Particle size measurements by NpEdC855

VIEWS: 4 PAGES: 25

									L U N D UN I V E R S I T Y




         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 methods
Microscopy

Principe of operation                           Benefits
                                                   – “Simple” and intuitive
    – Optic or electronic
                                                   – Give shape information
       measures
                                                   – Reasonable amount of sample
    – Two dimensional projection
             •   Projection screen or circles   •   Drawbacks
             •   Image analysing programs            – Statistic relevance “tedious” if
•   Measures                                           image analyse can not be used
                                                     – Risk for bias interpretation
     – Feret diameters
                                                     – Difficult for high concentrations
     – Equal circles
                                                     – Sample preparation might be
•   Size range- 0.001-1000 m                          difficult
•   Gives number average,or area
    average

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Visual methods
Estimations 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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Visual
Different 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 methods
Image 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             Benefits
     – stack of sieves that are          – “Simple” and intuitive
        mechanical vibration for         – Works well for larger particles
        pre-decided time and speed
     – Air-jet sieving - individual   Drawbacks
        sieves with an under              – Can break up weak
        pressure and and air stream         agglomerates (granulates)
        under the sieve which blows       – Does not give shape
        away oversize particles             information
•   Measures - Projected perimeter-       – Need substantial amount of
    square, circle                          material
                                          – Needs calibration now and
      –Size range - 5-125 000 m
                                            then
•   Gives weight average



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Separation methods
Powder 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 methods
Chromatography

•   Measures                                   •   Benefits
     – Hydrodynamic radius                          – Short retention times
                                                    – Separation of different
•   Principe of operation                             fractions
     – Size exclusion (SEC GPC):               •   Drawbacks
             •   porous gel beads                   – Risk for interaction
             •   Size range -0.001-0.5 m           – Need detector
      –   Hydrodynamic
          Chromatography (HDC)
             •   Flow in narrow space
                                                               QuickTime™ and a
                                                                 decompressor
             •   Size range capillary -0.02-             are neede d to see this picture.

                 50 m packed column
                 0,03-2 m

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Separation methods
FFF Field flow fractionation

•   Size range 30nm- 1m                      •   Benefits
•   Principe of operation                          – No material interaction

     – Flow in a chanel                            – High resolution

       effected by an external                     – Good for large
       field                                         polymers
             •   Heat                         •   Drawbacks
             •   Sedimentation                     – Few commercial
             •   Hydraulic                           instrument
             •   Electric    Field                 – Still in development
                                                     stage
                 QuickTime™ and a
                  decompressor
          are neede d to se e this picture.


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Separation methods
Cascade impactores

•   Measure- Aerodynamic volume,                         •   Benefits
•   Principe of operation                                     – Clear relevance for
     – The ability for particles to
        flow an air flow                                        inhalation application
•   Size range normally 1-10 m                               – Can analyse content of
                                                                particles
                              Qui ckTi me™ and a
                                                         •   Drawbacks
                               decompressor
                       are needed to see this picture.        – Particles can bounce of
                                                                the impactor or interact
                                                                by neighbouring plates
                                                              – Difficult to de-
                                                                aggregate particles
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Stream Scanning Methods
Coulter counter

•   Measures - Volume diameter                    • Benefits
•   Gives number or massavarge
                                                     – measure both mass
      –   Size range - 0.1-2000 m
      –   Principe of operation                        and population
                Measurement on a suspension            distributions
                that is flowing through a tube,
                when a particle passes                 accurately
                through a small hole in a
                saphire crystal and the
                                                  Drawbacks
                presence of a particle in the         • Risk for blockage by large
                hole causes change in electric          particles,
                resistance
                                                      – More than one particle in
                                                        sensing zone
                                                      – Particles need to
                                                        suspended in solution

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Methods to measure particle size
Light scattering

•   Measures - Area diameter or             •  Benefits
    volume diameter, polymers                   – Well established
    Radius of gyration or molecular
    mass                                        – instruments are easy to

•   Principal of operation                        operate
     – Interaction with laser light             – yield highly reproducible
        the light are scattered and               data
        the intensity of the scattered      Drawbacks
        light are measured                      • Diluted samples-changes in
     – Two principals                             properties
             •   Static light scattering    • Tendency to
             •   Dynamic light scattering
                                                – Oversize the large particles
      –   Size range- 0.0001-1000
          m                                    – 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 scattering
Small and large particles

•    Small particles one                                      • Large particles multiple
     scattering center < 10 nm                                  scattering centres
•    Scatter intensity                                      • Scattering depend on
     independent of scattering                             decompressor and gives
                                                                angle
                                                           are needed toand athis picture. diffraction
                                                           QuickTime™ see
     angle (Rayleigh                                            pattern
     scattering)
           QuickTime™ and a
             decompressor
    are needed to see this picture.

            Qu i ckTi me ™ an d a
               de co mp re s so r
    a re ne ed ed to se e thi s p i cture.          QuickTime™ an d a
                                                      decompressor
                                             are need ed to see this picture.




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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
                                                             QuickTime™ an d a
•   Related to diffusion of the                                 decompressor
                                                       are need ed to see this p icture .



    particle

                          QuickTime™ and a
                            decompressor
                    are neede d to see this picture.




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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 size
Sedimentation

•    Measures - Frictional drag          Benefits
     diameter, stoke diameter                – “Simple” and intuitive

               2d 2 g                      – Well established
           v
                 18                     Drawbacks
                                         • Sensitive to temperature due to
•    Gives weight average                   density of media
         –   Principe of operation       • Sensitive to density difference
            • Sedimentation in            of particles
                 gravitational field     • Orientation of particles to
              • Sedimentation due to        maximize drag
                 centrifugal force
                                         • bias in the size distribution
         –   Size range -0.05-100 £gm)      toward larger particle


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Methods to measure particle size
Sedigraph




                                   QuickTime™ and a
                                      decompressor
                             are need ed to see this picture.




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     Surface area analyse
     permeability

     •   Measures:                              •   Benefits
          – Specific area                            – Simple equipment
     •   Principe of operation                       – Relevant for many
          – Measures the pressure                      applications
            drop in a particle bed              •   Drawbacks
                              3  1   P             – Has to know
           s(m kg) 
                2
                           *        *
                     k 2v (1 ) 2 L                   •   Porosity
           –   Conditions                                •   Kozenys constant
                    •   Laminar flow                –   Needs uniform density
                                                      of particles
                    •   Know Kozenys constant
                    •   Homogenous particle
                        bed

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     L U N D UN I V E R S I T Y

     Surface area analyse
     Gas adsorption

     •   Principe of operation           •   Range
          – Measures the                      – 0.01 to over 2000 m2/g.
            adsorption of gas            •   Benefits
            molecules                         – Well established
                  •   Remove adsorbed         – High precision
                      molecules
                                              – Gives inner pores
                  •   Introduce gas
                                         •   Drawbacks
                  •   Measure pressure
                      differences             – Over estimation of available
                                                area
                 P        1   b 1 P
                                *           – Experimental difficulties
              n(P  P0 ) bnm bnm P0



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