As with polymers and pharmaceuticals, one of the most useful measurements made using fast scanning is that of the glass transition, and again with this is found to be enhanced at high rates; see Figure 1. In this case a shift in the position of the Tg is also observed, probably due to thermal lag associated with the large thermal mass of water/ice. Whist at very low mass the transition may not be any more scan rate dependent than the polymers, for most practical purposes the thermal mass of water/ice materials means that the onset temperature will be influenced to some extent by the mass of material present.

Figure 1. The glass transition from a 10-mL sample of a 5% sucrose solution scanned at 10◦C/min (lower trace), 50◦C/min and 100◦C/min (largest step). This shows the large increase in sensitivity possible, though water-based materials exhibit larger thermal lag effects which significantly shift the measured Tg as a function of scan rate.

With many life science materials where a Tg is to be measured, high rates are very helpful. However, where cell tissue may be destroyed by freezing there are limitations to the technique, since low temperatures may not be used at the start of a run. In many cases, this means that the duration of the initial transient is likely to overrun the transition of interest so lower scan rates may be needed. If a system can be frozen without harming the sample, the duration of the melting peak of water should be taken into account. This can mask events close to ambient because of its size and the time taken for it to complete, so this may limit the usefulness of the technique and mean that lower rates need to be chosen. However, even at rates of 50 or 100◦C/min more useful information may be gained than scanning at very slow rates.

Keeping materials of any sort absolutely dry is very difficult given that most materials absorb moisture from the atmosphere, and this is particularly true of amorphous materials where water molecules can more easily penetrate and surround the host molecules than in a crystalline material. Furthermore, freeze-dried or spray-dried materials are not completely dry when produced, and contain variable amounts of moisture depending upon the production conditions. This directly affects the stability of these materials, and it is important to measure the Tg, plasticized by the moisture content, to determine the actual value as it would be found in storage.

Using slow scanning rates a DSC may give some information, but in an unsealed pan a material is likely to dry out before Tg is measured, and moisture loss from the material prior to Tg will not only change the position of Tg but can mask it altogether. In a sealed pan there is no guarantee that moisture will remain in the sample, since it could come out of the sample, though remain in the pan. One approach is to measure the moisture content of the sample, and then to measure the Tg of the dried material. Using the Gordon–Taylor equation the Tg of the plasticized material can then be calculated.

With fast scanning the Tg of the non-dried material (complete with its moisture content) can easily be measured since the material does not have time to dry out. Furthermore, the peak from moisture loss is displaced to higher temperature so the Tg may be more clearly seen, as in the case of heparin, Figure 2. High scan rates therefore provide a simple and rapid method for the determination of stability of glassy materials affected by moisture.

Figure 2. Tg of heparin determined by HyperDSC. As the scan rate is increased moisture loss is delayed to higher temperatures revealing the glass transition, plasticized by the moisture.