Choosing the Right Baseline
Introduction
In thermal analysis, baselines are mostly used in connection with the integration of peaks. The peak area is determined by integrating the area between the measurement curve and a virtual or true baseline. In the same way, the peak temperature is defined as the point on the curve where the distance to the baseline is greatest.
Extrapolated baselines are important for the determination of glass transition temperatures and the onset temperatures of effects. In the literature and in standards, the term “baseline” is sometimes defined differently, or different terms are used for the same thing.
Introduction
In thermal analysis, baselines are mostly used in connection with the integration of peaks. The peak area is determined by integrating the area between the measurement curve and a virtual or true baseline. In the same way, the peak temperature is defined as the point on the curve where the distance to the baseline is greatest.
Extrapolated baselines are important for the determination of glass transition temperatures and the onset temperatures of effects. In the literature and in standards, the term “baseline” is sometimes defined differently, or different terms are used for the same thing.
The terms most frequently encountered have therefore been summarized together with some brief comments. A number of application examples are then discussed to illustrate the rules governing the choice of baselines and that show which type of baseline should be used for the optimum evaluation of a particular DSC curve.
Terminology
The terms used in thermal analysis are summarized and explained in various standards.
However, since the definitions are not always the same, the terms used have been summarized below for the discussion of baselines that follows. Further definitions can be found in the book by Höhne [1] as well as in the standards mentioned (ISO [2], DIN [3], ASTM [4, 5]). The preferred terms are highlighted, but other terms are also included.
Blank, blank curve, zero line [3], instrument baseline [2]: A thermal analysis curve measured under the same conditions as the sample but without the sample; the mass of the crucibles used must be the same. Blank curves are essential for specific heat capacity determinations. Comment: In some cases, the zero line [1] is also understood as a curve measured without the sample or crucibles.
Sample blank: A curve that is obtained from a “fully converted” sample. This is usually the second heating run of the same sample under the same conditions. The effect measured in the first heating run no longer appears.
Baseline (also sample baseline [2]): Part of the curve that does not exhibit any transitions or reactions. This is an isothermal baseline if the temperature is held constant. A dynamic baseline is obtained when the temperature is changed through heating or cooling.
The baseline depends on the heat capacity of the sample (with an empty reference crucible) and the blank curve. Comment: In practice, the term is also used to mean the virtual baseline used for integration. Virtual baseline [2]: An imaginary line in the region of a reaction or transition that the DSC curve would show if no reaction or transition enthalpy were produced. Interpolated baseline [1]: This is a line that joins the measured curve before and after the peak. Extrapolated baseline: This is a line that extends the measured curve before or after the thermal effect.
The types of virtual baselines normally used are explained in the applications. True baseline: In the region of the transition or reaction, the baseline can be calculated according to physical data or even measured.
Factors Influencing the Baseline
The influence of measurement conditions on the DSC curve and the baseline should always be taken into account when interpreting curves and evaluating numerical data. Furthermore, the course of the blank curve and its reproducibility should be known.
Possible important parameters that can change during a transition are [1]:
- Mass, shape and structure of the sample, e.g. powder or film;
- Thermal conductivity and contact of the sample with the bottom of the crucible, e.g. a powder liquefies during melting;
- Heat transfer from the crucible to the sensor, e.g. deformation of the crucible due to an increase in the internal pressure or through products escaping from the crucible;
- Heating rate, e.g. when it changes from dynamic to isothermal;
- Thermal history of the sample and measuring system. If it is difficult to choose the baseline, it often helps to examine the sample and crucible after measurement with regard to the above points.
If it is difficult to choose the baseline, it often helps to examine the sample and crucible after measurement with regard to the above points.
Principles for Constructing Virtual Baselines
The basic principle for constructing a virtual baseline can be summarized as follows:
 |
Conclusions
Whenever possible, physical changes must be taken into account when choosing the optimum baseline for an integration or onset determination.
Since jumps in heat capacity rarely occur, a virtual baseline should be constructed that is smooth and free from any irregularities or discontinuities. The correct choice of baseline assumes that the curve has been properly and consistently interpreted [7]. Furthermore, the integration limits must be carefully chosen depending on the information required.
The rules and types of baseline discussed here using DSC measurements as examples can be applied to other TA measurement techniques, e.g. for the integration of peaks from SDTA, DTG analyses and other mathematically derived measurement curves.
Choosing the Right Baseline | Thermal Analysis Application No. UC 251 | Application published in METTLER TOLEDO Thermal Analysis UserCom 25