Table of contents 1 Why is particle size important?
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- Điều hướng trang này:
- Static Light Scattering
| Dynamic Light Scattering
There is a well-known empirical correlation between the diffusion coefficient of a macromolecule and its molecular weight known as the Mark-Houwink-Sakurada equation. D t = kM Where: D t = diffusion coefficient k = empirical constant M = molecular weight α = an empirical constant The values for k and a are found empirically for polymer/solvent pairs. That is, they must be specified for the polymer, solvent, and temperature. These values can be found in the literature. The advantages of this technique are that polymer concentration need not be known and that molecular weight can be determined rapidly. It does, however, rely on empirical constants and the nature of the average molecular weight. a Static Light Scattering The SZ-100 can also be used in a static light scattering mode to measure the molecular weight of proteins, small particles, and polymers. These results are generated using a Debye plot (Figure 29) created by measuring the scattered light at a single angle (90°) at multiple sample concentrations. The intercept of the Debye plot is used to determine the molecular weight and the slope is used to calculate the second virial coefficient. Molecular weight from static light scattering experiments uses the Rayleigh equation given below: lim Kc = 1 + 2A 2 c Where: K = the Debye constant C = the sample concentration Rθ = the Rayleigh ratio Mw = the weight average molecular weight A2 = the second virial coefficient T he Debye constant is given by K=4π 2 n 2 (dn/dc) 2 /(λ 4 N A ) where n is the refractive index of the liquid, (dn/dc) is the refractive index increment, λ is the wavelength of light in vacuo, and N A is Avogadro’s number. In most cases, all of these values are independent of molecular weight. The limit given in the equation above deserves special attention. The equation only works at the limit of zero angle. One practice required for larger macromolecules is to use a multi-angle scattering instrument and extrapolate the result to zero angle. For smaller molecules (Rg<20nm), this is not necessary and data at a single angle can be used. However, this does introduce a systematic error that increases with angle used. That is, measurement results using back angle have about twice the systematic error compared to results obtained using scattering at right angle (90°). For this reason, the SZ-100 collects light scattering data at 90°. The advantage of this technique is that the results are well-defined and not reliant on empirical correlations, although it requires careful sample preparation and is a more time-intensive process. Δ R θ M w θg0 figure 29 | DEBYE PLOTS TO MEASURE MOLECULAR WEIGHT OF SEVERAL SAMPLES tải về 7.54 Mb. Chia sẻ với bạn bè của bạn:
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