Induced Grating Technology in Particle Size Analysis

By: Lab Manager Magazine -

Problem: Currently nanoparticles are measured by most particle size analyzers using scattered light; however in some cases this presents many physical restrictions and also requires the input of the refractive index as a measurement condition. Future trends in science indicate the need for accurate sizing of nano and even sub-nano particles, particularly in the area of drug development and pharmaceuticals.

Solution: Although traditional particle sizing methods using scattered light alone have not been ruled out as a solution to this problem, a new technology that has received considerable attention and an Editors’ Choice bronze award for best new product at Pittcon 2009 is the induced grating (IG) method from Shimadzu Scientific Instruments’ IG-1000. IG is a new technique for measuring the size of nanoparticles using dielectrophoresis and diffracted light that delivers excellent reproducibility and acquires stable data, particularly for sub-10 nm particles.
How it Works Particle size is measured using the diffusion rate of a grating that is composed of particles in the liquid. The diffusion rate of large particles is slow and that of small particles, especially nanometer particles, is fast. The diffusion behavior of particles can be monitored by detecting the change of primary diffracted light.
In the Shimadzu IG-1000, a diffraction grating is formed by drawing particles toward the electrodes when dielectrophoresis is on, and the diffraction grating disappears when the dielectrophoresis is turned off and particles are released away from the electrodes. The decay process of this particle density diffraction grating is measured via the change in intensity of the diffracted light, and a relationship between particle size and diffusion rate is then established.
Stable measurement with good reproducibility is possible because IG utilizes optical signals emitted by the diffraction grating formed by the particles and not scattered light emitted by the particles. Even in the single nano region, a good S/N ratio can be obtained.
The new measurement principle is resistant to contamination and, even if the sample is mixed with small amounts of foreign particles, information about the particles to be analyzed is captured reliably. The filtering of samples in order to remove coarse particles is not required.
An alternating voltage is applied to cyclically arranged electrodes, and a cyclic concentration distribution of microscopic particles is formed in the liquid by dielectrophoresis. Although the cyclic concentration distribution of microscopic particles acts as a diffraction grating (a particle concentration diffraction grating), if the alternating voltage is stopped, the grating diffuses and disappears (patent pending).
The cyclically arranged electrodes also function as a diffraction grating, although the light created is weaker than the diffracted light created by the particle concentration diffraction grating. The electrode configuration has been modified as shown in the figure so that the pitch of the electrode diffraction grating is half that of the particle concentration diffraction grating (patent pending). In this way there is a more precise measurement.
The IG method also ensures high reproducibility and the acquisition of stable data. In particular, high reproducibility for particle sizes of less than 10 nm removes the uncertainty of particle analysis in the single nano region.