1 l u n d u n i v e r s i t y methods to determine particle properties chapter 7

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L U N D U N 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. Sedimentation6. Surface methods (e.g., permeability, adsorption)

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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

Visual methodsMicroscopy

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- 1m• 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 stageQuickTime™ and a

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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

m

• 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

• 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

• 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

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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

dataDrawbacks

• 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 < 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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L U N D U N I V E R S I T YLight 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

€

v =2d2gΔρ

18μ

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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€

s(m2 kg) =1

kμρ 2v*

ε 3

(1−ε)2*

ΔP

L

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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

€

P

n(P − P0)=

1

bnm+b−1

bnm*P

P0

1 l u n d u n i v e r s i t y methods to determine particle properties chapter 7

Documents

u n d u n

v e r s i t y methods

v e r s i t y slide

v e r s i t y principles

different methods

surface methods

stream scanning methods

field scanning methods