Determination of absolute molecular mass

The application of a low-angle laser light-scattering (LALLS) detector for the measurement of absolute molecular mass of molecules eluted from SEC columns is very attractive, because the construction of a calibration curve in advance is not required. However, this method is not generally applicable to copolymers, because the intensity of light scattering is a function not only of the molecular mass of the copolymer but also of the specific refractive index in the solvent used as the mobile phase. The value of the specific refractive index for the copolymer is a function of composition. In general, the correct treatment of LS measurement for copolymers requires measurements in at least three solvents of different specific refractive index increments. 

Based on the theory of LS, the apparent molecular mass of a copolymer M is expressed with the true molecular weight and the heterogeneity parameters P and Q as follows  

An SEC system with three different detectors has been applied for the characterization of copolymers EPM and EPDM an evaporation detector (ED) to measure the concentration AC, a differential refractive index detector (RI) to measure the refractive index difference An between the solution and the solvent (the mobile phase), and a LALLS detector to measure the corresponding molecular weight of the eluting solutes, in the effluent from a column [37]. All three detectors were interfaced with a microcomputer, and computations were based on the following equations  

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