Thermal Analysis
Information for Users User Com
25 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
Choosing the correct baseline is crucial for the determination of the enthalpy of a transi-
tion or a reaction. The baseline represents the DSC curve that would be measured if no
transition or reaction occurred. The examples described in this article illustrate how to
choose the right baseline for a particular evaluation.
Dear Customer,
We are very pleased to receive more and more articles from you for publication in UserCom. Thanks to new
techniques and better performance, thermal analysis is being used in an ever-increasing number of scien-
tific fields. Hyphenated techniques such as evolved gas analysis, microscopy and chemiluminescence yield
much more information about samples and very often greatly simplify the interpretation of measurement
results.
We think this issue of UserCom will once again give you ideas for applications in new and interesting areas
using the multitude of techniques now available.
Choosing the right baseline
Dr. Rudolf Riesen
Contents 1/2007
TA Tip
- Choosing the right baseline 1
Applications
- Determination of the Noack
evaporation loss of lubricants
by TGA 7
- The characterization of poly-
morphs by thermal analysis 9
- Analysis of melting processes
using TOP EM® 13
- Characterization of delivery
systems by thermogravimetry 18
Tips and hints
- Detection and evaluation of
weak sample effects in DSC 21
Dates
- Exhibitions 23
- Courses and Seminars 23
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and the onset temperatures of effects. In the literature and in standards, the term “baseline” is sometimes defined differ-ently, or different terms are used for the same thing. The terms most frequently encountered have therefore been sum-marized together with some brief com-ments. 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 al-ways 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], in-strument baseline [2]: A thermal analysis curve measured under the same condi-tions as the sample but without the sam-ple; 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 meas-ured 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 tempera-ture is changed through heating or cool-ing.
reaction mass
The baseline depends on the heat capac-ity of the sample (with an empty refer-ence 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 tran-sition 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 af-ter 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 base-line
The influence of measurement condi-tions 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 cruci-ble 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.
Principles for constructing virtual baselines
The basic principle for constructing a virtual baseline can be summarized as follows:
The interpolated baseline for the de-termination of the transition enthalpy or the reaction enthalpy leaves the DSC curve tangentially before the thermal effect and joins the curve again tangen-tially after the effect.
A good example to illustrate this is the take-off and landing of an aircraft. In special cases there are some exceptions to this that will be described in the ex-amples. Figure 1 shows how these princi-ples are applied.
1 nonsensical;
2 good (Line ),
1 unsatisfactory (horizontal straight line);
2 good (integral baseline, pos-sibly Spline ),
1.2.3.4.5.a)b)T A  T i p
Figure 1.
Drawing interpo-lated DSC baselines (the endothermic
direction is upward).
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METTLER TOLEDO  UserCom 1/2007
The mass of the sample changes dur-ing the transition.
The S TA R e  software provides several dif-ferent types of baseline to accommodate the changes shown by the DSC curve dur-ing a transition.
Table 1 describes the baselines and their typical applications.
The extrapolated virtual baselines are the tangents to the measured curve at the evaluation limits, just as they are used for interpolation with the baselines. Typi-•good (Integral tangential , or pos-sibly Spline ),
melting with exothermic decomposi-tion, 1 good (straight line to the point of intersection with the DSC curve);  2 rather arbitrary because the DSC curve is the sum of simultaneously  occurring processes,
two overlapping peaks, e.g. eutectic and melting peak of the main com-ponent, 1 good for the total integral, 2 good for the integration of the first peak (peak interpreted as sitting on the main peak, Spline ).
The transition line from one tangent to
c)d)e)another can have different shapes and be displayed as a straight line or as a sig-moidal curve (S-shaped function). The type of interpolated baseline chosen de-pends mostly on the physical conditions or chemical changes involved, for exam-ple:
The specific heat capacity of the sample, c p , hardly changes during the transition or it changes linearl
y with temperature.
The transition is accompanied by a significant change in the heat capacity The heat transfer to the sample changes during the transition.
•••Table 1.  List of virtual baseline types for
integration.
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cal applications of extrapolated baselines are for the determination of the:glass transition temperature
extrapolated onset temperature (also as first deviation from the measured curve)step height.
In all evaluations in which extrapolated tangents are used, one must make sure that artifacts on the measured curve or signal noise do not interfere with the de-termination of the slope of the tangent. This would result in the tangents being in the wrong place.
