A NEW APPROACH TO ESTABLISHING UNIVERSAL CALIBRATION CURVES FOR SIZE-EXCLUSION CHROMATOGRAPHY

In size exclusion chromatography (s.e.c.) analysis both the universal calibration curve (UCC) and the secondary calibration curve (molecular weight calibration) can be constructed using the Mark-Houwink-Sakurada (MHS) relation, [eta] = KM(a). Over a very wide range of molecular weights neither K nor a is constant; therefore, the interpretation of results using one set of K and a values may not be correct (i.e. in the case of commercial resins). An alternative model which relates molecular weight (MW) to [eta] over a very wide range of molecular weights is [eta] = K(theta)M(r)1/2BAR + K'M(w)BAR where K(theta) and K' do not change. The intrinsic viscosities of polystyrene standards with MWs of 10(3) to 2 x 10(6) were measured and the results were used to establish a UCC for a multi-detector s.e.c. system. From the UCC two calibration curves were established using the MHS relation and the alternative model. Commercial polystyrene and polyethylene resins were analysed using the same analytical system. At the higher molecular weight end one observes a deviation between the results from absolute MW detectors and the calibration curve based on a particular set of K and a values. On the other hand, MWs obtained from the calibration curve using the alternative model were consistent with results from absolute molecular weight detectors. This indicates that the use of the alternative model will provide a more accurate calibration for s.e.c. analysis when the molecular weight distribution is relatively wide.