Reading a DSC trace: transitions, artefacts and the baseline

Which features on a differential scanning calorimetry curve are the sample, which are the instrument, and the baseline discipline that separates the two. Written for anyone about to interpret their first trace.

The first thing to check on any differential scanning calorimetry plot is which way is up. Some instruments plot endothermic events upwards, some downwards, and the software default is not universal. A melting peak read as an exotherm has started more than one wrong conclusion. Find the axis label, or the endo arrow the manufacturer prints on the plot, before interpreting anything else.

What the curve is

The instrument holds a sample and a reference under the same temperature programme and records the difference in heat flow required to keep them together. When the sample absorbs energy, the trace deviates one way; when it releases energy, the other. The horizontal axis is temperature or time, and which one you plot changes what you can read off it, so keep both in the exported data.

The features that are real

• A step in the baseline with no peak is a change in heat capacity. In a polymer or a glass that is the glass transition, and it is quoted as a midpoint temperature rather than an onset.
• An endothermic peak is energy going in: melting, a solid state transition, loss of water or solvent, or the start of decomposition. For a melting point, quote the extrapolated onset rather than the peak maximum, because the onset is much less sensitive to sample mass and heating rate.
• An exothermic peak is energy coming out: crystallisation, curing, oxidation, or a phase change into a more stable form. A cold crystallisation exotherm just above the glass transition, followed by a melting endotherm, is the signature of a polymer that was quenched from the melt.
• Peak area is enthalpy, once the instrument has been calibrated with a standard of known heat of fusion at a comparable heating rate. Without that calibration the area is an arbitrary number.

The features that are the instrument

A trace has features that belong to the equipment rather than to the material, and they are recognisable once you know them.

• The startup transient. At the beginning of a ramp the instrument has to establish a steady heat flow, and the first part of the curve is that settling process. Start the programme well below the temperature of interest and ignore the initial section.
• Spikes and steps. A sharp discontinuity is almost always mechanical: a crucible lid seating, a sample shifting or cracking, a bubble escaping. Real thermal events have width.
• A sloping or curved baseline. Usually asymmetric crucibles, a contaminated sensor, or a mass difference between sample and reference pans. It is correctable by subtraction and it should be corrected, not fitted around.
• Peaks that move with heating rate. Kinetically controlled events shift to higher temperature as you ramp faster, and poor thermal contact does the same thing. If a transition temperature matters, measure it at more than one rate and say which you quoted.
• An apparent exotherm from sample movement. A sample creeping up the crucible wall or shrinking away from the base changes the thermal contact and produces a broad deviation that looks like a transition and is not.

Baseline discipline

Three habits remove most misreadings, and all three cost only instrument time.

1. Run an empty baseline with the same crucibles, the same programme and the same purge gas, and subtract it. Repeat it whenever the crucible type or gas changes.
2. Run the sample twice. The first heating carries the sample's thermal and processing history, including moisture, residual stress and any metastable structure. The second heating, after a controlled cool, is the one that compares meaningfully with another sample. Report both if the history is the point of the experiment.
3. Check for mass loss. An endotherm that could be evaporation, dehydration or decomposition should be confirmed on a thermogravimetric run, or on a simultaneous instrument that records mass and heat flow together. Weigh the crucible before and after as a minimum.

The rule that holds all of it together is simple enough to put on the wall above the instrument. A DSC peak tells you that energy moved and at what temperature. It does not tell you what happened. Naming the event needs mass loss data, an evolved gas measurement, a diffraction pattern on the recovered sample, or knowledge of the material that you can defend. Report the temperature and the enthalpy from the calorimeter, and get the identification from somewhere else.

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