1

This paper is published in Thermochim. Acta, 2013, 566, 71–76.

Optimization of DSC calibration procedure

Alexey Shimkin*

All-Russian scientific research institute of aviation materials, 17 Radio Str., 105005 Moscow, Russian Federation

The influence of calibration of differential scanning calorimeters on data precision could not be underestimated. The

more precise the calibration procedure is the more time is required for proper calibration of the DSC instrument.

Since for each change of the experimental conditions (pan material, heating rate, purge gas, etc.) a calibration is

needed, time spent on calibration of one calorimeter is often measured in dozens of hours per year. This paper

describes the strategy for speeding up temperature and heat flow calibrations of differential scanning calorimeters.

The amount of time saved by different modifications of the procedure as well as the influence of the approach on

accuracy of calibration data is estimated.

Keywords: differential scanning calorimetry, DSC, calibration, optimization.

1. Introduction

Differential scanning calorimetry (DSC) is widely used for investigation and quality control of

metal alloys, organic compounds, polymers (including bio-polymers) and polymer composites,

etc [1–3]. Temperature and heat flow calibrations play a very important role in accuracy of data

obtained by DSC [4–6]. According to international standards calibration should be performed

regularly, or after a significant change in the experimental conditions (including cleaning the

measuring cell). The use of several calibration substances and the necessity of carrying out

measurements under various conditions (heating rate or atmosphere) result in increasing of

instrument time spent on calibration. This is particularly important for kinetic studies which

demand performing a calibration for each heating rate that could vary from high to very low.

Meanwhile, the main goal of an operator aside from obtaining reliable data is the reduction of a

‘dead’ time so he could perform more measurements, and when routine measurements are

carried out one may choose speed over precision. This paper describes possible ways to reduce

time spent on calibration and discusses their potential influence on accuracy of the

measurements.

There are quite a few procedures for temperature and heat flow calibrations of differential

scanning calorimeters. Most of them require two heating-cooling cycles for each sample of

calibration material, only the second being used for calibration. International standard ISO

11357-1 [7] insists on performing cooling and heating runs at the same rate as will be used for

sample measurement while ASTM E 967 [8] and E 968 [9] allow rapid heating of calibration

material to 30 °C below the calibration temperature with subsequent stabilization. German

Society of Thermal Analysis (GEFTA) proposed a calibration procedure which offers

determination of extrapolated peak onset temperatures as a function of the heating rate and linear

extrapolation to zero heating rate [10, 11]. Later International Union of Pure and Applied

Chemistry (IUPAC) added to this procedure measurements of two samples of each calibration

substance with masses differ by a factor of at least 3 [12, 13]. All these calibration procedures

aim at higher precision but the cost of it (measured in ‘dead’ time) is very high, unreasonably for

routine measurements.

2. The strategy

To avoid unnecessarily spent time while keeping data accuracy satisfactory we propose the use

of ‘smart’ temperature programs based on the following rules:

highest possible starting temperature

* Tel.: +7 499 263 8996.

E-mail addresses: shimkin_aa@viam.ru, alexshimkin@gmail.com.

2

highest possible heating rate where nothing interesting happens

highest possible cooling rate

combination of measurements at all required heating rates, starting from the highest

only one dummy run for the whole temperature program

adjustment of the temperature program.

Though this strategy is the most effective when applied to kinetic studies where several

calibrations differ only by heating rate are needed, the first three principles could be applied to

single calibrations as well. It will be of particular use when high temperatures or low heating

rates are involved. Let us investigate this strategy in detail.

2.1.Highest possible starting temperature

Usually only data obtained from second measurement of calibration material is used. So why

cool sample to room temperature? It is logical to perform cooling only to temperature T3 below

transition that is low enough to get linear baseline on subsequent heating (TP1, Fig. 1).

Fig. 1. Temperature program TP1 with increased starting temperature.

The realization of this principle could reduce time spent on one cooling/heating cycle by:

where v is heating/cooling rate.

