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Fast, high-resolution measurements of multimodal particle size distributions
By Judith Hadley
Agilent Application Scientist
Colloidal particle suspensions often include a wide range of particle sizes. Accurate determination of the size of these nanoscale particles is important to ensure the stability, reliability, and efficacy of the suspensions. For example, to effectively adhere and cover the surface, coatings require multiple particle sizes. In contrast, for consistent taste and texture, artificial flavorings in foods and beverages must have only one size of particles. Both applications need an accurate particle size analyzer that can distinguish the presence or absence of multiple sizes. Techniques like laser diffraction that average the results cannot effectively find the details of a particle size distribution. An emerging technique based on ultraviolet-visible (UV-Vis) spectrophotometry measures multimodal particle size distributions in a fraction of the time and with much better resolution.
How does this particle size analyzer work?
Particle size is often determined by measuring light that is scattered by the particles. The Agilent 7010 Particle Size Spectrophotometer uses a proven UV-Vis platform that shines light onto particles in suspension and measures the scattered light using a highly sensitive photodiode array. The Agilent 7010 uses Maxwell’s equations to calculate the particle sizes based on the UV-Vis scattered light, as described in Agilent Application Note (5989-8805EN).
Laser diffraction instruments measure light scattering as a function of the angle of detection. The Agilent 7010 Particle Size Spectrophotometer measures scattered light at a fixed angle as a function of the wavelength of the incident light. Using a wide range of wavelengths from 190 nm to 1100 nm, this technique correctly measures particle sizes for mixtures of large and small particles. It requires only minimal sample preparation and cleanup, and is much faster than other particle size measurements.
Rapid identification of multiple size populations
A common challenge for light scattering techniques is the measurement of small particles in a population of large particles. Typically, the presence of a few large particles can scatter too much light, hiding the presence of small particles. As demonstrated in a previous newsletter article, the UV-Vis spectroscopic technique used by the Agilent 7010 is able to resolve bimodal and trimodal particle size distributions. The instrument requires only 10 seconds to accurately resolve multiple size populations.
Excellent resolution, even when sizes are very close
The results in the previous article demonstrated that the 7010 Particle Size Spectrophotometer can resolve populations that differ greatly in particle size, as well as in their volume proportions. The following examples demonstrate how close in size two populations can be, and still be resolved by the Agilent 7010. Polystyrene at 200 nm was mixed first with 1 µm polystyrene, and then with incrementally smaller polystyrene particles. Figure 1 illustrates the size resolution of 50%/50% polystyrene mixtures: 200 nm/1000 nm (blue trace); 200 nm/800 nm (red); 200 nm/600 nm (green); and 200 nm/500 nm (black).
In fact, the Agilent 7010 has resolved two different size populations that are as close as a ratio of 1:1.5. This is demonstrated in Figure 2 for 1 µm and 2 µm (blue), 2 µm and 3 µm (red), and 3 µm and 5 µm (green) polystyrene particles.
New approach brings better resolution and faster analyses
Unlike laser diffraction, the Agilent 7010 Particle Size Spectrophotometer measures scattered light at a fixed angle as a function of the wavelength of the incident light. The advantages of its extended wavelength range include the ability to measure smaller particles, and to correctly measure particle sizes for mixtures of large and small particles.
Because of the fast detection with a photodiode array detector, particle size measurements are performed in seconds, opening up new applications – for example, in samples that rapidly settle or in unstable systems. In summary, the Agilent 7010 Particle Size Spectrophotometer provides fast, accurate results over a wide concentration range and in a size range of practical interest to many researchers. For more information on using the Agilent 7010 for accurate measurement of particle sizes of polymers, download the full Application Note (5989-9976EN). |