Quick and Easy Membrane Characterization with GPC-SEC

Size exclusion chromatography is a powerful tool for the characterization of molecules differing in size and molar mass. It is widely used and well known for the determination of molar mass distributions and molar mass averages. For membranes GPC-SEC is a useful characterization method as it can measure the membrane characteristic pore size distribution, average pore size and molecular cut-off significantly faster than other methods.

The procedure is based on the comparison of the chromatograms of a stock solution and the filtered stock solution (filtrate, permeate) after passing through the membrane. Stock solution and filtrate are then measured on a conventional GPC-SEC system.

Results and Discussion

When the dextrane stock solution is filtered, larger molecules will not pass the membrane but stay in the retentate. This is clearly visible when overlaying the GPC-SEC elugrams of the stock solution and filtrate.

From these chromatograms sieve curves can be calculated using, for example, the following equation:

P(M) = I(M)filtrate/I(M)stock solution

This process can be automated for several membranes to show an overlay and direct comparison of several sieve curves.

As typical for GPC-SEC molar masses can be obtained using a dextrane calibration curve. The membrane cutoff values are then available for every sieve curve.

Figure 1

From molar masses the average pore size and the pore size distribution can be determined based on Rg-M relation by van Dijk et al (1). Figure 2 shows the sieve curves of three membranes clearly indicating their differences in selectivity and filtration properties.

Figure 2

Conclusion

Qualitative and quantitative characterization of membranes can be easily achieved using standard GPC-SEC equipment. This methodology offers a fast and easy route to membrane characteristics such as pore size distribution, average pore size and molecular cut-off.

References

(1) van Dijk et al., Macromolecules, 25, 3585 (1992).

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