REPORT ON COOPERATIVE DETERMINATION OF MO LECULAR-WEIGHT AVERAGES OF POLYMERS BY SIZE-EXCLUSION CHROMATOGRAPHY .4. A REPORT ON THE 2ND ROUND-ROBIN TEST (NO-2) - ON THE CONSTRUCTION OF BASE-LINE

At 2nd round robin test (RR-2), the problem on the baseline constructi on was considered to be the main reason of high relative standard devi ations (RSD) of number-average molecular weight (M(n)). There are two methods to construct baseline of an SEC chromatogram. Method A is the extrapolation from the baseline at the beginning of the polymer chroma togram to the baseline after the solvent related peaks and method B is the draw of a linear baseline between the beginning of the polymer ch romatogram and the end of the chromatogram before the solvent related peaks (the lowest point between them). Laboratories at RR-2 were divid ed into two groups, group A which employed method A and group B which employed method B. After evaluating all data of M(n) and removing inva lid data, RSDs of M(n) were calculated separately. RSDs in group A wer e between 5.2 and 7.1% and those in group B was between 7.3 and 9.3%. The average values of M(n) in group B were 1.13 to 1.23 times higher t han those in group A. It is essential to standardize the procedure of the baseline construction in order to obtain better RSD. Ideally, meth od A is preferable, but there were several invalid data due to the dis turbance of solvent related peaks and the low setup of cutoff molecula r weight which he between 2500 and 3000. Method B is more appropriate, but tends to neglect low molecular weight components.