Scattering large-particle

Distribution of radiation for (a) Rayleigh scattering and (b) large-particle scattering. 

Scattering by Large Molecules With large particles, scattering ean be different in different dircetions (Mie scattering). Measurements of this type of scattered radiation arc used to determine the size and shape of large molecules and colloidal particles (see Chapler 34). 

FIGURE 34-2 Scattering from particles of different sizes, (a) Large-particle scattering showing interference. Rays emitted in a backward direction observed at point D can destructively interfere because of the large path difference between the route SXD and the route SYD. Rays emitted in the forward direction observed at position D and traveling routes SXD and SYD are more likely to constructively interfere because of the much smaller path difference. 

Guinier showed that a characteristic shape-independent geometric function called the radius of gyration can be measured from large particle scattering. It is defined... 

Large particles scatter a greater amount of light but through a smaller angle. 

PCS measurements are more difficult around 1 /i. There are many reasons for this. The particle may be so dense and large that it sediments before a proper measurement can be made. Diffusion slows down inversely with size what was a repeatable 2 minute experiment at 0.1 is a 20 minute experiment at 1 fjL, provided the intensity remained constant. It doesn t. Large particles scatter more in the forward direction. There is, however, a more subtle effect that limits measurements on large particles. 

Most modern spectrometers provide a visual display of the log-normal distribution that has the same mean radius and variance as obtained from the moments analysis of the data. This can be somewhat misleading if the true size distribution of the sample is not close to being log-normal and can be a serious misinterpretation if the sample is multimodal. Some spectrometers do, however, provide enhancements of the above procedure if a sample is suspected to be multimodal. Further, since no consideration has been given to the fact that large particles scatter more light than small ones, then all the results of moments analysis are intensity weighted or -averages quantities. 

Watson, R. M. J., Jennings, B. R., Large Particle Scattering Factor for Flat Particles, J. Colloid Interface Set, 1991, 142,244-250. 

Shortly after the consumption of a fatty meal, the presence of chylomicrons is very apparent in a sample of plasma (lipaemia). The very large particles scatter light giving the plasma an opalescent appearance. The particles can be isolated as a floating layer after a short low speed centrifugation and can be withdrawn from the top of the tube with a Pasteur pipette for analysis. 

---