Conventional SEC/GPC setup employs a concentration detector (Refractive Index or UV-vis) to obtain the relative molecular weights (Mn, Mw, Mz) and molecular weight distribution (polydispersity index, PDI) of polymers. This technique requires column calibration using known narrow molecular weight distribution standards. In order to obtain accurate molecular weight values, the standards have to be of the same or similar chemical structure as that of the sample. Depending on the sample molecular weight distribution, typically 8 to 12 standards can generate a reasonable calibration curve.
Conventional SEC/GPC is the most accurate method for determining molecular weight distribution (PDI) due to the sensitivity of the concentration detector to measure very low molecular weight fractions (<500 Da) of a polymer sample often underestimated by other detectors such as light scattering. For instance, when analyzing a broad polystyrene standard, the PDI calculated was 1.50 which is representative of a broadly distributed polymer.
It is also most useful when comparing supposedly identical products or to identify variations between different lots of the same product. For instance, in the polymer processing industry slight variation in distribution can separate “in-spec” from “off-spec” batches of samples having the same technical specifications. As such, this technique is widely used for routine quality control and batch-to-batch monitoring of known products (e.g. polyethylene, polypropylene, polystyrene, polyethylene oxide, polymethyl methacrylate, etc.).