Laser diffraction has become a standard technique for routine particle size analysis in a variety of industries. The strengths of laser diffraction have been so compelling, it has begun to displace more traditional methods for particle size analysis, such as sieving.
Making this transition requires some assessment of the strengths of laser diffraction and how it compares with sieving. The following points are important to weigh when considering laser diffraction as an alternative or replacement for sieving.
The repeatability and reproducibility of results with diffraction is markedly better than with sieving.
Typical laser diffraction measurements feature reproducibility of better than 1% and repeatability of better than 0.5%, with little variability contributed from the operator. This is in stark contrast to sieving methods, where reproducibility can vary strongly with operator and even the age of the sieve stack. The statistical advantages of laser diffraction offer one of the most important arguments in moving to an analytical method from sieves.
Laser diffraction requires a smaller representative sample than sieving, but is also capable of using a larger sample.
Sample size for a dry-dispersed laser diffraction experiment ranges from 10 mg to 30 g of material to obtain robust statistics, with a typical experiment using grams of material. The only practical limit on sample size for laser diffraction is the length of the experiment, as the instrument can be continually fed or even placed online. This is in contrast to sieves, where a practical limit is dependent on the amount of material each sieve can handle. Thus, the sample size required for laser diffraction offers considerable flexibility to handle the most polydisperse material.
Laser diffraction experiments take seconds, compared to minutes for sieve testing.
In modern material testing, especially for process monitoring, speed is the single most important factor in selecting an analysis method. The difference between a few seconds and a few minutes can often be measured in thousands of dollars in wasted material due to suboptimal process conditions. A routine laser diffraction experiment takes on the order of 15 seconds for 10 g of material, meaning the measurements can be conducted almost in real time without the need for process instrumentation, and the results are available as soon as the run is complete. This also means that a trained operator can run more than 100 samples in an eight hour shift with little effort. Sieve methods requiring a few minutes each, plus cleanup, often constitute a bottleneck in the analysis lab, resulting in wasted material and limiting the number of samples that can be run in a single shift. Laser diffraction provides not only an improvement in the control over process conditions, but also improves lab efficiency and throughput.
The cost of ongoing operation for a diffraction system is minimal compared to sieve replacements.
Laser diffraction systems require simple window cleaning, and sometimes window replacement, in order to maintain the system. For a dry dispersion system the vacuum bag must also be changed regularly to ensure performance. Contrast this with a sieve stack, where wire breakage and tearing will eventually require the sieves to be replaced to ensure continuity with earlier measurements.
Customizing reports in the Mastersizer range allows easy sieve data interpretation if needed.
The Mastersizer 3000 software suite allows for simple customization of reports to provide the most relevant information to the user. When correlating to a sieve stack, simply select the corresponding size ranges and add them directly to the report to obtain percent-in-range. Particle size distribution and common statistical parameters can also be easily displayed on a single report to assist in specification transfer for demanding customers. Data may also be quickly exported though customizable export templates to common Excel-compatible formats.
The size range of diffraction spans 0.1 - 3500 µm, far better than sieves.
Laser diffraction instruments, such as the Mastersizer 3000, cover a huge dynamic range in a single measurement. Sieves are limited to ranges that cover tens to hundreds of microns, with poor performance under 45 microns. Sieves also sample the middle dimension of particles, e.g., plate-like or needle-like particles may have dimensions many times larger than the sieve openings, but can still pass through. Such a situation can result in misrepresentation of the true size of particles in a sample.
Inter-sample maintenance and cleaning of diffraction is faster, simpler, and easier.
Dry dispersion laser diffraction methods require, at most, a quick brushing of the system before the next sample can be loaded and ready to measure. The Aero S disperser available for the Mastersizer 3000 even has a pre-coded cleaning routine included for users. Typical turn-around time for cleaning is about 30 seconds between samples meaning less dead time between experiments. Sieves require more extensive cleaning and weighing of the screens between experiments that can add ten minutes or more to the total turn-around time. Laser diffraction provides much better up-time and less exhaustive cleaning requirements when directly compared to sieve methods.
Automation of test conditions ensures ideal sample presentation in the Mastersizer products.
Malvern's entire line of laser diffraction technologies is designed around the Standard Operating Procedure (SOP), minimizing inter-user variance while maximizing efficiency. Once an SOP has been established for measuring a sample the user only needs to load and properly name the sample and the instrument will do the rest. Iterative development of an SOP will include setting of all computer controlled parameters, cleaning, report printing, and results export, and can be accomplished quickly without expert attention. Automated sieve methods are available, but still suffer from longer cleaning times, operator variance, and longer experiment times. The Mastersizer line provides the fastest and most accurate method for getting particle size results.
Sample containment with a vacuum in dry laser diffraction methods negates potential contact hazards.
The Mastersizer line of dry dispersion accessories is completely sealed from the sample loading area through to the vacuum bag. Utilization of a closed system means that exposure hazards to users are minimized. The use of a vacuum collection system makes disposal quick and easy. In addition, the instrument is small enough to fit into a hood or enclosure for maximum safety when running dry samples. Sieve methods are more prone to exposure risk due to their open architecture. Additionally, there is no direct collection of the sample into a convenient container for disposal.
Diffraction makes your lab quieter while taking up a small footprint of bench and cabinet space.
The Masterizer 3000 laser diffraction system requires half the space of its predecessor, around 27" total. The system can run on almost any internal air supply, and the vacuum can be located outside of the lab to minimize noise. Contrast this with a sieve assembly that is noisy, dirty, vibrates, and takes up a larger amount of bench space. Laser diffraction has a clear advantage in making your lab a better place to work.
