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
Description Sieve diameter ?m Sieve that do not allow more than 40 to pass ?m
Coarse 1700 355
Moderate coarse 710 250
Moderate fine 355 180
Fine 180
Very fine 125
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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