In DLS because of this intensity bias it is difficult to exceed a practical resolving power of 1:4 i.e 100 nm particles can be resolved from 400 nm particles. For bimodal samples the particle-by-particle approach allows resolution of discrete populations with a size ratio of 1:1.33 (for example 300 nm particles could be discriminated from 400 nm particles). The NTA approach does not give an average particle size and hence is not biased towards the larger particles. DLS produces an average particle size due to the ensemble measurement (all particles measured at the same time) and is biased towards larger particles within the sample (by virtue of the fact they scatter light more intensely than the smaller particles). What Does This Mean in Practise? Polydisperse Sampleįor polydisperse samples (samples containing a range of particle sizes) in general the NTA approach is better suited due to the particle-by-particle measurement.
#DYNAMIC LIGHT SCATTERING SIZE RANGE ISO#
Through application of the auto-correlation function and subsequent calculation of the exponential decay, average particle size can be calculated from time-dependent fluctuations in light intensity (refer to ISO 13321 for further details).įigure 1. DLS observes the time dependent fluctuations in scattering intensity caused by constructive and destructive interference resulting from the relative Brownian movements of the particles within a sample. NTA measures this movement through image analysis (fig.1) tracking the movement of the particles on a particle-by-particle basis, this movement can be related to particle size. hard sphere, globular, dendrimer, chain stiffness, and degree of branching).Sponsored by Malvern Panalytical Jan 21 2009īoth techniques measure Brownian motion and relate this movement to an equivalent hydrodynamic diameter, with the motion of smaller particles becoming more exaggerated. Hydrodynamic sizes are more easily measured than radii of gyration and can be measured over a wider range of sizes. The conversion from hydrodynamic radius to radius of gyration is a function of chain architecture (including questions of random coil vs. The hydrodynamic radius is not the same as the radius of gyration. Radius calculations are the same except for a factor of two.Īlso, a note to those interested in polymer size. That is, the determined particle size is the size of a sphere that diffuses the way as your particle.įor those who work with protein sizing and other areas where hydrodynamic radius is more commonly used, note that the development here is around diameter. Finally, and most importantly, it reminds the analyst that the particle size determined by dynamic light scattering is the hydrodynamic size. Temperature is even more important due to the viscosity term since viscosity is a stiff function of temperature. The first is that sample temperature is important, at it appears directly in the equation. However, the equation does serve as important reminder about a few points. The calculations are handled by instrument software. T is thermodynamic temperature (we control this).k B is Boltzmann’s constant (we know this).
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