1. Purge gas

One of the main issues involved in fast scanning DSC is getting good thermal transfer between the analyzer and the sample and in practice this can be achieved by using helium as a purge gas. Helium has much higher thermal conductivity than nitrogen, air or oxygen and results in much better performance. It is not worth using nitrogen unless unavoidable. At higher temperatures, instrumental limitations may not permit the use of helium and gas mixtures such as helium/neon have been used to give both high performance and a broad temperature range. The rate of flow of purge gas used is normally very low, e.g. 30–40 mL/min, so the costs involved are small. After a sample is loaded into the furnace, sufficient time should be given for all air to purge from the system before the run begins; noting that vented pans should also be given time to fully purge. This usually takes a few minutes, and can be gauged by the stability of the heat flow signal. A delay of a few minutes can be inserted into a standard method, and a typical SOP should suggest 3–4 min.

2. Sample pans

Sample pans should have a flat base with the sample well pressed down to give good thermal contact. Pan selection is likely to be dependent upon the type of transition being measured. When looking for a glass transition resolution issues are generally irrelevant, so the size and thickness of pan have little influence on the result. Large, thicker aluminum pans are usually best because they hold large amounts of sample and in general retain a flat base; the flatness of the base is far more important than thickness of the pan. If melting transitions are to be measured then the traditional flat type of pan is best, or if using dish shaped autosampler pans make sure of good internal contact. Very thin aluminum pans are available for HyperDSC work where resolution issues are important and where thickness of pan is suspected of having an effect, but for most work this is unlikely to be the case. In effect, the same pans are used at high scan rates as for low scan rates, flatten or discard pans that do not have a flat base, and use good sample loading techniques.

Beware of contamination of pans. Contamination will have more effect at high rates because of increased sensitivity. Pre-heat pans to 300◦C to volatilize off potentially contaminating oils before looking for very small transitions.

3. Sample size

The best size of sample to use is often difficult to predict since there are so many potentially influencing factors. However, the main issue is to decide what type of transition is involved. Minimize sample size when melting a material; 1–2 mg is probably a good starting point, and avoid decomposition and resulting contamination issues. The faster the scan rate the smaller the sample needed. Maximize sample size when looking for a small Tg. The actual mass will vary greatly with density of sample. It may not be possible to optimize conditions such that satisfactory measurement of a Tg and a melting profile can be obtained from a single run. Do not try to melt large sample sizes which will potentially collapse and so give poor data, apart from any contamination issues.

4. Scan rate

As shown with most applications in this chapter, a wide range of scan rates can be very useful, from very slow for reactions which need more time to complete (not excluding isothermal work either) to very fast. The biggest question is where to start. Generally, it is best to avoid extremes and for much of the work shown here a scan rate of 300◦C/min was chosen for initial investigations. For small hard-to-find glass transitions 500◦C/min may be immediately chosen, though for melting profiles, particularly of polymers, this may prove too rapid for optimal results. Remember to calibrate appropriately for the scan rates selected.

5. Instrumental settings

In general, work with the widest dynamic range available since transients and melting peaks can be very large, and set data point collection rates to be as fast as possible. For most work in this chapter, data were collected at a rate of 20 data points per second.

6. Cleanliness

As remarked with regard to sample pans, small contamination effects which are not noticeable at low rates can become evident at fast rates; hence the need for a clean and careful technique, ensuring that the working environment is also clean. Small amounts of volatilization or sublimation can cause contamination around the furnace area which may be noticeable on subsequent runs, so bear in mind the need to check and clean furnaces between runs, particularly when melting materials. If there are any anomalies in the instrumental baseline it may be useful to use a baseline subtraction technique, particularly where very small glass transition measurements are being made. Subtraction should only be used if the analyser is clean and the baseline is totally reproducible, but baseline subtraction is normally worth using since it only takes a minute or two to perform at high rates.

7. Getting started

Start with simple samples that have known transitions and will not easily decompose and so cause contamination issues. Look at the effects of increasing scan rate and varying sample weight and begin to get a feel for the practice of the technique. Do not start with the most difficult or challenging sample that is available.