Coupling with Molar-Mass-Sensitive Detectors

Coupling with Molar-Mass-Sensitive Detectors 

As has been pointed out, for SEC of complex polymers no simple correspondence exists between elution volume and molar mass. It is, therefore, useful to determine the molar mass not via a calibration curve but directly from the SEC effluent. This can be done by using molar-mass-sensitive detectors based on Rayleigh light scattering or intrinsic viscosity measurements [45]. 

In a plot of K c/R(Q) vs. sin2(0/2), Mw can be obtained from the intercept and the radius of gyration from the slope. A multi-angle measurement provides additional information. 

In most cases the injected concentration is small and A2 can be neglected. Thus, if the optical properties (nQ and Ant Ac) of the polymer solution are known, the molar mass at each elution volume increment can be determined. 

If a low-angle light scattering instrument is used, P(0) is close to unity and Mw i can be calculated directly. For a multi-angle light scattering instrument, the mean-square radius of gyration Pg2 at each elution volume can also be obtained from P(0)  

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Coupling with Molar-Mass-Sensitive Detectors... 

The most useful approach for the molar mass analysis of complex polymers is the coupling of molar mass-sensitive detectors to the SEC. Since the response of such detectors depends on both concentration and molar mass, they have to be combined with a concentration-sensitive detector... 

High-temperature SEC is the premier technique for information regarding molar masses. A number of different concentration detectors as well as molar mass sensitive detectors can be used. The coupling of SEC with spectroscopic techniques like FTIR and H-NMR reveals the chemical composition across the MMD of the sample. 

Beckett described inductively coupled plasma mass spectrometry (ICP-MS) as an off-line detector for FFF which could be applied to collected fractions [ 149]. This detector is so sensitive that even trace elements can be detected making it very useful for the analysis of environmental samples where the particle size distribution can be determined together with the amount of different ele-ments/pollutants, etc. in the various fractions. In case of copolymers, ICP-MS detection coupled to Th-FFF was suggested to yield the ratio of the different monomers as a function of the molar mass. In several works, the ICP-MS detector was coupled on-line to FFF [150,151]. This on-line coupling proved very useful for detecting changes in the chemical composition of mixtures, in the described case of the clay minerals kaolinite and illite as natural suspended colloidal matter. 

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