methods and has a limited size range of below 10μm, restricting its use for certain samples. Additionally, cascade impactors only have calibrated performance at certain air flow rates, an issue with the potential to cause data inaccuracies. Laser diffraction In contrast to the preceding methods, laser diffraction uses the principle of light scattering to generate particle size distribution data. Laser diffraction determines particle size distribution by measuring the angular variation in intensity of scattered light as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam; small particles scatter light at larger angles. The pattern of scattered light produced from the dispersion is detected and, using an appropriate light scattering model, such as the Mie Theory, is converted into a particle size distribution. Figure 2: Laser diffraction measurements exploit the relationship between the scattered light pattern produced by a sample and the size of particles present. Laser diffraction systems can measure a large number of particles across a wide dynamic range, as seen in Figure 2. Those configured for sprays typically measure particles from 0.1μm to 2,000μm in size. Capable of performing rapid, high throughput measurements, modern instruments are able to make particle size measurements in real time during a spray event under atmospheric conditions and with no interference with the spray particles. The measurement zone can be configured to accommodate almost all spray geometries and methods are easy to develop and validate with accuracy and reproducibility. A good understanding of the influences of the refractive index for both the sample and dispersant is required in order to apply the Mie Theory correctly and achieve successful analysis. These values are freely available for the dispersant and easily determined for the sample, either through estimation based on available guidance or experimentally, using a refractometer. The high data acquisition rates of laser diffraction deliver insight into spray behavior that cannot be assessed via alternative methods. Such information allows developers to rapidly and efficiently learn how changes in design, such as propellant formulation or the mechanism of the valve, influence the size of the droplets produced and, consequently, the safety and performance of a product. Case Study: Using laser diffraction for real time spray plume particle sizing While aerodynamic particle sizing techniques have been widespread within spray development for some time, laser diffraction has been more recently embraced as a valuable addition to the analytical toolkit that offers a new approach to spray testing. To demonstrate the technique’s applicability for realtime spray analysis, an experiment was carried out to investigate how variables in aerosol design directly effect the particle size distribution of spray droplets. A Spraytec laser diffraction system (Malvern Instruments) was used to analyze the particle size distribution of droplets within the spray plume of a typical aerosol, tested with different propellant concentrations (70% ethanol: 30% propellant and 30% ethanol: 70% propellant) and different valve configurations (standard and vapor tap). The experiment was set up with a 15cm measurement distance from the nozzle to the beam and an extractor behind the measurement zone. Figure 3 shows the change in particle size over time following actuation of a spray product, measured using laser diffraction. These data are better illustrated by Figure 4a and Figure 4b, which show the particle size distribution within the spray at the very start of the spray event and again once a stable particle size distribution has been reached. The particle size distribution is shown to have changed markedly during plume development and is significantly narrower, only 1.5 seconds after actuation. The effect of the propellant composition was assessed by examining the transmission profiles produced during actuation. Transmission is a measure of the amount of source light detected and is therefore directly related to concentration, a crucial parameter when monitoring the delivered dose. Figure 5 shows that the 30% ethanol: 70% propellant Figure 3: Changes in particle/droplet size during spray plume development, measured using laser diffraction. May 2014 Spray 43
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