Latex Beads Size Measurement Techniques
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In this new-letter we explain the technique of Latex beads size measurement. Accurate particle size measurement of latex requires understanding particle characteristics, as different sizes need different methods. In in vitro diagnostics, particles typically range from micron to nanometer scale, with focus on average size and distribution rather than individual microspheres. Key factors include particle shape, size distribution, measurement method, and data processing. Common measurement methods include microscopy, sieving, sedimentation, laser diffraction, photon correlation spectroscopy, and Coulter counting, with varying results depending on the method used.
1. Microscopic Method
The microscopic method allows for direct observation and measurement of individual particles, and it provides valuable information about particle size, shape (e.g., spherical, square, needle-like, irregular polygon), structural condition (e.g., solid, hollow, loose, porous), and surface morphology. Particles in the range of 100 μm to 0.01 μm can be measured using optical or scanning electron microscopes. This is the most intuitive measurement method and is often used for calibration and standardization of other measurement techniques.
To begin the test, the sample is dispersed onto a microscope slide, which is then placed on the microscope stage. By selecting the appropriate objective and eyepiece magnification, and adjusting the focal length, the particles are brought into clear focus. The size of the particles is then measured using a calibrated eyepiece micrometer scale.
T get the particle size distribution of the particles, you typically need to observe about 70,000 particles. This process usually takes 2-3 days. While the microscopic method is highly accurate, it is also time-consuming and labor-intensive. It is best suited for measuring the particle size of a limited number of particles or for samples where the particle sizes are relatively homogeneous. In cases with heterogeneous samples, this method can be inconvenient and may lead to inaccurate measurements due to variations in the field of view.
2. Sieving Method
The sieving method is one of the simplest, earliest, and most widely used techniques for particle size determination. It includes both dry and wet sieving. This method can utilize a single sieve to control the passage rate of a specific particle size, or multiple sieves stacked together to evaluate the passage rates of various particle sizes simultaneously and calculate their respective percentages.
The sample to be tested is passed through a series of standard sieves with different aperture sizes. Particles are compared against the sieve apertures—those larger than a given aperture are retained on the sieve, while smaller particles pass through to the next sieve. This process separates particles into different size classes, each of which is weighed to determine the particle size distribution expressed as a mass fraction.
The sieving method is suitable for particle size distribution measurements ranging from approximately 10 mm to 20 μm. If using electroformed sieves (microporous sieves), the sieving size can be as small as 5 μm or even smaller. The sieve analysis method is suitable for particle size distribution measurements between about 10 mm and 20 μm. If electroformed sieves (microporous sieves) are used, the sieve size can be as small as 5 μm or even smaller.
3. Sedimentation Method
The sedimentation method is based on Stokes' law, which allows for the determination of particle size distribution by measuring the rate at which particles settle in a fluid over time. The particle size determined through this method is referred to as the Stokes diameter.
Stokes' law defines the calculation of the drag force overcome by a spherical object in a viscous laminar flow.
Stokes' law [F=6πηυR]
Where R is the radius of the sphere, (υ) is the velocity of the microsphere relative to the fluid,(η) is the viscosity coefficient of the fluid.
Particles suspended in a liquid, under the action of the gravitational field to overcome the resistance to produce settlement. Particles of the same substance, large particle size settles quickly, small particle size settles slowly.
4. Laser Diffraction Method
Generally, the particle size and size distribution are calculated from the scattered light energy distribution of the particles to be measured within a small angle in the forward direction.
Light travels in a uniform medium in a straight-line direction. When encountering tiny particles, the incident light will partially deviate from its original direction of propagation and be projected in other directions, which is called light scattering.
When the particles are far enough away from each other, the light scattering produced by individual particles does not cancel out due to coherence and can be considered independent of each other. And the angle at which the light is produced to deviate is directly related to the size of the particle diameter. The size of the particles used in the immune response is basically around 1 um, which is in the MIE scattering mode. For particles with larger diameters, it is also called the diffraction method because the scattering is dominated by diffraction in the forward small angle range.
The laser diffraction method requires that the concentration of the particles to be measured is low enough to reduce the coherence of the scattered light between the particles.
5. Photon Correlation Spectroscopy
Photon Correlation Spectroscopy (PCS), developed based on the principle of Dynamic Light Scattering (DLS), has become one of the most important means of measuring the particle size of nanoparticles after more than 30 years of development. It is based on the speed of particles moving in liquid, the law that large-size particles move slowly, and small-size particles move quickly to find the particle size distribution. It is applicable to the range of 1nm-1μm.
Particles suspended in a liquid undergo disordered Brownian motion, and when a beam of light at a fixed angle is shone on these particles, the moving particles cause a shift in the intensity of the scattered light. The faster the motion, the faster the change in light intensity caused, and the smaller the particle size, the faster the motion. Therefore, by analyzing the change of scattered light intensity, the speed of Brownian motion of the particles can be obtained, and then the particle size distribution can be obtained by using the Stokes-Einstein equation.
6. Coulter Counting Method
The Coulter counting method is a particle size measurement method invented in the 1950s by a man named Coulter, which utilizes voltage pulses generated by a change in electrical resistance for particle size measurement. It is commonly used for counting blood cells. It is an individual and three-dimensional measurement of particles or cells, which can not only accurately measure the particle size distribution of the particles to be measured, but also do the absolute number and concentration of particles.
As shown in the figure below, the small hole is immersed in the electrolyte with an anode and cathode inside and outside, and the current flows from the anode to the cathode through the small hole as the electrolyte moves. And when the particles suspended in the electrolyte pass through the small hole, the cross-section of the electrolyte flowing through the small hole decreases, the resistance between the two stages increases, the voltage rises, and a voltage pulse is generated. The voltage pulse is proportional to the volume of the particles within a certain range. The peak value of the voltage pulse allows the size of the individual particles to be calculated and thus the particle size distribution of the particles can be counted.
There are also other methods to measure the particle size of micro- and nanoscale particles. Besides the previously mentioned techniques, other methods such as gas permeability and ultrasonic methods also exist. These methods differ in their measurement principles and ranges. Selecting the appropriate measurement method requires a thorough understanding of the particles' characteristics and careful consideration of the laboratory conditions.
Hope that every researcher experiences smooth experiments.
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