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##### Methods to determine particle properties

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**Methods to determine particle properties**Chapter 7**What ranges do we need to measure**Particle Characterization: Light Scattering Methods**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)**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 Visual methodsMicroscopy**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**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)**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**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 Separation methods Sieving**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 Separation methodsChromatography**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 Separation methodsFFF Field flow fractionation Field**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 Separation methods Cascade impactores**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 Stream Scanning MethodsCoulter counter**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 Methods to measure particle size Light scattering**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)**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 Light scatteringSmall and large particles**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**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**Particle size is determined by correlating variations in**light intensity to the Brownian movement of the particles Related to diffusion of the particle Light scattering Dynamic light scattering**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**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 Methods to measure particle sizeSedimentation**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 Surface area analysepermeability**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 Surface area analyseGas adsorption