For heating rate of 10 K/min it gives 20 min for each 100 °C temperature difference (T3 – T1).

Though it is not always clearly defined in calibration procedures, this principle is de facto widely

used. So the temperature program shown in Fig. 1 will be considered hereafter as ‘usual’, other

rules applying to it.

2.2.Highest possible heating rate where nothing interesting happens

Since the region before transition is of no interest, heating rate in this area could be increased to

reduce time spent on measurement. Similar approach is used in ASTM E967 standard. Usually

T3 could be chosen as the point where heating rate changes, i.e. T4 = T3 (TP2, Fig. 2).

Fig. 2. Temperature program TP2 with increased heating rate on initial step.

3

The only difference between two temperature programs (Fig. 1 and 2) is the initial heating step.

If v1 and v2 are heating rates of initial ramps for the first and the second temperature programs,

the time difference could be calculated as:

If heating rate in the region before transition is increased from 10 to 20 K/min then:

That gives 5 min for each 100 °C temperature range (T4 – T1). This difference may seem

insignificant but for materials which transitions are located at high temperatures (such as gold or

nickel) time spent on measurement could be reduced by more than 1 h. The use of higher heating

rates will lead to further increase in productivity.

2.3.Highest possible cooling rate

Time spent on cooling segments could be reduced as well because they are of no practical use for

calibration except for preparation of unified sample for each heating step (TP3, Fig. 3).

Fig. 3. Temperature program TP3 with increased cooling rate.

The gain in time in this case will be:

or 5 min for each 100 °C temperature range (T2 – T3) when cooling rate is increased from 10 to

20 K/min. Since the ‘working’ range between T2 and T3 rarely exceeds 150 °C, this optimization

is only important when low heating/cooling rates are used.

2.4.Combination of measurements at all required heating rates, starting from the

highest

When several calibrations at different heating rates are needed it is possible to accelerate

procedure by combining all the necessary segments in one measurement (TP4, Fig. 4). To reduce

time spent on initial heating of the sample the segment with the highest heating rate should be

placed at the beginning of the temperature program.

4

Fig. 4. Temperature program TP4 for two calibrations with different heating rates.

In this case we also need not cool down to room temperature and can start new measurement

with different heating rate directly from T3. The time difference between this combined

temperature program and two separate measurements will be:

where v1 is the quenching rate for the first measurement, and v2 is the heating rate for the second

measurement.

If v1 and v2 are equal 20 and 5 K/min respectively, that is often the case, then the gain in time

will be:

or 25 min for each 100 °C temperature range (T3 – T1). Since this difference can exceed 1200 °C,

an operator could save as much as 5 h that is often enough for several measurements. The time

savings would be even more impressive if more than two calibration measurements are

combined.

2.5.Only one dummy run for the whole temperature program

In the previous temperature program TP4 the first and the third heating segments have not been

used for data acquisition. The initial heating is necessary to melt calibration sample which is

essential for accuracy of data obtained. But the subsequent measurement does not require such

step because the sample is already recrystallized. So the third heating segment could be removed

from the temperature program (TP5, Fig. 5).

Fig. 5. Temperature program TP5 for two calibrations with single initial heating segment.

The realization of this approach will reduce time spent on two calibration measurements

compared with temperature program TP4 by:

That gives 1 h for heating rate of 5 K/min and 150 °C ‘working’ range between T2 and T3.

5

2.6.The ideal temperature program

In summary, the ideal temperature program for three simultaneous calibrations could look like

Fig. 6 (TP6). For heating rates of 20, 10 and 5 K/min (which are often used in kinetic studies)

calibration for indium using this temperature program will require two times less time than three

separate measurements, saving 1–2 h depending on parameters.

Fig. 6. The ‘ideal’ temperature program TP6 for three calibrations with different heating rates.

Since optimal temperatures T2 and T3 could vary for different segments, it is possible to use

lower T2 and higher T3 for runs with lower heating rates. The optimal values could be determined

only experimentally, and the difference between T''3 and T3 could be as much as 150 °C. The

possible difference between highest temperatures T''2 and T2 is usually much smaller (up to 30

°C) because it is not necessary to have a linear baseline in the cooling segments of temperature

program.