Application examples
The different types of baseline presented in Figure 1 are illustrated in the follow-•••ing practical examples. Figure 2 shows the most frequently used virtual base-lines:a) Spline : This is used to determine the reaction enthalpy of a postcuring reac-tion that is overlapped by the begin-ning of decomposition.b) Horizontal right : Isothermal cur-ing of an epoxy resin at 140 °C. When the reaction has finished the DSC curve is horizontal. The baseline can be drawn horizontally through the last measured points.c) Integral horizontal : The DSC curve of 1.162 mg water, which on heating evaporates through a 50-µm hole in the crucible lid. The loss of mass causes a change in the sample heat capacity, which is reduced proportion-ally to the amount vaporized. At the end of the measurement the crucible
is empty and the DSC signal is practi-cally 0 mW.
d) L ine : The DSC curve shows a glass transition of the amorphous part of
the polyethylene terephthalate (PET) followed by cold crystallization and
melting of the crystallites. The straight baseline is the virtual extension of the DSC curve after the glass
transition to the curve after the melting and shows the trend of the curve without crystal-lization and melting. The integral of the two effects yields 22.8 J/g as the difference between the exothermic and endothermic processes. This means that crystallites were already present at the beginning of the measurement. In relation to the melting enthalpy of 100% crystalline PET, this shows that the degree of crystallization of the sample was initially about 16% and was therefore not fully amorphous.
Figure 3 discusses how to draw the interpo-lated baseline if the baselines before and after the peak are at different levels, for example because the specific heat capac-ities of ice (2.1 J/gK) and water (4.2 J/gK) are very different. The figure shows four identical curves of the same part of the DSC melting peak of 1.87 mg water measured at 5 K/min (melting enthalpy 333 J/g). Each curve has a different base-line type.
The Horizontal left  baseline does not take the change in heat capacity in the evaluation range into account and yields a peak area that is too large.The Line  baseline is clearly unfa-vorable and contradicts the basic principles (no tangents, it crosses the DSC curve). In reality, the change in heat capacity is of course not linear with temperature between the evalua-tion limits as the dotted line wrongly shows.
The Spline  baseline is somewhat bet-ter, but also crosses the DSC curve. In this case, the  Integral horizontal  baseline is optimal. It draws the
baseline proportional to the peak area from the level before the peak to the level after the peak and so takes into account the change in heat capacity.
1)2)3)4)T A  T i p
Figure 2.
Examples of fre-quently used types of baseline.
Figure 3.
Example for the change of the heat capacity during the
transition ice/water.
In the first three cases, the result of the integration can be improved by choos-ing better limits, but even so, the virtual baselines do not correspond to the physi-cal facts.
Overlapping thermal effects are usually the most difficult with regard to choosing a realistic virtual baseline.
Figure 4 shows how a second heating run of the reacted sample helps to locate the exact position of the baseline. An epoxy resin was partially cured at 100 °C for 80 min, causing the material to vitrify [6]. The DSC curves shown in Figure 4 were then measured at 5 K/min. The postcuring reaction begins at the glass transition (curve 1). Curve 2 shows the DSC second measurement of the same, fully cured, sample. The straight line 3 (dotted) describes the course of the DSC curve after complete curing above the glass transition. It therefore represents the baseline for the integration and serves as a tangent for the determination of the glass transition temperature. It can be assumed that the behavior of the heat capacity above the glass transition during postcuring is about the same as that of the fully cured sampl
e.
The postcuring enthalpy is determined as follows: The dotted line (curve 3) is subtracted from curve 1 yielding curve 4. The peak in this curve is integrated using the Zero line baseline type within the
limits shown. Separation of the partially overlapping peaks could also be achieved using temperature-modulated DSC.
The purpose of the example in Figure 5 is to show how important correct inter-pretation of the DSC curve is. The choice of the integration limits and the baseline type should be good enough to obtain re-sults that provide consistent information for further investigations.
Figure 5 shows the DSC curve of a 40% solution of sucrose in water measured at 5 K/min after slow cooling. The glass transition occurs at about −45 °C and the ice that had crystallized out melts in the sucrose solution in the range −37 °C to 0 °C. Integration from −29 °C (dotted line) would assume that the specific heat
capacity decreases, which is not the case
here. The Line and Spline baselines
yield enthalpy values that are 5% too low.
Only the Integral tangential baseline
from −37 °C gives the correct value that
can be used for a consistent quantitative
evaluation.
Although one thinks mostly about inter-
polated baselines, extrapolated baselines
are in fact just as important.
This is shown in the examples in Fig-
ure 6:
Oxidation induction time (OIT) of a
mineral oil, measured at 180 °C at an
oxygen pressure of 3.5 MPa.
Melting point of benzoic acid deter-
mined as the extrapolated onset.
a)
b)
Glass transition temperature of poly-
styrene determined as the midpoint
according to how the tangents are
constructed (depends on the particu-
lar standard).
At the glass transition, the specific
heat capacity, c p, increases leading to
a step in the c p curve. The step height
is characteristic of the amorphous
content of the sample.
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 oc-
cur, a virtual baseline should be con-
c)
d)
Figure 4.
Example showing
a special baseline
(Polygon using just
two points, X) to
determine the post-
curing reaction. The
Zero line baseline
was used to inte-
grate the peaks in
curve 4.
Figure 5.
Curve interpretation
and the choice of
integration limits
and baselines.
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METTLER TOLEDO UserCom 1/2007

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