Transcript Particle Size - International Islamic University Malaysia

Particle Size
Sizing Technique 1:
Coulter principle
Kausar Ahmad
Kulliyyah of Pharmacy, IIUM
http://staff.iiu.edu.my/akausar
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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.
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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
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From: http://www.cyto.purdue.edu/flowcyt/educate/ee520/sld008.htm
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Principle
A current, between two electrodes, 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
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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.
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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.
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Converting Signals to Particle Diameter
Calibrate instrument using 10 or 20 µm
polystyrene standards
Obtain the ‘Kd’
• 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.
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Calibration of instrument
A monodisperse standard is used
Pulses on oscilloscope of uniform size
Concentration used is very low so that coincidence effects are less
than 2%
Instrument is adjusted/calibrated to give the ‘Kd’
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Relationship between
electrical signal and volume of particle
Voltage proportional to volume of particle:
U = constant x V
constant = r0if / 2R4
U=amplitude of voltage pulse
V=particle volume
r0=electrical resistivity
i= aperture current
f= particle ‘shape’ factor
R=aperture radius
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must not
be dirty
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Sample Concentration
If more than one particle passes through the aperture at
exactly the same time (coincidence effect), the reading is
not accurate.
Therefore, sample must be reasonably diluted and should
be within the specified range as indicated by the
instrument.
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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.
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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 clean
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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
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Application
Counting algae
Counting bacteria
Counting cells
In industries
To standardise
standards !
• To detect contaminant in
petroleum
• Electronic: TV screen, CRT
(Dots per inch = DPI)
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References
JZ Knapp, TA Barber & A Lieberman, Liquid and SurfaceBorne Particle Measurement Handbook, Marcel Dekker,
New York (1996).
T Allen, Particle Size Measurement 4th. Ed., Chapman and
Hall, London (1990).
Beckman-Coulter Multisizer 3 operation manual (lab)
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