3. Adjustment of the temperature program

Now it is time to discuss the pitfalls of this approach, and the necessity of adjustment of the

temperature program to specific conditions (calibrant properties, heating/cooling rates,

instrument features, etc.).

The first rule (TP1) has seemingly no influence on accuracy of calibration and can be applied

without restrictions. The only question is where to place T3. It should be positioned as high as

possible to shorten measurement time but if it will be too high there could be not enough time for

temperature curve to become linear making heat flux calibration wrong. The position of T3 will

be affected by cooling and heating rates, instrument inertia, pans material, and even atmosphere.

One should also take into account whether the instrument is equipped with forced cooling device

or no. Usually, when an instrument with low thermal inertia and heating/cooling rate of 10

K/min are used, this point could be placed to 30 °C below the calibration temperature as ASTM

E967 recommends. But in real life there are tasks that require different heating rates, and some

(even state-of-the-art) instruments are very inertial (for example, having large measuring cell and

high temperature furnaces, as high temperature DSCs or simultaneous thermal analyzers do). In

this case not only 100 °C gap may be required but also addition of isothermal segments to

stabilize temperature.

Similar problems await an operator applying the second rule (TP2). How high the heating rate in

initial step could be? If the heating rate will be too high there could be significant overheating,

and for some instruments equipped with inertial furnaces the transition could occur before the

real heating segment even starts. Also at high temperature rates there could be not enough time

for temperature curve to become linear. The practice shows that use of heating rate of 20–30

K/min is usually quite safe but the upper limit is strongly instrument-dependent.

This is true for cooling rates as well (TP3) except when non-metallic calibration standards are

used. Unlike crystallization of metals that takes place quickly solid–solid transitions often occur

in several steps [14–16]. In this case cooling rates as low as 5 K/min and even lower T3 may be

6

required to get correct data. There are opposite cases as well: when ammonium nitrate is cooled

down at 80 K/min, 5 transitions could be observed while cooling at 2 K/min leads to only 4

transitions at subsequent heating [17].

One more problem is related to assignment of end temperature T2. Usually transitions peaks of

calibration samples are rather sharp and extra 20–30 °C are quite enough for baseline to re-

establish. But high inertia [18] or high heating rates [19, 20] may lead to peaks becoming so

wide that T2 should be placed to 50–100 °C higher than transition temperature.

Combination of measurements (TP4, TP5) is only possible when characteristics of calibration

sample are not changed upon multiple heating/cooling cycles. ISO 11357-1 insists on using fresh

calibration samples for each measurement though in the annex the recommendation not to ‘re-

use calibration materials that have, or might have, reacted with the crucible material’ could be

found that implies a possibility of reuse when there are no changes. Actually, some materials

(such as rubidium nitrate, silver or nickel) degrade quickly and cannot be measured twice. But

commonly used adamantane, indium, bismuth, zinc, tin, gold and many others are stable enough

if not over-heated to be utilized in multiple-segment calibration procedure. Perhaps it is not the

case when high precision measurements are to be made but time saving is mostly important for

routine work where some degree of inaccuracy is allowed.

This approach also lacks uncertainties resulting from variation of the crucible position. These

uncertainties could be estimated to be not more than 0.8 K and 2.5 % which are defined by ISO

11357-1 for temperature and sensitivity calibrations respectively.

So, if one wishes to make correct measurements, he should choose experimental parameters very

carefully. Using the described optimization procedure or no, the temperature program should be

adopted for specific conditions (instrument capabilities, heating rates, atmosphere, etc.). This is

the implementation of ‘smart’ approach to compose temperature program for DSC calibration.

