A comparison of calibration procedures for the analysis of broad molecular weight distributions using size exclusion chromatography with multiple detection

A range of well characterized broad polymer standards has been analysed using size exclusion chromatography (s.e.c.), with differential refractive index, low angle laser light scattering (LALLS) and differential viscometry (DV) as on-line detection methods. This study highlights the importance of accurate and precise estimation of the inter detector delay (IDD) parameter and shows that the conventional methods of IDD measurement will lead to errors in calculation of the molecular weight (MWT) moments and also large errors in calculated Mark-Houwink-Sakurada (MHS) parameters. The method of IDD calculation suggested by Suddaby el al. (Suddaby, K. G., Sanayei, R. A., O'Driscoll, K. F., Rudin, A., Makromol. Chem., 1993, 194, 1965) is found to be superior, and like the earlier workers we found the IDD to be MWT sensitive for both the LALLS and DV detectors. This work shows that if the MWT dependence of IDD is ignored, then this leads to a distortion in the measured relationship between intrinsic viscosity (IV) and molecular weight. This work also highlights the shortcomings of using the MHS relationship to represent the IV-MWT relationship, and we clearly show that the Stockmayer-Fixman (SF) equation is a better relationship for s.e.c. calibration over a broader MWT range. The SF equation is also recommended, as K-theta can be readily estimated from group contribution calculations thus allowing the IV-MWT relationship to be defined with one adjustable parameter, K' (or B), which itself has a thermodynamic significance. A computer program, which adds seamlessly to existing commercial software (PL Caliber), has been written to implement these improved calibration methods. MHS constants, K and alpha, and SF constants, K' and K-theta, are reported for a number of polymers. (C) 1997 Elsevier Science Ltd.