Newer techniques of particle system analysis are emerging, in contrast to size analysis, the only one previously available. This is in response to the need felt for a full analysis of the particle shape and size. Image analysis technology is being applied in ways which can help analyze particles with more parameters for both industry and research. This will in turn speed up the discovery, identification and solution of many problems which would have been missed if only one size parameter was used.
The most useful among the newer techniques is laser diffraction (LD) particle size measurement. It is in widespread use among industry and research suppliers and users of particle systems, providing the standard for quality control measurements of both intake and output materials through its capabilities in measuring the equivalent spherical diameter (ESD) in volume percentage over a particle size range of less than 10 nm to 2-3 mm.
Dynamic image analysis (DIA) technology-based measurement of particle morphology functions as a synergistic technique which adds significantly to the value of LD reporting. This article describes how one sample in the same workflow and the same sample cell can be measured using both LD and DIA simultaneously, with the novel Microtrac Sync instrumentation.
Combination Instrument of LD and DIA
The principle of LD is the scattering of coherent light by particles to different degrees depending upon the particle size. Thus small particles scatter the photons of larger wavelengths more and to greater angles of deviation, compared to larger particles. The LD instrument directs a coherent laser beam across a stream of particles which scatters it. The scattered light in turn reaches an array of diode detectors located at angles of zero to 160 degrees away from the angle of the laser beam. The detectors measure the distribution of light flux in order to assess the size distribution of the scattering particles.
The DIA principle depends upon the image capture of particles streaming past a digital camera with high resolution. The illumination is provided by a high-speed strobe backlight. The images are fed into a video file. Using the pixel size and the number of pixels in each image, the software provides a calculation of multiple size and shape parameters as well as the light intensity from the particles.
The FlowSync has the capability to handle the sample with precision and consistency through a number of operations, such as filing, de-aeration, pre-circulation and circulation. The automated nature of the operation lends it a high degree of repeatability with respect to the final data output. The FlowSync is so designed that it keeps the stream in turbulence, thus keeping the particles in constant motion within the streaming flow without having to stir them.
Any collection of particles is broken up by an inline ultrasound probe which can vary its power setting as required. This provides for consistent dispersion of the sample throughout the measurement process. The sample container has a “wash” feature to clean off the vessel walls during the “rinse” cycle, preventing any materials from being carried over to the next measurement cycle.
The TurboSync makes sure the uniformly dispersed sample reaches the measuring container or cell. This is essential to ensure repeatable and unvarying analysis of the particle size for dry powders, even for volumes as low as 0.1 cc, making it useful when the sample is costly or rare.
The use of compressed air and settable flow conditions mean that dry samples, even of alumina and similar materials which typically have a high degree of agglomeration, are distributed optimally, as usually expected only with fluid dispersal systems. Even if the material is delicate, the conditions for optimum dispersion can be set with precision. The TurboSync autoscan takes about 10 seconds to complete a measurement.
These two instruments work in a complementary manner in the Microtrac Sync to make up a single step system with an intuitive connect/disconnect setup. In other words, it is easy to switch from wet to dry analysis, because there is no need to change wiring or tubing connections around, all that is needed is to change the sample module out for the appropriate one. The instrumentation is equipped with Microtrac FLEX software which can rapidly and simply set up a program for each measurement cycle. The data is then saved on the same computer system and can be exported to other LIMS systems or user networks.
Laser Diffraction Measurement Results
The original Fraunhofer theory algorithm used in LD has now been replaced by the Mie theory which compensates for transparency. The Microtrac Sync has an LD mode which uses a specialized algorithm set in order to offset the inability of the Mie theory to measure non-spherical particle shapes. This limitation is common to both theories and restricts their measurement to spheres as a result.