4. Verification

The approach proposed has been checked by calibration of Netzsch DSC 204 F1 apparatus using

standard calibration set (adamantane, biphenyl, potassium nitrate, indium, tin, bismuth, zinc, and

cesium chloride). Theoretical values for temperatures and enthalpies of transitions were taken

from the manufacturer’s certification provided by Netzsch. Data given in the Tables 1 and 2

summarizes results on extrapolated temperatures of the beginning and peak areas of the

transitions obtained for different calibration materials using optimized temperature programs like

shown in Fig. 7 for indium. The parameters were as follows:

heating rate of 20 K/min on initial step

two measurement runs for each heating rate (20, 10 and 5 K/min) as recommended by

manufacturer

cooling rate of 10 K/min for every cooling step.

The heating rate of 20 K/min that was used in the first run has been considered high enough, so

heating rate on initial step has been chosen equal to heating rate of the first measurement step.

The use of heating rate of 30 or even 40 K/min on the initial step will reduce the time of the

measurement by only several minutes, and the use of a single heating rate allows to avoid extra

programming. As the instrument used has very low thermal inertia, high and low temperatures T2

and T3 have not been varied depending on heating rate though Fig. 5 shows some possibilities for

further optimization.

For heating rate of 20 K/min there are two RSD (relative standard deviation) values calculated

from 1, 2 and 2, 3 runs correspondingly. It is easy to notice that the first RSD value is almost

everywhere higher than the second one. It is because a calibrant could possibly have not good

contact with pan bottom, and after it was melted its properties slightly change. Almost all RSD

values are rather small which implies good repeatability. The exception is the first calibration

experiment for potassium nitrate. This measurement has been done with standard cooling rate of

10 K/min, but this cooling rate seems too high for potassium nitrate. Indeed, when cooling rate

7

has been lowered to 5 K/min, RSDs became quite normal. Figure 8 shows the difference between

two cooling runs of potassium nitrate at different cooling rates. For 5 K/min there are two peaks

while for 10 K/min only one, the second just beginning. The other way to correct the calibration

is to lower T3, giving more time for the transition to take place.

Table 1. Temperature calibration data (°C).

Calibrant 20 K/min 10 K/min 5 K/min

1 2 3 % RSDa % RSD

b 1 2 % RSD 1 2 % RSD

Adamantane –61.3 –61.4 –61.3 0.12 0.12 –62.3 –62.3 0.00 –62.5 –62.6 0.11

Biphenyl 70.0 70.1 70.2 0.10 0.10 69.8 69.8 0.00 69.5 69.5 0.00

KNO3 132.0 131.7 131.2 0.16 0.27 130.7 130.5 0.11 129.9 129.6 0.16

KNO3c 132.4 131.9 131.7 0.27 0.11 130.6 130.6 0.00 129.9 129.8 0.05

In 157.4 157.4 157.4 0.00 0.00 157.2 157.2 0.00 157.1 157.1 0.00

Sn 234.7 232.9 232.9 0.54 0.00 232.4 232.4 0.00 232.2 232.2 0.00

Bi 272.4 272.1 272.1 0.08 0.00 271.8 271.9 0.03 271.6 271.6 0.00

Zn 420.3 419.8 420.0 0.08 0.03 419.7 419.6 0.02 419.4 419.4 0.00

CsCl 478.4 478.2 478.2 0.03 0.00 477.1 476.7 0.06 476.4 476.2 0.03 a For measurements 1 and 2.

b For measurements 2 and 3.

c Cooling rate of 5 K/min.

Table 2. Heat flow calibration data (μV s/mg).

