EVALUATION OF LIGHT-SCATTERING DETECTORS FOR SIZE-EXCLUSION CHROMATOGRAPHY .1. INSTRUMENT PRECISION AND ACCURACY

A systematic evaluation of two types of light-scattering detectors for size exclusion chromatography (SEC) was completed. The two detectors were the low-angle laser light scattering photometer (LALLS) and the multiangle laser light-scattering photometer (MALLS). Instrument evaluations were performed at both room (30-40-degrees-C) and high (135-145-degrees-C) temperatures using the polystyrene standard, NBS 706, at room temperature and the polyethylene standard, SRM 1476, at high temperature. Results of the evaluation showed that when experimental uncertainties were taken into account LALLS and MALLS demonstrated equivalent precision and accuracy for molecular weight determination. The main source of inaccuracy found (particularly for SRM 1476) was the sensitivity difference between the light-scattering and the concentration (DRI) detectors; i.e., the DRI detector was unable to measure very low concentrations of very high molecular weight material present in SRM 1476, whereas the light-scattering detectors respond strongly. It was shown that for LALLS the overall weight-average molecular weight (M(w)BAR) for the whole polymer calculated using an equation that did not require the DRI detector output circumvented this sensitivity problem while assuming that the low angle used was sufficiently close to zero. Use of this equation for MALLS is possible by extrapolating data from all angles used to obtain a light-scattering chromatogram at zero angle. However, this possibility was not examined here. A particular advantage of MALLS over LALLS is that MALLS can provide the z-average root mean square radius (commonly referred to here and in other light-scattering literature as the ''radius of gyration'') values from the same data as those used to obtain molecular weight values. Although the radius of gyration values at each retention volume were not as precise as the corresponding weight-average molecular weights, at room temperature, precision was better than 2% for a significant portion of the chromatogram. (C) 1993 John Wiley & Sons, Inc.