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Physical Pharmacy 21 Particle Size Sizing Technique 1: Coulter principle Kausar Ahmad Kulliyyah of Pharmacy, IIUM

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Presentation on theme: "Physical Pharmacy 21 Particle Size Sizing Technique 1: Coulter principle Kausar Ahmad Kulliyyah of Pharmacy, IIUM"— Presentation transcript:

1 Physical Pharmacy 21 Particle Size Sizing Technique 1: Coulter principle Kausar Ahmad Kulliyyah of Pharmacy, IIUM

2 Physical Pharmacy 22 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.

3 Physical Pharmacy 23 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

4 Physical Pharmacy 24 From:

5 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 Physical Pharmacy 25

6 6 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.

7 Physical Pharmacy 27 Advantages of Technique capable of counting thousands of particles per secondResults are not affected by: Colour Composition Refractive index Or other light interaction effects Absolute sample volume.

8 Physical Pharmacy 28 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.

9 Physical Pharmacy 29 Calibration of instrument A monodisperse standard is usedPulses 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

10 Physical Pharmacy 210 Relationship between electrical signal and volume of particle Voltage proportional to volume of particle: U = constant x V constant = if / 2 R 4 U=amplitude of voltage pulse V=particle volume 0 =electrical resistivity i= aperture current f= particle shape factor R=aperture radius must not be dirty

11 Physical Pharmacy 211 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.

12 Physical Pharmacy 212 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.

13 Physical Pharmacy 213 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 accurateAperture should be clean

14 Physical Pharmacy 214 Results Generated Results can be displayed in terms of number, volume, surface area against particle size. Size axis can be linear, logarithmic scaleDistribution can be differential or cumulative data Cumulative data can be oversize or undersize

15 Physical Pharmacy 215 Application Counting algaeCounting bacteriaCounting cells To standardise standards ! In industries To detect contaminant in petroleum Electronic: TV screen, CRT (Dots per inch = DPI)

16 Physical Pharmacy 216 References JZ Knapp, TA Barber & A Lieberman, Liquid and Surface- Borne Particle Measurement Handbook, Marcel Dekker, New York (1996). T Allen, Particle Size Measurement 4 th. Ed., Chapman and Hall, London (1990). Beckman-Coulter Multisizer 3 operation manual (lab)

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