Calibrant 20 K/min 10 K/min 5 K/min

1 2 3 % RSDa % RSD

b 1 2 % RSD 1 2 % RSD

Adamantane –73.83 –74.49 –74.47 0.63 0.02 –74.85 –75.15 0.28 –74.63 –74.88 0.24

Biphenyl –448.50 –445.80 –445.90 0.43 0.02 –443.80 –443.60 0.03 –441.90 –441.40 0.08

KNO3 –174.00 –169.80 –138.20 1.73 14.51 –171.80 –161.10 4.55 –171.20 –152.00 8.40

KNO3c –173.40 –173.30 –172.00 0.04 0.53 –171.80 –171.50 0.12 –171.50 –171.40 0.04

In –99.31 –99.23 –99.35 0.06 0.09 –99.41 –99.40 0.01 –99.49 –99.50 0.01

Sn –189.60 –189.80 –189.90 0.07 0.04 –190.50 –190.30 0.07 –190.90 –190.90 0.00

Bi –155.70 –155.70 –155.70 0.00 0.00 –156.00 –156.00 0.00 –156.30 –156.20 0.05

Zn –283.10 –283.00 –283.10 0.02 0.02 –283.50 –283.60 0.02 –283.90 –283.80 0.02

CsCl –43.17 –43.12 –42.92 0.08 0.33 –43.12 –43.04 0.13 –43.15 –43.40 0.41 a For measurements 1 and 2.

b For measurements 2 and 3.

c Cooling rate of 5 K/min.

8

Fig. 7. The temperature program for indium calibrations with heating rates of 20, 10 and 5 K/min.

Fig. 8. Cooling segments for potassium nitrate at 10 and 5 K/min.

Table 3 summarizes mean values of measured onset temperatures. The largest deviations from

theoretic values are observed for adamantane and potassium nitrate. On the basis of experimental

data coefficients of determination R2 have been calculated that proved to be rather high (Tables

3).

9

Table 3. Mean values and coefficients of determination for temperature calibration data (°C).

Calibrant Theoretic 5

K/min

10

K/min

20

K/min R

2

Adamantane –64.5 –62.6 –62.3 –61.4 0.982

Biphenyl 69.2 69.5 69.8 70.2 0.979

KNO3 128.7 129.9 130.6 131.8 0.997

In 156.6 157.1 157.2 157.4 1.000

Sn 231.9 232.2 232.4 232.9 0.997

Bi 271.4 271.6 271.9 272.1 0.964

Zn 419.5 419.4 419.7 419.9 0.964

CsCl 476.0 476.3 476.9 478.2 1.000

Temperature calibration curve shows perfect linear fit (Fig. 9).

Fig. 9. Temperature calibration curves.

Sensitivity coefficients for heat flow calibration have been calculated by dividing determined

peak areas measured in μV s/mg by enthalpy of the transition, units J/g (Tables 4).

The heat flow calibration curves (Fig. 10) were calculated by Netzsch software and have

ordinary shapes. Mean deviations of calculated curve from experimental values are 1.8 % for all

three calibrations, with maximum deviations of 5.2–5.4 % for bismuth.

Table 4. Sensitivity data for Netzsch DSC 204 F1.

Calibrant Theoretic

enthalpy, J/g

Sensitivity, μV/mW

5 K/min 10

K/min

20

K/min

Adamantane –22.00 3.393 3.381 3.346

Biphenyl –120.50 3.665 3.682 3.700

KNO3 –50.00 3.429 3.433 3.453

In –28.60 3.479 3.476 3.472

Sn –60.50 3.155 3.147 3.138

Bi –53.10 2.943 2.938 2.932

Zn –107.50 2.640 2.638 2.633

CsCl –17.20 2.516 2.502 2.501

10

Fig. 10. Heat flow calibration curves.

5. Conclusions

The approach for speeding up calibration of differential scanning calorimeters has been

proposed. The strategy is based on adjustment of temperature programs to lower time spent on

calibration taking into account instrument features, experimental conditions, and calibrant

properties. Though the application of this approach requires high skills of the operator it could

save many hours of working time having almost no effect on the precision of the calibration data.

References

[1] A.A. Elkordy (Ed.), Applications of Calorimetry in a Wide Context – Differential Scanning

Calorimetry, Isothermal Titration Calorimetry and Microcalorimetry, InTech, Rijeka, New

York, 2013, p. 474 p.

