Interdetector volume calculations

Methods that calculate average polymer properties without the DRI are significant because they circumvent complications that arise from the measurement of the interdetector volume between the light-scattering and concentration detectors 10, 11). It is necessary, however, that each local signal,, be divided by the computed particle scattering function, P 6)i. 

The term dn/dc refers to the change in RI of a polymer relative to its concentration. The LS detector responds to M, the VISC detector responds to the intrinsic viscosity ([t]]), which is inversely proportional to molecular density, and the RI detector monitors concentration (C). A single narrow standard is used to determine the offset constants related to the interdetector volume for a given three-detector system [5]. Either C or dn/dc of a polymer sample must be known a priori in order to calculate the other variable using the RI detector [Eq. (3)]. Once both dn/dc and C are known, the LS and VISC [Eqs. (1) and (2)] can be solved to determine M and [77], respectively, for a polymer sample [3]. Structural information, such as chain flexibility, branching, and intramolecular interactions are all related to [7]]. Several key polymer properties related to [iq] are as follows ... 

A different procedure was used by Lecacheux and Lesec (33) for determining interdetector volume for both a viscometer and a light-scattering detector. In this approach, an excluded monodisperse polymer standard is injected. When the correct interdetector volume is selected, the calculated intrinsic viscosity, or molecular weight, is equal to the expected value and remains constant as a function of elution... 

When a molecular weight-sensitive detector is added as a second detector to an SEC system, it is essential that the dead volume in the connecting tubing between the measurement points of the two detector cells be known precisely. If this is not done, the calculated values contain significant errors. In particular, the measured polydispersity and Mark-Houwink coefficients are extremely sensitive to errors that may be incurred in the interdetector dead volume. 

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