The Coulter Principle
While under contract to the United States Navy in the late 1940s, Wallace H. Coulter developed a technology for counting and sizing particles using impedance measurements. The technology was principally developed to count blood cells quickly by measuring the changes in electrical conductance as cells suspended in a conductive fluid passed through a small orifice. This approach is known as the Coulter Principle and over the past seventy-five years the technology has also been used to characterize thousands of different industrial particulate materials with over 8,000 references to the uses of this technology have been documented.
Beckman Coulter systems which utilize this principle are called COULTER COUNTER® instruments. Drugs, pigments, fillers, toners, foods, abrasives, explosives, clay, minerals, construction materials, coating materials, metals, filter materials, and many other sample types have all been analyzed using the Coulter Principle. This method can be used to measure any particulate material that can be suspended in an electrolyte solution. Particles as small as 0.2 µm and as large as 1600 µm in diameter can routinely be measured using the latest Multisizer 4e system.
How it Works
In a COULTER COUNTER instrument, a tube with a small aperture on the wall is immersed into a container in which particles are suspended in a low-concentration electrolyte (Figure 1). Two electrodes, one inside the aperture tube and one outside the tube create a current path through the electrolyte when an electric field is applied. The impedance (effective resistance) between the electrodes is then measured. The aperture therefore creates a "sensing zone" and particles suspended in the electrolyte can be counted by passing them through the aperture.
As a particle passes through the aperture, a volume of electrolyte equivalent to the immersed volume of the particle is displaced from the sensing zone. This causes a short-term change in the impedance across the aperture which can be measured as a voltage or current pulse. The pulse height is proportional to the volume of the sensed particle.
Using count- and pulse-height analyzer circuits, the number and volume of each particle passing through the aperture can be electronically recorded and digitized along with several key parameters that describe each pulse such as pulse height, pulse width, time stamp, pulse area, etc.
Saved pulse data can be used to monitor sample changes over time to follow changes in the sample such as fragmentation or aggregation. Generally particle volume is usually represented in terms of equivalent spherical diameter which can be then used to obtain particle size distribution.
In modern COULTER COUNTER instruments, such as the Multisizer™3 and 4e particle counter precise control and measuring of the volume of liquid passing through the aperture allows the sample concentration to be determined.
It generally takes less than one minute to perform a measurement with a COULTER COUNTER instrument given typical sampling rates up to 10,000 counts per second.
Available aperture sizes range from 10—2000 µm and can be used to measure particles within a size range of 2 to 80% of the nominal diameter with accuracy better than 1%. Therefore, an overall particle size-range of 0.2—1600 µm is feasible. For example, a 30 µm aperture can measure particles from about 0.6 to 18 µm in diameter. A 140 µm aperture can measure particles from about 2.8 to 84 µm.
If the particles to be measured cover a wider range than a single aperture can measure, two or more apertures can be used and the test results can be overlapped to provide a complete particle size distribution.
Highest Resolution for Particle Size Analysis
The number of pulses detected during measurement gives the particle count with the amplitude of the pulse giving the volume of the particle. Because this is a single-particle measurement process, it yields the highest resolution that any particle characterization technique can achieve. The advantages of such high resolution include the capability to display details of a particle size distribution, finer differentiation between particle sizes and more accurate statistical values calculated from the distribution.