The larger the difference of the heat flows between the reference and sample sides of the detector outside regions in which thermal events occur, the greater the noise and the steeper the slope of the DSC curve.
The different heat flows are a result of thermal asymmetry between the sample and the reference sides due to the heat capacity of the sample itself. This asymmetry is greater, the heavier the sample, the larger its specific heat capacity, and the faster the sample is heated or cooled. The unequal heat capacities can be compensated by using an appropriate reference material.
In the ideal case, the differential heat flow signal outside regions in which thermal effects of the sample occur is then zero. Noise and slope are thereby reduced to the level of that obtained with empty sample and reference crucibles (see Figure 1).
In general, the reference is an empty crucible identical to the sample crucible. The reference can however be adapted to the sample properties in order to achieve better thermal symmetry, i.e. similar heat capacities with similar temperature dependence on the sample and reference sides.
This is achieved by filling the reference crucible with a reference material whose thermal mass is equivalent to that of the sample. The compensation of the heat capacity of the sample by the reference material at a particular temperature is given by the equation
cpS (T) * mS = cpR (T) * mR
The mass of the sample, mS, multiplied by its specific heat capacity, cpS, should be equal to the specific heat capacity of the reference material, cpR, multiplied by its mass, mR.
Example:
To compensate the heat capacity of a sample of polystyrene (30.0 mg) using aluminum oxide powder as reference material on the reference side:
Sample:
polystyrene, mS = 30 mg,
cp = 1.17 J/g K (at room temperature)
Reference material:
aluminum oxide powder, mR,
cp = 0.78 J/g K (at room temperature)
The calculation according to eq (1) yields: 30.0 mg * 1.17 J/g K = mR * 0.78 J/g K The required mass of the reference material, mR, is therefore 45.3 mg.
An ideal reference material:
Heat capacity compensation using a reference material in the reference crucible is a simple and effective method to reduce both the signal noise and the slope of the DSC curve. This is especially useful if very weak thermal effects in large samples at high heating rates have to be measured, i.e. under conditions that increase the noise and slope.
Lower noise is equivalent to improved sensor sensitivity. Together with reduced slope, this allows weak effects to be detected and evaluated with confidence.
The results show that denaturation in very dilute protein solutions and weak glass transitions in reinforced carbon-fiber composite materials can only be reliably detected and evaluated if compensation is used.
Detection and Evaluation of Weak Sample Effects in DSC | Thermal Analysis Application No. UC 256 | Application published in METTLER TOLEDO Thermal Analysis UserCom 25