[2] N.V. Antyufeeva, V.M. Aleksashin, G.F. Zhelezina, Yu.V. Stolyankov, Thermoanalytical

methodical approaches for estimation of prepregs and carbon-based composites properties ,

All the Materials. Encyclopaedic Reference Book (Vse Materialy. Entsiklopedicheskij

Spravochnik), 4 (2012) 18–27.

[3] Yu.V. Loschinin, S.I. Pakhomkin, A.S. Fokin, Study of the heating rate effect on phase

transformations of Al alloys by the differential scanning calorimetry method, Aviation

Materials and Technologies (Aviacionnye materialy i tehnologii), 2 (2011) 3–6.

[4] P. Gabbott, A Practical Introduction to Differential Scanning Calorimetry , in: P. Gabbott

(Ed.), Principles and Applications of Thermal Analysis, Wiley-Blackwell, Oxford-Ames-

Carlton, 2007, pp. 12–17.

[5] E. Gmelin, S.M. Sarge, Temperature, heat and heat flow rate calibration of differential

scanning calorimeters, Thermochim. Acta 347 (2000) 9–13.

[6] C.A. N. de Castro, M.J.V. Lourenço, M.O. Sampaio, Calibration of a DSC: its importance for

the traceability and uncertainty of thermal measurements, Thermochim. Acta 347 (2000) 85–

91.

[7] ISO 11357-1:2009, Plastics – Differential scanning calorimetry (DSC) – Part 1: General

principles, 2009.

[8] ASTM E 967-08, Standard Test Method for Temperature Calibration of Differential

Scanning Calorimeters and Differential Thermal Analyzers, 2008.

[9] ASTM E 968-02, Standard Practice for Heat Flow Calibration of Differential Scanning

Calorimeters, 2008.

[10] G.W.H. Höhne, H.K. Cammenga, W. Eysel, E. Gmelin, W. Hemminger, The temperature

calibration of scanning calorimeters, Thermochim. Acta 160 (1990) 1–12.

[11] S.M. Sarge, W. Hemminger, E. Gmelin, G.W.H. Höhne, H. K. Cammenga, W. Eysel,

Metrologically based procedures for the temperature, heat and heat flow rate calibration of

DSC, J. Thermal Anal. 49 (1997) 1125–1134.

11

[12] E. Gmelin, St. M. Sarge, Calibration of differential scanning calorimeters, Pure Appl.

Chem. 67 (1995) 1789–1800.

[13] G.D. Gatta, M.J. Richardson, S.M. Sarge, S. Stølen, Standards, calibration, and guidelines

in microcalorimetry. Part 2. Calibration standards for differential scanning calorimetry, Pure

Appl. Chem. 78 (2006) 1455–1476.

[14] E. Charrier, E.L. Charsley, P.G. Laye, H.M. Markham, B. Berger, T.T. Griffiths,

Determination of the temperature and enthalpy of the solid–solid phase transition of caesium

nitrate by differential scanning calorimetry, Thermochim. Acta 445 (2006) 36–39.

[15] V. V. Deshpande, M.D. Karkhanavala, U.R.K. Rao, Phase transitions in potassium nitrate, J.

Thermal Anal. 6 (1974) 613–621.

[16] G. Hakvoort, C.M. Hol, DSC Investigation of some testing and calibration compounds,

particularly during cooling, J. Therm. Anal. Calorimetry 52 (1998) 195–202.

[17] D.M. Price, Temperature calibration of differential scanning calorimeters, J. Thermal Anal.

45 (1995) 1285–1296.

[18] E. Moukhina, Enthalpy calibration for wide DSC peaks, Thermochim. Acta 522 (2011) 96–

99.

[19] C.I. Braga, M.C. Rezende, M.L. Costa, Methodology for DSC calibration in high heating

rates, J. Aerosp. Technol. Manag. 3 (2011) 179–192.

[20] G.V. Poel, V.B.F. Mathot, High-speed/high performance differential scanning calorimetry

(HPer DSC): temperature calibration in the heating and cooling mode and minimization of

thermal lag, Thermochim. Acta 446 (2006) 41–54.