Collected Applications
Thermal Analysis
PHARMACEUTICALS
172
174
176
178 °C
Preface
Thermal Analysis (TA) is the term used to describe all the analytical techniques that measure the physical
and chemical properties of a sample as a function of temperature.
The potential applications of thermal analysis in the pharmaceutical industry are numerous on account of
the different chemical-physical aspects of investigations. Amongst others these include method development,
characterization and specification of active and inactive ingredients, safety analysis or routine analysis in
quality control and stability studies.
This booklet describes applications of thermal analysis in the pharmaceutical industry with the help of
selected examples i.e. the possible uses of TA in the development of new pharmaceutical substances through
to the quality control of commercial products.
In particular we would like to thank Dr. Danièle Giron and Dr. Sabine Pfeffer (Novartis Pharma AG,
previously TRD, Sandoz Pharma, Basle, Switzerland) for their expert assistance and support which contributed
greatly to the success of this booklet, and also to thank Dr. Thomas Gübeli for making facilities available in
the Chemical Development Department, Analytical Research, Sandoz Pharma, Basle, Switzerland.
We thank Professor Dr. P. C. Schmidt of the Department of Pharmaceutical Technology, Eberhard-KarlsUniversity Tübingen, Germany for preparing numerous application examples.
In addition we would like to thank all the other people who were involved in the preparation of this booklet
including Helga Judex who was responsible for the layout.
Elisabeth Schwarz, Basle
Dr. Jürgen de Buhr, Schwerzenbach
This application booklet presents selected application examples. These have been tested with the utmost care using the
analytical instruments mentioned in the booklet. The experiments were conducted and the resulting data evaluated according
to the current state of our knowledge.
The application booklet does not however absolve you from personally testing the suitability of the examples for your own
methods, instruments and purposes. As the use and transfer of an application example are beyond our control, we cannot
accept any responsibility.
When chemicals and solvents are used, the safety rules and instructions of the manufacturer must be observed.
® ™ All names of commercial products can be registered trade marks even if they are not denoted as such.
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Content
Preface .......................................................................................................................................................... 1
Introduction to Thermal Analysis ................................................................................................................. 4
Application Overview Pharmaceuticals ....................................................................................................... 8
Some Comments on the Pharmaceutical Industry ........................................................................................ 9
Applications of Thermal Analysis in the Pharmaceutical Industry ............................................................ 10
Applications and Techniques ...................................................................................................................... 15
1.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sample Preparation and Method Choice
DSC Calibration, Temperature and Heat flow .................................................................................... 16
TGA Calibration, Temperature ........................................................................................................... 18
DSC Calibration, Heating Rate Independence .................................................................................... 20
The Influence of Heating Rate on the Detection of Polymorphism, Butylated Hydroxyanisole........ 22
Influences on Crystallization Behavior, Saccharose Solutions ........................................................... 24
Influence of the Heating Rate on Moisture Content Determination, an O/W Cream ......................... 26
Influence of the Heating Rate on Decomposition, Metolazone .......................................................... 27
Influence of the Pan on Dehydration, Glucose Monohydrate ............................................................. 29
Sample Preparation, Butylated Hydroxyanisole ................................................................................. 31
Influence of the Sample Weight, Butylated Hydroxytoluene ............................................................. 32
Influence of the Pan on the Determination of Moisture Content, Cellulose ....................................... 33
Sample Storage and Hygroscopic Effects ........................................................................................... 35
Oxidation Stability of Oils .................................................................................................................. 36
Influence of Thermal History and the Evaluation of the Glass Transition, Polystyrene ..................... 38
2.
15
16
17
18
19
20
21
22
23
Identification and Characterization
Melting Point Determination, Vanillin ................................................................................................ 40
Characterization of the Melting Behavior, Vanillin ............................................................................ 41
Phase Changes, Cholesteryl Myristate ................................................................................................ 42
Identification Based on Melting Behavior, Polyethylene Glycol........................................................ 44
Melting Point Depression of Water, Sugar Solutions.......................................................................... 46
DSC 'Fingerprint', O/W Cream ........................................................................................................... 47
Glass Transition, Poly (D,L-lactide)-Co-Glycolide (DLPLGGLU) ................................................... 48
Glass Transition and Moisture Content, Hydroxypropoxymethylcellulose Phthalate (HPMC-PH) .. 49
Quality Control, PE Films ................................................................................................................... 51
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3.
