Microtrac’s Tri-Laser Particle Sizer

Microtrac, Inc. has developed a multilaser particle sizing analyzer and sample delivery controller designed to provide better resolution at lower particle sizes in both wet and dry mediums.

The new method still uses a forward laser, but improves on particle size limit and resolution through the use of two additional high-angle lasers. The combined tri-laser system multiplies the number of logarithmic detectors available for light detection, and increases light scatter detection from near zero degrees to 160 degrees in a continuous pattern.

The resulting improvement in particle size identification enables measurement of particles from 2,860 to .02 microns in size.

The Microtrac S3500 Tri-Laser particle analyzer provides accurate measurement of submicronic particle size without the use of distribution modeling, curve fitting or assumptions.

The unit is capable of differentiating between individual modes (particle sizes) throughout the submicron size range through the use of unified scatter technique. The sample delivery controller (SDC) is designed to facilitate and control delivery of the sample material to the S3000 Analyzer by software control.

Background on Light Scatter Technology Particle Counters

Leeds and Northup (L&N) developed models of commercial particle counter instruments based on light scattering technology for particles sizes ranging from 2 to 200 microns using Fraunhofer diffraction methods.

Improvements utilizing Mie scatter theory and differential polarization techniques allowed L&N to push the lower resolution to fractions of microns, achieving .1 micron resolution in the 1980s.

Unified scatter technique was developed to replace differential polarization in the early 1990s. The benefits associated with the unified scatter included elimination of discontinuities associated with two different scatter methods integrated to provide single sample readings.

Principle of Light Scatter Technology

Light has properties that can be used for determining particle size and particle size distributions. The angle at which the light is diffracted depends upon the wavelength of the light and the particle size.

The angle of diffraction is measured to determine particle size. For a particular particle size, should the wavelength change, the angle will change. Frequency is a measure of how many waves pass through a given point during one second.

The more waves that cross the point, or the closer the distance between the waves, the higher the frequency. Frequency change or shift information is used in dynamic light scattering.

Light which is scattered by particles produces patterns that are proportional to the cross-sectional area of the particle. Low angle scatter (10 to 15 degrees) can accommodate size ranges from hundreds to several microns.

Scattered light intensities decrease as the angle of deflection increases. Detector sizes should become progressively larger as the scatter angle increases.

Characteristics of the Microtrac S3500

Users of this technology prefer readings in terms of amount of particulate based on volume rather than area. The Microtrac S3500 uses a unique detector geometry that produces signals proportional to the volume of the particulate material rather than the area of the particulate.

The S3500 also uses a logarithmic array instead of a linear array. Scatter patterns produced by the logarithmic array are identical in shape by particle size, but register at different locations on the log angle axis. This difference optimizes the process of separating and analyzing signals.

The key feature is the use of multiple laser light sources to enhance detection across a much broader selection of diffraction angles. As noted previously, single laser systems provide measurement up to roughly 15 degrees. The S3500 provides measurement up to 160 degrees.

The primary laser produces scatter from nearly on-axis to about 60 degrees. The second laser, which is off-axis, is positioned to produce scatter beyond 60 degrees. The third laser is positioned to produce backscatter. The second and third lasers use common detectors.

The addition of the off-axis detectors appreciably increase the angular range and enable the size range to extend to .1 micron. Figure 1 shows how the configuration of lasers and photo detectors align with one another to produce the range and resolution.

Figure 1. Top-down view of the optical configuration used in the Microtrac S3000. Provides optimized efficiency of lasers, lenses and detectors.

Sample Delivery Controller

The SDC was developed to be an integrated companion to the S3000 family of TRI-LASER analyzers. The SDC is a modular system that can accommodate most sampling requirements. The sample is equipped with a built-in ultrasonic probe to disperse the particles as they flow through the systems.

The fluid handler will de-aerate, pre-circulate, activate analysis, drain and rinse automatically, and is designed to accommodate any common laboratory solvent.

Technical Editor’s Note:
The S3500 does not report particle concentration for given sizes. The technology represents improvements in particle size recognition that would be useful in improving particle size recognition for use in industrial particle counting applications.