LD has become deservedly popular for the quality control specification of particle size in most industrial fields because of its ease, rapidity of results and established accuracy and consistency. Figure 3 shows a detailed and complete LD size quality control report using the Sync. It shows, in clockwise order beginning at the top left corner, the following:
- size distribution table
- standard operating procedure (SOP)
- measurement Information
- size percentiles
- summary data
Dynamic Imaging Analysis Measurement Results
The measurement of particle size and shape using DIA yields a wealth of additional information about the physical attributes of the particles. This is important because many particle systems, and the products they are used to manufacture, display changed key characteristics even though the LD size distribution remains unchanged. This type of problem is best identified by analysis of particle morphology, which will result in a solution.
The principle of DIA is simple, but the measurements are then analyzed using a very sophisticated and flexible software which rapidly identifies and solves problems. The primary features of the post-measurement DIA software are described below.
DIA X-Y Graph and Table
Figure 4 shows the X-Y graph and table derived from the DIA analysis. The software allows the format of the curve to be varied from logarithmic, linear, volume percentage, count percentage, differential, cumulative coarser, or finer, as required. Each particle in the stream is individually measured, for infinite resolution just as in single particle counting, rather than in ensemble counting techniques. The number percentage is also very accurate, as a result, whereas ensemble technology has to fall back on volume distributions from which the number distributions are back-calculated.
Figure 4 shows the View Particles screen, in this case derived from a sample of fused glass beads, individual glass beads and sand. The callouts 1, 2, and 3 indicate the three particle types. The Search Particles button (4) will result in opening the Particle Query (5), in which window a query has been typed in. This will include all particles which have an area below 10 square microns but over 2 microns in equivalent diameter.
In callout 6, a search is displayed showing that the particles searched for comprised 99.91% and 99.37 % by volume and number respectively. The Data tab (7) enables the opening of the image file in the form of a spreadsheet which displays all particles as rows against the full array of 33 parameter columns. Using this, it is possible to select the Scatter Diagram display instead.
The scatter diagram (SD) is a way to represent the location of all the particles in a search listing any two of the morphology parameters in a two-dimensional manner. This helps to rapidly evaluate the presence of multiple modes in any distribution along the X or Y axis, as seen in Figure 9, which shows a pair of SD displays.
The first plots size against sphericity, while the second, on the right, plots size against transparency. The size is in terms of Da. Modes occur in areas of high concentration, which are shown as dark blue. Three separate modes may be seen in each SD. The first, mode 1, has a very fine particle size, making up a small part of the total volume of the sample. It is thought to be due to the measurement of disrupted beads.
Mode 2 shows a bimodal size distribution, probably because there are separate beads of high sphericity centered at just over 400 microns, as well as fused beads of between 500-1100 microns. Mode 3 shows lower transparency as well as sphericity ranges, with a size centering around 300 microns, and is probably due to the sand particles.
Another function is the Filter Function which provides a quantitative measurement of the constituents in a mixture. This is seen to be applied in Table 2, which was set for glass bead measurement. The possible categories are as follows:
- Transparent Particles
- Good Beads
- Bad Beads
The original distributions measured are preserved under all conditions. After measuring the components, the filters may be applied one by one if desired. The table can be incorporated into the SOP so that without searching for each component for individual identification, the table itself is used to analyze the measurement data.
LD has been around for decades as the most popular technique for measurement of particle size, both in research as well as in industry, because of its robust and accurate measurements coupled with simplicity and long history of usefulness.
DIA is now being used more and more to detect particle size, shape and other morphological features in particle systems.
The combination of LD and DIA is much more useful than either alone, by yielding information that could be missed by LD alone.
The use of analytical image software which can visualize, sort, identify and measure constituents of a mixture using up to 33 parameters of shape and size for each particle separately allows a huge volume of additional data to be obtained regarding the materials used in the process.
Microtrac’s instrument makes use of both these technologies in a single-step method which allows the same sample in the same sample cell during the same measurement cycle to undergo two different measurements simultaneously, within just a few minutes.
This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.
For more information on this source, please visit Microtrac, Inc.