The use of narrow-MMD calibrants

An SEC system can be calibrated relatively simply by plotting the elution volume (or time) of the peak maxima of a series of calibrants with polydispersities (ratio of to M ) of less than 1.1, against their molecular masses. The use of such narrow MMD calibrants means that sensible calibrations will be obtained regardless of which molecular mass average is used for the plot in practice, the manufacturers of these calibrants now usually supply a peak molecular mass to be used in the calibration. How this calibration is defined will be considered later. 

The main problem associated with using narrow-MMD calibrants is the restricted range of polymer types available. Poly(styrene) calibrants are the main type used, and a very wide range of molecular masses is available. A small number of other polymer types is obtainable, but the molecular mass range is often rather limited. Poly(ethylene oxide) and poly(ethylene glycol) calibrants deserve special mention, because they can frequently be employed where poly(styrene) behaves anomalously, for instance in polar solvents such as dimethylformamide. Some of the narrow-distribution polymer calibrants available are listed in Table 3.1 and Appendix 2. 

The narrow molecular mass distributions are usually obtained by polymerizing these materials directly to obtain a narrow distribution (i.e. by ionic polymerization techniques), rather than by using any fractionation techniques. As these polymerization techniques are limited in their applicability, there are technical as well as commercial reasons for the limited range of polymer types for which narrow MMD calibrants are available. The manufacturers of these narrow-distribution calibrants will undoubtedly seek to extend the number of chemical types and the molecular mass range available, but commercial considerations will inevitably limit this expansion. 

To actually carry out the calibration, the calibrant polymers can be run individually, in various combinations or as a cocktail of a number of, or all of, the calibrants in a single solution. However, a number of important points should be adhered to if the calibration is not to be misleading  

Generally, the calibrants can be run at very low concentrations, as it is only necessary to identify the peak maxima. 

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