Applying Blue Laser Technology to Microtrac Unified Scatter Technique for Full-Range Particle Size Measurement

The first truly commercial light scattering instrument [1] was developed in the U.S. by Leeds & Northrup more than three decades ago. This was based purely on Fraunhofer diffraction and measured particle sizes in the approximate range of 2 to 200 µm. At this time there existed a number of experimental laboratory and prototype units; however, these units were generally manually operated devices with extremely limited capability [3,4]. The measurement range was extended toward smaller sizes by L&N development teams in the late 1970s and early 1980s. This was achieved by employing Mie scatter theory and also differential polarization techniques in order to extend the size range down to 0.1 µm. At that time this technology was considered to be state of the art; filling many particle-sizing requirements during the 1980s.

A new "Unified Scatter Technique" was introduced by L&N development teams during the 1990s. This technique replaced the older differential polarization method with its associated problem of discontinuities when incorporating two different scatter methods into one integrated particle size distribution. This new method made use of both a high-angle and a forward detector array, combined with a unique and new Tri-laser System, which efficiently multiplies the logarithmic detectors available for scattered light detection. Scattered light was produced by the three laser arrangement through an angular range from extremely close-to-zero degrees up to 160 degrees, in one continuous pattern.

The Tri-laser system has been upgraded; adding blue lasers to bring improvements in sensitivity and resolution in the nanosize and submicron ranges. The concept of utilizing logarithmic detector design to provide shift-invariance described for Tri-laser systems is maintained and extended to the combined use of blue and near-infrared lasers

Volume Response

Patterns, proportional to the cross-sectional area of the particles, are produced by light which is scattered by particles. Users of particle measurement systems would usually prefer results in terms of the "amount" of particulate material, such as volume, rather than area. A unique detector geometry, which is capable of producing signals proportional to the volume of particulate material rather than area, was employed by early Microtrac Analyzers. Previously this detector geometry was utilized in all MICROTRAC Analyzer systems. The geometrical configuration refers to a rectangular line array of non-linear detectors, which transforms scattered light flux into signals that are directly proportional to particle volume. It is possible to readily convert these signals into a volume particle size distribution using a high-resolution iterative deconvolution algorithm.

Logarithmic Vs Linear Array

Scattered light, which is produced by particles in a laser beam, is not a linear phenomenon. In practical system designs, the low angle scatter (less than 10 or 15 degrees) is capable of accommodating size ranges from hundreds of microns down to several microns. It requires very large angles beyond that to provide scattered light information for sizes below several microns down to 10 nm. For this reason, and also because scattered light intensities decrease with increasing angles, it is essential for the detectors in a measurement system to grow progressively as the scatter angle increases. For this reason MICROTRAC systems have always utilized a logarithmic progression of detectors [5].

An advantage of the nonlinear detector array can be viewed in the first 2 figures. The scattering functions and their plots show that the linear array (Figure 1) generates scatter intensities that are quite different for particles of varied sizes (50 micron and 20 micron particles used as examples). In contrast, patterns that are identical in shape for the different sizes, but simply displaced on a log-angle axis, are produced by the logarithmic, array (Figure 2). The data processing task of deconvolving a particle size distribution from scattered light flux is optimized by this convenient shift-invariant functional relationship.

Linear Line Array

Figure 1. Linear Line Array

Logarithmic Line Array

Figure 2. Logarithmic Line Array

Single Laser Systems

Detectors of different sizes and shapes are employed by instruments which utilize light scattering techniques. These detectors range from small to extremely large and some take on extremely strange-looking configurations. Additional enhancements such as polarization and side scatter techniques are used by a few instruments. One common aspect found in all these instruments is the utilization of one laser as the source.

MICROTRAC also employed a single laser process until the advent of the Tri-laser System, with the exception of progressing from a gas laser to a solid state laser for prolonged dependability and analyzer size reduction. A single laser with manipulation of detector geometry is capable of producing real modal information in particle size distributions down to about 0.5 micron. If distributions are relatively wide, well-behaved, and have a single mode, it is possible to mathematically extend the small particle response in single laser systems to approach the 0.1-micron limit that many systems claim.

However, if sample materials comprise of components that contain modes below about 0.5 micron, the single laser systems will not be able to distinguish those modes from the main distribution, or from other modes at larger sizes. Additional information is often required in the form of blue LEDs and blue filters, possibly along with polarization techniques, the disadvantages of which are discussed earlier, or a whole new approach provided in the multiple laser system as discussed in this article.

The Tri-Laser System – Advanced Use of Blue and Red/NIR Lasers

MICROTRAC has developed an advanced Tri-laser System, which allows light scattering measurements to be produced from the forward low angle region to almost the whole angular spectrum (approximately zero to 160 degrees). Until the introduction of commercial lasers, “diffraction” measurements were complex, due to beam divergence, intensity, bandwidth, and coherence. Similar issues are experienced with filters and LEDs. Sources that used only laser technology were incorporated by the original Tri-laser system. The utilization of blue lasers (shorter wavelength) in the advanced system provides distributions of improved sensitivity and resolution, especially in the submicron and nanosize regions of the particle spectrum. The application of blue laser technology, combined with highly developed mathematical analysis, improves the ability of instruments to measure submicron and multi-modal particles with unsurpassed sensitivity and resolution. Scatter from nearly on-axis to about 60 degrees is produced by the primary laser (red/on-axis) with the help of a high angle array and forward array, both of which are logarithmic. The second laser (blue/off-axis) is positioned to generate scatter beyond the 60 degree level using the same detectors (Figure 3). The third laser (blue/off-axis) is positioned to produce backscatter, again using the same detectors. This technique multiplies the number of sensors that are available for detection of scattered light in an effective manner.

