Particle Size Sizing Technique 1: Coulter principle

1
Particle Size Sizing Technique 1 Coulter
principle

• Kausar Ahmad
• Kulliyyah of Pharmacy, IIUM

2
Topics on Sizing Techniques

• Electrical sensing zone method
• Sieving method
• Microscopy
• Scanning Electron Microscopy (SEM)
• Hydrodynamic Chromatography
• Laser diffraction technique
• Photon Correlation Spectroscopy

3
Industries Involving Particles

• Electronics
• Mineral and metal industry
• Polymers, plastics and composite fillers
• Textile products
• Wood and paper products
• Cosmetics

• Pharmaceuticals
• Soaps, detergent and abrasives
• Processed food
• Cement and concrete, glass and ceramics,
  chemicals, paints, coatings, inks, agricultural

4
Particle-related Processes

• Grinding
• Ion exchange
• Precipitation
• Water clarification
• Road surfacing

• Chromatography
• Detergency
• Emulsion polymerisation
• Food processing

5
Properties of Particles

• Size
• Shape
• Nature soft, hard
• Particle-particle interactions
• Particle-solvent interactions

• Surface properties
• Surface charge
• Specific surface area
• Pore size

6
Techniques to Identify Particles

• Visual inspection
• Optical microscopy
• Polarised light microscopy
• Crystal morphology
• Scanning electron microscopy
• Surface morphology
• Transmission electron microscopy

• Infrared micro-spectrophotometry
• Structural information
• Mass spectrometry
• Mass-specific molecular data
• X-ray crystallography
• Crystal structure
• X-ray spectrometry
• Elemental composition

7
Electrical Sensing Zone Method

• A.k.a. the Coulter principle
• Basic method of counting and sizing based on the
  detection and measurement of
• changes in electrical resistance,
• produced by a particle or biological cell,
• suspended in a conductive liquid,
• traversing through a small aperture.

8
Wallace Coulter - Coulter orifice (1956)

• (as early as 1948) - measured changes in
  electrical conductance as cells suspended in
  saline passed through a small orifice
• Cells are relatively poor conductors
• Blood is a suspension of cells in plasma which is
  a relatively good conductor
• Previously it was known that the cellular
  fraction of blood could be estimated from the
  conductance of blood
• As the ratio of cells to plasma increases the
  conductance of blood decreases

9
Slide 8 of 83
Slide 8 of 83
10
Continue Electrical Sensing Zone Method

• Current flow in a conducting liquid
• A current, which is maintained between two
  electrodes, will create a sensing zone around the
  aperture
• When passing through the aperture, the magnitude
  of the current is ca. I mA
• When a particle passes through the aperture, it
  causes changes in electrical impedance
• Each particle will trigger a voltage pulse
  indicating a depression in current flow
• The magnitude of the decrease depends on size of
  particle

11
Interpreting the results

• Amplitude of the pulse is proportional to the
  volume of the particle
• Hence, can determine diameter of particle
• Each pulse represents one particle
• Hence, can determine the number of particle
• And therefore, Coulter counter.
• The voltage pulses will be counted, amplified and
  allocated to the right size class.

12
Advantages of Technique

• capable of counting thousands of particles per
  second
• Results are not affected by
• Colour
• Composition
• Refractive index
• Or other light interaction effects
• Absolute sample volume. Why is this an advantage?

13
Converting Signals to Particle Diameter

• Calibrate instrument using 10 or 20 µm
  polystyrene standards
• Obtain the Kd ?slide
• The Kd is used to convert the amplitude of the
  pulse in volt to volume of the particle (this is
  a linear response)
• From the volume, the diameter can be calculated.

14
Calibration of instrument

• A monodisperse standard is used
• Concentration used is very low so that
  coincidence effects are less than 2
• Pulses on oscilloscope of uniform size
• Instrument is adjusted so that height is 40 of
  maximum by controlling gain and currect
  selector settings
• This gives the Kd. see next slide

15
Diameter Calibration Factor, kD

• Kd 6WVm1012/?VT?(?nV)1/3
• W mass of sample in beaker (g)
• VT volume of electrolyte solution in which W is
  diluted
• Vm manometer volume (cm3)
• immersed density of the particles (g/cm3)
• ?n number of particles in a size interval
• V arithmetic mean volume for that particular
  size interval, in instrument units (e.g. product
  of threshold value, aperture current, and
  attenuation)

16
Relationship between electrical signal and
volume of particle

• Voltage proportional to volume of particle
• U constant x V
• constant r0if / ?2R4
• Uamplitude of voltage pulse
• Vparticle volume
• r0electrical resistivity
• i aperture current
• f particle shape factor
• Raperture radius

must not be dirty
17
Sample Concentration

• If more than one particle passes through the
  aperture at exactly the same time, the reading is
  not accurate.
• Therefore, sample must be reasonably diluted and
  should be within the specified range as indicated
  by the instrument.
• Exercise For a given volume, the smaller the
  particles, the lesser is the sample required. WHY?

18
Sample Condition

• It is important that only one particle passes
  through the aperture.
• There should not be any aggregation or
  flocculation.
• For detecting a stable suspension, the particles
  must exist as discrete individual entities.
• A dispersant must be used
• Samples dispersed in the electrolyte must be
  stirred during measurement, especially if it is a
  solid dispersion, to prevent settling.

19
Aperture

• The aperture comes in different sizes
• E.g. an aperture of 100 µm can detect particles
  within 2 to 60 µm
• Outside the range, the measurement is not
  accurate
• Aperture should be cleanwhy?

20
Results Generated

• Results can be displayed in terms of number,
  volume, surface area against particle size.
• Size axis can be linear, logarithmic scale
• Distribution can be differential or cumulative
  data
• Cumulative data can be oversize or undersize

21
Application

• Counting algae
• Counting bacteria
• Counting cells
• To standardise standards !
• In industries
• To detect contaminant in petroleum
• Electronic TV screen, CRT (Dots per inch DPI)

22
References

• JZ Knapp, TA Barber A Lieberman, Liquid and
  Surface-Borne Particle Measurement Handbook,
  Marcel Dekker, New York (1996).
• T Allen, Particle Size Measurement 4th. Ed.,
  Chapman and Hall, London (1990).
• http//www.cyto.purdue.edu/flowcyt/educate/ee520/s
  ld008.htm
• Beckman-Coulter Multisizer 3 operation manual
  (lab)