering

• Measures - Area diameter or volume diameter,
  polymers Radius of gyration or molecular mass
• Principal of operation
• Interaction with laser light the light are
  scattered and the intensity of the scattered
  light are measured
• Two principals
• Static light scattering
• Dynamic light scattering
• Size range- 0.0001-1000 ?m

• Benefits
• Well established
• instruments are easy to operate
• yield highly reproducible data
• Drawbacks
• Diluted samples-changes in properties
• Tendency to
• Oversize the large particles
• Over estimates the number of small particles

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Static light scattering

• Particle size information is obtained from
  intensity of the scattering pattern at various
  angles.
• Intensity is dependent on
• wavelength of the light
• Scattering angle
• particle size
• relative index of refraction n of the particle
  and the medium.

Micromeritics Technical Workshop Series (Fall
2000)
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Light scatteringSmall and large particles

• Small particles one scattering center lt 10 nm
• Scatter intensity independent of scattering angle
  (Rayleigh scattering)

• Large particles multiple scattering centres
• Scattering depend on angle and gives diffraction
  pattern

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Light scattering Mie theory

• The complete solution to Maxwells equation for
  homogeneous sphere
• Incident light of only a single wavelength is
• considered.
• No dynamic scattering effects are considered.
• The scattering particle is isotropic.
• There is no multiple scattering.
• All particles are spheres.
• All particles have the same optical properties.
• Light energy may be lost to absorption by the
  particles.
• Applicable for all sizes
• Needs to know the refractive index to calculate
  the size

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Light scattering Fraunhofer theory

• Treats that the particle as completely adsorbing
  disc
• does not account for light transmitted or
  refracted by the particle.
• Only applicable to particles much larger than the
  wavelength of the light
• Do not need to know the refractive index
• Much simpler math

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Light scattering Dynamic light scattering

• Particle size is determined by correlating
  variations in light intensity to the Brownian
  movement of the particles
• Related to diffusion of the particle

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Light scattering Dynamic light scattering the
decay function

• Monodisperse particles gives a single exponential
  decay rate
• Polydisperse samples the self diffusion
  coefficient is defined by a distribution function
  that includes
• number density of species
• mass M
• particle form

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Methods to measure particle sizeSedimentation

• Measures - Frictional drag diameter, stoke
  diameter
• Gives weight average
• Principe of operation
• Sedimentation in gravitational field
• Sedimentation due to centrifugal force
• Size range -0.05-100 gm)

• Benefits
• Simple and intuitive
• Well established
• Drawbacks
• Sensitive to temperature due to density of media
• Sensitive to density difference of particles
• Orientation of particles to maximize drag
• bias in the size distribution toward larger
  particle

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Methods to measure particle sizeSedigraph
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Surface area analysepermeability

• Measures
• Specific area
• Principe of operation
• Measures the pressure drop in a particle bed
• Conditions
• Laminar flow
• Know Kozenys constant
• Homogenous particle bed

• Benefits
• Simple equipment
• Relevant for many applications
• Drawbacks
• Has to know
• Porosity
• Kozenys constant
• Needs uniform density of particles

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Surface area analyseGas adsorption

• Principe of operation
• Measures the adsorption of gas molecules
• Remove adsorbed molecules
• Introduce gas
• Measure pressure differences

• Range
• 0.01 to over 2000 m2/g.
• Benefits
• Well established
• High precision
• Gives inner pores
• Drawbacks
• Over estimation of available area
• Experimental difficulties