Report on the cooperative determination of molecular-weight averages of polymers by size-exclusion chromatography

V. Dependence of molecular weight averages on detectors and mobile phases

Abstract

The effects of the differences in detectors and mobile phases on the calculated molecular-weight averages are discussed based on the data obtained from 1st and 2nd round-robin tests. Although the weight-average molecular weights (Mw) of polystyrene (PS) samples remained unchanged with the detectors, the number-average molecular weights (M_n) obtained with an ultraviolet absorption detector (UV) were 7488% of those with a differential refractometer (RI). The fractions at a lower molecular weight range of the differential molecular-weight distributions, calculated using UV chromatograms, were higher than those based on RI chromatograms, which caused the M_n, obtained by a UV detector to have smaller values than those by an RI detector. Peak broadening due to the cell structure or the difference in the molecular-weight dependence of the response coefficients of UV and RI detectors were negative as the main reason. The most probable assumption was that the end-structure of the PS samples affected the increase in the UV absorption. When a comparison of the M_n, values between laboratories is required, the type of detector for size-exclusion chromatography must be the same. The values of Mn and M_w for poly (methyl methacrylate), obtained with chloroform mobile phase, were 525% higher than those with the tetrahydrofuran mobile phase. PS standards were used for the calibration; in this case, the change in the values of M_nand M_w for other types of polymers than PS using different mobile phases had to be taken into account.