Developing a Laser Diffraction Particle Size Analyzer for Current Customer Requirements

Particle size analysis has traditionally been dominated by historical techniques such as sieving or sedimentation procedures. Over the past 10–15 years, laser diffraction (static light scattering) has taken over a large part of the market because of its speed of analysis, ease of use, reproducibility, and efficiency. For most applications in the 100-nm to 1000-μm range, laser diffraction’s advantages far outweigh the initially higher cost of an advanced optical instrument.

As the technologies have matured, the instruments offered by various competitors have, to a great degree, converged into a similar design approach and similar performance. Differences among various manufacturers may be based on historical developments or on the product being optimized for a perceived target market. As the technique becomes more widely accepted, new applications arise that have different requirements and require a different approach. The challenge to the manufacturers is to identify these needs and design a product that will satisfy a wide range of customer requirements.

Figure 1 - Partica LA-950 particle size analyzer.

The Partica LA-950 (Figure 1) laser diffraction particle size analyzer (Horiba Instruments, Inc., Irvine, CA) was developed as a solution to customer requirements for existing markets as well as emerging ones. After the company performed a thorough market study and discussed requirements with end users, the design targets were formulated to provide the greatest utility and to be a step forward for users of particle size analyzers.

The new optical system design provides a very wide particle size measurement range (0.01–3000 μm), while maintaining high performance in the submicron range and increasing the large size limit in order to extend the use of the technique to new markets.

In addition, particular attention has been paid to the user’s ability to generate accurate, reliable data quickly and easily. Sample-to-sample analysis time has been reduced to 1 min total, and the built-in automation of the sampling systems and software eliminates time-consuming manual tasks such as filling with dispersant fluid, selecting conditions, and initiating each step of the analysis sequence. Combined with the outstanding precision of the analysis system, reproducibility and repeatability from site to site have been improved dramatically.

The submicron measurement improvement is made possible by development of the company’s existing high-angle scattering measurement technology. Since the scattered signal from particles smaller than the wavelength of the incident light are at higher scattering angles, low intensity and not highly resolved like those from larger particles, an increased number of more sensitive high-angle detectors and a higher intensity and shorter-wavelength solid state, light-emitting diode (LED) second light source has been added to increase the intensity of the scattered signal. This results in increased sensitivity, accuracy, and resolution for smaller particles, thus allowing measurement of a wide range of new nanosized materials that would normally require a different analytical technique, such as photon correlation spectroscopy (PCS). The end user benefits because this analytical capability is combined with the system’s larger size capabilities, possibly avoiding the need to obtain a second analyzer.

The larger size measurements are made possible by a reverse-Fourier optical design with a folded optical path, which achieves the longer focal lengths necessary while reducing the footprint to almost half of that required by systems with an equivalent upper size limit. The optical bench itself is made of cast aluminum for rigidity and thermal stability, which contributes to the system’s precision compared to competitive optical systems with components mounted on sheet metal or on circuit boards.

Since market size does not often allow for a range of models for specific applications, the Partica LA-950 was designed as a wide-measurement-range, general-purpose analytical system. In addition to the dramatic improvement seen in measurement technology, the main opportunities for advancement in laser diffraction particle size analyzers include usability, analysis speed, and the ability to tailor the system for specific applications.

Different applications require alternative sample dispersion systems for optimum analytical performance. For users with multiple types of samples, the traditional approach has been to make the change from one sampling system to another as easy and fast as possible. This usually requires removing a measurement cell, disconnecting the sampling system, and replacing the components for the alternate type.

Figure 2 - The Partica’s measurement chamber showing the multiple cell positions and ease of access for cleaning.

The Partica LA-950’s optical bench mounts the optical components vertically, allowing the measurement cell to be mounted on a sliding cell tray that passes through an opening in the bench. This allows for up to three different measurement cells to be mounted simultaneously, and for the sampling systems to remain in place. Changing from one measurement method to another is accomplished in as little as 5 sec by moving the cell tray from one position to the next (Figure 2). The sampling system connections remain in place at all times. The instrument automatically recognizes the sampler type and configures the software for the sample operation. This allows for a single instrument to be used for multiple types of analysis without lengthy reconfiguration or operator involvement.

Customer requirements call for a range of sample dispersion and measurement systems for a vast array of sample types. These include dry powder analysis, small-volume samplers for precious or toxic samples or for dispersing media, large-volume systems for increased sample weights to improve sampling, autosamplers for maximum sample throughput, and organic solvent compatible systems.

The standard fluid circulation system includes a high-volume fill pump that can fill the flow system in 3 sec, controlled by a multilevel fill sensor. The high-efficiency circulation pump is powerful enough to suspend even the largest dense particles, but can be adjusted to be gentle enough for friable particles or emulsions. A built-in ultrasonic probe is included to allow in-system dispersion of agglomerates, thus avoiding the need for external sample preparation.

The best return on investment is provided when the greatest number of samples can be run on a single system. The analysis time for most laser diffraction instruments is on the order of 3–5 min. The Partica LA-950 has been designed to accomplish a sample-to-sample time of 1 min total, including filling with dispersant, alignment, recording of background, circulation of sample, analysis, and drain and rinse cycles. The careful design of the dispersant filling system, alignment mechanism, electronics, and optical system can provide high precision, resolution, and accuracy over the entire size range, thus making the return on investment much more attractive.

For applications requiring toxic dispersants or precious samples, or in situations where waste disposal is an issue, small-volume sampling options are available with as small as 15 mL total volume. The measurement performance is equivalent to full-volume systems, while preserving precious samples.