The Methods of Gel Permeation Chromatography
In this particle size analysis article, we’ll take a look at two brilliant new methods of gel permeation chromatography (shortened to GPC). Specifically, we’ll look at the benefits of using light scattering detectors and viscosity detectors, as well as how these methods can be combined with more traditional GPC methods such as refractive index detection in order to get the best results.
What is GPC?
In its traditional form, gel permeation chromatography (GPC) is a proven and effective method for determining average molecular weight of polymers and small molecules. It also enables us to ascertain the overall molecular weight distribution. However, conventional GPC does have its limitations. For example, the molecular masses provided by conventional GPC are all relative. In GPC molecules are separated according to hydrodynamic volume, not molecular weigh. Molecular weights and molecular weight distributions are determined by comparing retention times with molecular weight standards. A calibration curve is utilised in order to achieve this.
Since this really depends on the polymer that is being used, true molecular masses can only be determined if the samples are precisely the same structure. This shortcoming can lead to large deviations occurring in branched samples, as their molecular density is significantly higher than that of linear chains.
The most common detectors used in conventional GPC are:
- Refractive Index (RI)
- Ultraviolet (UV)
However, their signals rely on concentration only, not on polymer size or molecular weight. As we’ve already mentioned earlier in this article, light scattering detectors and viscosity detectors have been shown to fix the problems that have for a long time been associated with conventional GPC.
Light scattering detectors
The signals that are provided by static light scattering detectors are directly proportional to the molecular weight of the polymer, as well as concentration of molecules and their refractive index. So the advantage of using static light scattering detectors in GPC is that molecular weight can be gleaned without the need to create a calibration curve.
The signal provided by viscosity detection is relative to intrinsic viscosity and polymer concentration. Intrinsic viscosity is inverse to molecular density, so measurement of intrinsic viscosity gives a good indication of molecular structure.
The famous Mark-Houwink plot shows the double logarithmic plot of molecular weight against intrinsic viscosity. It’s an important plot when it comes to polymer structure analysis, as it mirrors structural changes in the polymer (such as chain rigidity).
By their powers combined
By using the advantages of refractive index detectors, light scattering detectors and viscosity detectors, triple detection can be achieved. Light scattering enables accurate molecular weights to be determined; intrinsic viscosity provides structure information; and concentration gives quantitative ratios of different species. It also enables the differentiation of monomers, dimers, trimers and aggregates.
ATA Scientific offers scientific instruments that can be used for GPC and other associated processes. Contact us today for more information on how we can help you find the right instrument.