The use of broad-MMD calibrants

An alternative approach to calibrating an SEC system has been to use a single broad-MMD calibrant. The most satisfactory approach for this form of calibration is to use a calibrant which has been particularly well characterized. This would require that the calibrant be fractionated and that each of the many fractions be carefully examined by absolute techniques in order to build up a detailed molecular weight profile of the whole distribution [4]. 

Clearly, the characterization of a broad-MMD calibrant requires a considerable amount of effort and expertise, and they have been mainly used where there are problems in producing narrow-MMD calibrants. In practice, the main applications of this approach have been the use of NBS Standard Sample 706 (polystyrene) and, in particular, NBS Standard Sample 1475 (linear polyethylene). SEC has been a very valuable technique for examining the MMDs of polyethylene, but as there has only been limited success in producing narrow-MMD polyethylene calibrants, NBS 1475 has been widely used for calibrating high-temperature SEC systems used for this application. Unfortunately, the molecular mass range of NBS 1475 is rather low and is not really appropriate for many samples. Barlow et al [5] used NBS 1475 and extended the calibration range with additional polymers (see Chapter 4, section 4.5.1.1). 

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