|
Static light scattering is a technique in physical chemistry that measures the intensity of the scattered light to obtain the average molecular weight ''Mw'' of a macromolecule like a polymer or a protein in solution. Measurement of the scattering intensity at many angles allows calculation of the root mean square radius, also called the radius of gyration ''Rg''. By measuring the scattering intensity for many samples of various concentrations, the second virial coefficient ''A2'', can be calculated. Static light scattering is also commonly utilized to determine the size of particle suspensions in the sub-μm and supra-μm ranges, via the Lorenz-Mie(see Mie scattering) and Fraunhofer diffraction formalisms, respectively. For static light scattering experiments, a high intensity monochromatic light, usually a laser, is launched in a solution containing the macromolecules. One or many detectors are used to measure the scattering intensity at one or many angles. The angular dependence is required to obtain accurate measurements of both molar mass and size for all macromolecules of radius above 1-2% the incident wavelength. Hence simultaneous measurements at several angles relative to the direction of incident light, known as multi-angle light scattering (MALS) or multi-angle laser light scattering (MALLS), is generally regarded as the standard implementation of static light scattering. Additional details on the history and theory of MALS may be found in multiangle light scattering. To measure the average molecular weight directly without calibration from the light scattering intensity, the laser intensity, the quantum efficiency of the detector, and the full scattering volume and solid angle of the detector needs to be known. Since this is impractical, all commercial instruments are calibrated using a strong, known scatterer like toluene since the Rayleigh Ratio of toluene and a few other solvents were measured using an absolute light scattering instrument. ==Theory== For a light scattering instrument composed of many detectors placed at various angles, all the detectors need to respond the same way. Usually detectors will have slightly different quantum efficiency, different gains and are looking at different geometrical scattering volumes. In this case a normalization of the detectors is absolutely needed. To normalize the detectors, a measurement of a pure solvent is made first. Then an isotropic scatterer is added to the solvent. Since isotropic scatterers scatter the same intensity at any angle, the detector efficiency and gain can be normalized with this procedure. It is convenient to normalize all the detectors to the 90° angle detector. where ''IR(90)'' is the scattering intensity measured for the Rayleigh scatterer by the 90° angle detector. The most common equation to measure the weight average molecular weight, ''Mw'', is the Zimm equation: where and with and the scattering vector for vertically polarized light is with ''n''0 the refractive index of the solvent, λ the wavelength of the light source, ''N''A Avogadro's number (6.022x1023), ''c'' the solution concentration, and d''n''/d''c'' the change in refractive index of the solution with change in concentration. The intensity of the analyte measured at an angle is ''IA(θ)''. In these equation the subscript A is for analyte (the solution) and T is for the toluene with the Rayleigh Ratio of toluene, ''RT'' being 1.35x10−5 cm−1 for a HeNe laser. As described above, the radius of gyration, ''Rg'', and the second virial coefficient, ''A2'', are also calculated from this equation. The refractive index increment ''dn/dc'' characterizes the change of the refractive index ''n'' with the concentration ''c'', and can be measured with a differential refractometer. A Zimm plot is built from a double extrapolation to zero angle and zero concentration from many angle and many concentration measurements. In the most simple form, the Zimm equation is reduced to: for measurements made at low angle and infinite dilution since ''P(0) = 1''. There are typically a number of analyses developed to analyze the scattering of particles in solution to derive the above named physical characteristics of particles. A simple static light scattering experiment entails the average intensity of the sample that is corrected for the scattering of the solvent will yield the Rayleigh ratio, ''R'' as a function of the angle or the wave vector ''q'' as follows: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「static light scattering」の詳細全文を読む スポンサード リンク
|