Gel permeation chromatography

Gel permeation chromatography (GPC) involves steric separation of a sample, i.e., separation on the basis of size. Different detectors are used to analyze the resulting size fractions in order to quantify molecular weight and distribution, molecular size, and intrinsic viscosity. UV detection is used routinely to identify chemically different species as they elute. This is especially valuable for the analysis of copolymers and in the development of smart materials, since unique electrical, thermal, or photochromic properties often correlate directly with UV absorption characteristics. The difference in absorption profiles between polymers with similar molecular weights, refractive index increments, viscosities, or hydrodynamic radii can be significant; thus a UV detector can often differentiate when others cannot. However, a conventional system measures spectra only at a single, predetermined wavelength in the UV-VIS range. This limitation is overcome with the photodiode array (PDA) detector (Figure 1) for the TDAMax instrument (Viscotek Corp., A Malvern Company, Houston, TX).

The instrument is a comprehensive GPC system with an integrated triple or tetra detector array that includes low-angle light scattering, a differential refractive index detector, and a four-capillary differential viscometer. The PDA consists of 256 diodes and simultaneously collects data at wavelengths in the range 190–500 nm. With typical measurement times in the 20–40 min range producing a very data-rich analysis, the technique is extremely productive.

The UV cell of the PDA sits in the temperature-controlled zone of the system, which operates at temperatures up to 80 °C. A fiber-optic link to the PDA and then onto the powerful OmniSEC™ software package enables the captured data to be displayed as information-rich, easy-to-interpret 3-D images that provide insight into the nature of the sample being analyzed.

The PDA measures the complete absorption spectrum of the sample eluting at every time slice of the chromatogram, giving a fingerprint of each component in the matrix. Because it captures data across the UV-VIS range, the user does not need to select the wavelength of interest before measurement. This makes it much easier to carry out more open-ended investigative analysis into a complex polymerization reaction or when examining material about which little is known.

The software package is equally important since it simplifies data manipulation and presentation while controlling the chromatography. Looking at the full absorption spectra for each “slice” makes it easy to see what components— monomers, oligomers, different polymeric species—are eluting, giving information about molecular size and structure.