TRI-LASER System with Blue Lasers

Figure 3. TRI-LASER System with Blue Lasers

The laser can be motor-driven to different locations to alter the incident angle for illuminating the particles, as shown in Figure 4a. However, this movement proves to be impractical due to the requirement of mechanical movement and the attendant issues of mechanical failure or fatigue. The lasers in the Tri-laser system are solidly positioned at correct locations to modify the beam’s incident angle. Relocation of the wider angle scattering pattern on the same detector is enabled via the use of a second laser, thus enabling efficient use of one detector and two lasers. This concept is extended by the Tri-laser system by using three lasers and two detector arrays, as shown in Figure 3.

Moving a single laser to new location or applying a second laser in a different location allows the same detector to be used to measure wider angles for smaller particles.

Figure 4a. Moving a single laser to new location or applying a second laser in a different location allows the same detector to be used to measure wider angles for smaller particles.

Figure 4b provides a better understanding of the extent of the angular range. A relatively small fraction of the total scattered light is collected by the axis detector, which however represents a large fraction of the total size range. The angular range is markedly increased by the addition of an off-axis detector. The sensitivity at wide angular ranges is tremendously increased by the full TRI-LASER capability, incorporating blue lasers, which also provides an extremely significant extension to full-capability response to submicron and nanosize particles. This very large angular response, incorporated with blue laser technology, provides the resolution capability to readily separate individual modes down to the absolute limit of size response of the system.

Angular range of detection using 1 laser and multiple detector arrays. Near zero to 160º. The same can be accomplished using 3 lasers and limiting detector systems to 2.

Figure 4b. Angular range of detection using 1 laser and multiple detector arrays. Near zero to 160º. The same can be accomplished using 3 lasers and limiting detector systems to 2.

Data

Figure shows comparison and capability of Bluewave to measure nano particles.

Figure shows comparison and capability of Bluewave to measure nano particles.

Figure shows a comparison of different size polystyrenes with a bimodal mixture of the two. Note that size in mixture is same as individual polys.

Figure shows a comparison of different size polystyrenes with a bimodal mixture of the two. Note that size in mixture is same as individual polys.

Figure shows that Bluewave is sensitive to changes of volume percent of nano size particles in a tri-modal particle size distribution.

Figure shows that Bluewave is sensitive to changes of volume percent of nano size particles in a tri-modal particle size distribution.

Figure shows size sensitivity of particles in a mixture compared to individual certified polystyrene samples.

Figure shows size sensitivity of particles in a mixture compared to individual certified polystyrene samples.

Conclusion

An advanced Tri-laser particle size analysis system design (Microtrac BLUEWAVE), capable of measuring particle size down to 0.010 microns and as large as 2816 microns with no assumptions or curve fitting of the distribution, has been described in this article. Blue lasers, in combination with advanced mathematical analysis, have the potential to readily discern individual modes throughout the submicron region by means of a "Unified Scatter Technique," without resorting to more than one scatter mechanism such as differential polarization. A continuous pattern of scattered light flux, from near forward to near backward components with no discontinuities, is unified by this technique.

These Bluewave instruments have been developed to improve the precision of wide range “diffraction” particle size measurement, especially in the nanosize and submicron region. For extremely fine particulate sample materials, including biotechnology samples and those that do not need dilution, the choice should be the Nanotrac Analyzer with its measuring range of 0.8 nm to 6.5 µ, using the principle of heterodyne Dynamic Light Scattering [6]. The instrument has the potential to perform measurements using the unique “Dip–n-Run” particle size technique.

References

[1] Weiss. E. L. & Frock, H. N., Powder Technology, 14 (1976) 287 "Rapid Analysis of Particle Size Distributions by Laser Light Scattering."

[2] Comillault, J., Appl. Optics, Vol. 11, No. 2, Feb. 1972, p. 265, "Particle Size Analyzer".

[3] Gravatt, C. C., Optical Spectra, Jan. 1973, p. 35, "Light Scattering with Laser Sizes Particulate Matter."

[4] Shofher, F. M., et. al., ISA Transactions, Vol. 12 No. 1 (1973), p. 56 "Design Considerations for Particulate Instrumentation by Laser Light Scattering."

[5] Freud, P. J., Trainer, M. N., & Weiss. E. L., "Particle Measurement by Linear System Modeling and Inversion of Scattered Light", Presented to the 1990 Pittsburgh Conference & Exposition for Analytical Chemistry and Applied Spectroscopy; Symposium on "Powder Characterization and Particle Size Analysis", New York, NY, March 5-9, 1990.

[6] Freud, P. J., Trainer, M. N., "A New Approach to Particle Sizing by Dynamic Light Scattering", Presented to the 1990 Pittsburgh Conference & Exposition for Analytical Chemistry and Applied Spectroscopy; Symposium on "Powder Characterization and Particle Size Analysis", New York, NY, March 5-9, 1990.

[7] Freud, P.J., Trainer, M.N.,"Unified Scatter Technique for Full-Range Particle Size Measurement", American Laboratory, November 1994

This information has been sourced, reviewed and adapted from materials provided by Microtrac.

For more information on this source, please visit Microtrac.

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