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The NanoFlowSizer is a recent developed equipment to measure particle size distributions in turbid systems. The particle size distribution is for a broad range of applications an important quality parameters and usually estimated using light scattering techniques with well-known equipment like the NanoSizer and MasterSizer. The new method, based on optical coherence tomography, is especially suitable for fast and reliable measurement of particle size distributions of concentrated turbid systems, either inline, online, or offline as well as in flow. The NanoFlowSizer is suited for a broad range of applications and contributes to finding answers to relevant research questions.

Technical Details

Particle size distribution is for various materials an important quality parameter. This includes the droplet size distribution in emulsions, bubble size distribution in aerated systems as well as molecular, particle, and aggregate size distribution in suspensions. Applications are found in many disciplines like food, pharma and (bio)chemistry. A well-known technique for particle size measurement is light scattering, where the intensity of light of a specific frequency is measured that is scattered by the particles at different angles and/or times. Static light scattering (SLS) is often used for determination of particle sizes in the micrometer to millimeter range. Typical instrumentation for measuring particle size distribution using SLS is the Mastersizer (Malvern). For particles in the nanometer to micrometer range, dynamic light scattering (DLS) is used. Typical equipment for particle size determination using DLS is the Zeta or Nano-sizer (Malvern). In general, for a correct interpretation of the angle dependence of the scattered light intensity (SLS) and autocorrelation function of the scattered light intensity at one or more scattering angles (DLS) it is important that the scattered light is due to the scattering of only one scattering event (single light scattering). For turbid systems this is often a problem because due to the large concentration of scattering particles, light entering the detector is mainly the result of multiple scattering. To minimize the multiple scattering, samples are often diluted. This, however, is mostly unwanted because particle size distribution can change due to dilution and change of environment and equilibrium between the particles, like e.g. is the case for protein solutions. A way around could be the use of diffusing wave spectroscopy (DWS) which corresponds to dynamic light scattering in the multiple-scattering regime. However, for DWS interpretation of the scattering autocorrelation function is tedious and complicated, which makes the estimation of particle size distribution in concentrated turbid systems problematic.

Recently a new light scattering technique has been developed based on optical coherence tomography (OCT) using a broad-band light source in combination with a Michelson interferometer, that is especially suitable for fast and reliable measurement of particle size distributions of concentrated turbid systems. The technology can be described as Spatially Resolved Dynamic Light Scattering (SR-DLS) and can be used either inline, online, or offline. Furthermore, particles size distributions can be determined when the system is in motion (flow) in contrast to existing particle size measurement techniques. The instrumentation is called the NanoFlowSizer.

The science behind the NanoFlowSizer is the application of low coherence interferometry that allows characterization of the scattered light at high frequency as function of (penetration) depth in the sample. This yields multiple correlation functions, each characterizing intensity fluctuations at a particular depth or path length. The spatially resolved character allows distinction between single and multiple scattered light, because shorter path lengths will contain typically single scattered light. This allows direct measurement of relatively high concentrated nano-suspensions (typically 50-100x more concentrated compared to standard DLS).