This is the diameter of spherical particle that will give similar fluctuations. So only a ‘sort of’ diameter can be calculated. Solution Non spherical particles can not be compared with analytical solutions. You can try other DLS instrument which claim to handle concentrated solutions, but I don’t have any experience with them. some sample property of interest depends on dilution), then you can’t use our DLS. Conc Dilute Until “counts vs dilution” remains stable OR counts decrease with dilution. Note : Counts may appear to be low to begin with! Dilute and remeasure. If they increase, you have too much concentration. A general rule of thumb is “dilute 5 times and measure”. If not, it means that the solution is dirty (i.e.large particles are present) Dust Correlation curve (good sample) Correlation curve (BAD sample) Issues Correlation curve should go to zero on the right side, for good samples. The correlation curves may look OK, but the diameter measured over time will change Unstable Counts will be low and decrease (or remain stable) with dilution Single X Particles settling over time give ‘unstable’ suspension. Issues Too low a concentration gives very noisy correlation curves, and possibly incorrect diameter. If particle dia is 200 nm, the diameter given by the instrument may be 20 or 50 nm! X Single Non spherical particles can not be compared with analytical solutions. Issues Too high a concentration gives very nice looking correlation curves, but incorrect (usually less than actual) diameter. high concentration) give incorrect results Single Non spherical particles can not be compared with analytical solutions. Model Assumptions Single particle scattering effects can be compard with analytical solutions. Usually it will be in kcps (kilo cps) or Mcps. Measurement The number of signals (counts per second) is also given. Using a model, the particle size distribution is calculated Measured Particle Size Distribution Correlation curve Model Measurement Auto correlation vs time is meaured. This is useful for particles less than 3 micron size Dynamic A fast response detector, at a fixed angle, is used. This is not the same, but somewhat similar in idea. The scattered light fluctuates because of movement of particle scattering it. This is useful for particles over 1 micron size Static Intensity vs time is measured at a fixed angle (usually 90 degrees). Theory Many detectors are placed at various angles and the “intensity vs angle” data is analysed. Here, light detector is placed at some angle, and the intensity of scattered light is measured for some time and the average value is used Static Scattering This happens when the refractive index of the particles are different from that of the liquid Scatter Intensity vs angle is called static light scattering. Motion Light passing through the suspension (particle + liquid) will be scattered by the particles. Theory Small, spherical particles (less than 3 micron size) in liquid undergo brownian motion. Engg., IIT-Madras Simplifiedtheoretical background Theory Problems faced during experiments Issues Suggested solutions Soln
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