24
25
26
27
28
29
30
Stability
Decomposition, Hydrocortisone ......................................................................................................... 53
Decomposition at the Melting Point, Dihydroergotamine Mesylate .................................................. 54
Melting Behavior and Decomposition, Aspartame ............................................................................. 56
Total Decomposition, Malonic Acid ................................................................................................... 58
Kinetic Analysis of Decomposition, ................................................................................................... 59
Hydrate Stability, Theophylline .......................................................................................................... 61
Moisture, Starch/NaCMC (Primojel) .................................................................................................. 63
4.
31
32
33
Polymorphism
Polymorphism, Tripalmitin ................................................................................................................. 65
Polymorphism, Tolbutamide ............................................................................................................... 66
Polymorphic Modifications by Annealing, Butylated Hydroxyanisole .............................................. 68
34 DSC 'Fingerprint', Magnesium Stearate .............................................................................................. 70
35 Polymorphism, L-Polylactide ............................................................................................................. 71
36 Polymorphism, Sulfapyridine ............................................................................................................. 73
5. Pseudopolymorphism
37 Pseudopolymorphism, Glucose Monohydrate .................................................................................... 75
6. Enantiomers
38 Optical Purity, Ibuprofen ..................................................................................................................... 77
7.
39
40
41
Purity
Purity using DSC and HPLC, 4-Hydroxybenzoic Acid and its Esters................................................ 79
Purity Determination, Phenacetin + 4-Aminobenzoic Acid ............................................................... 81
Purity and Recrystallization, Cholesterol ........................................................................................... 83
8. Phase Diagrams
42 Phase Diagram, Tolbutamide and PEG 6000 ...................................................................................... 84
43 Eutectic Composition, Methyl-4-Hydroxybenzoate and 4-Hydroxybenzoic Acid ............................. 86
9.
44
45
46
47
Quantification/Detection
Solvent Detection by means of TG-MS, Pharmaceutically Active Substance .................................... 88
Quantification, O/W Creams with Different Water Content ............................................................... 90
Quantification,Theophylline Monohydrate ......................................................................................... 92
Determination of an Active Substance, Alcacyl .................................................................................. 94
Literature .................................................................................................................................................... 96
Index ........................................................................................................................................................... 98
Notes ......................................................................................................................................................... 100
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1 DSC Calibration, Temperature and Heat flow
Sample
Indium (calibration standard, purity > 99.999 %)
Application
Standard for calibration
Conditions
Measuring cell:
Pan:
Sample preparation:
DSC measurement:
Atmosphere:
Interpretation
The DSC curve shows the melting of indium. A pure substance melts at an exactly defined
temperature, its melting point. The melting point is taken to be the start or onset of the
melting process which is defined as the temperature given by the intercept of the extrapolated
slope of the melting curve and the continuation of the base line.
Evaluation
The onset temperature and the heat of fusion of indium are determined. The fully automated
evaluation performs a validation which compares the measured values with literature values.
If, as in this case, the values lie within the allowed limits then the message ‘The DSC module
is within specifications’ is displayed.
Melting point (onset)
Heat of fusion
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DSC821e
Aluminum 40 µl, with pierced lid
Indium pellet, pressed flat , premelted
Heating from 120 °C to 180 °C at 10 K/min
Nitrogen, 50 cm3/min
PHARMACEUTICALS
Measured
156.75
28.42
Ref. value
156.60
Tolerance
± 0.3 ° C
28.45
± 0.6 J/g
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Conclusion
The so-called indium-check is a quick and easy method to check the temperature and heat
flow calibration of an instrument. The results are automatically compared with reference
values. The instrument displays the appropriate message if an adjustment of the instrument
is required.
If the instrument is frequently used in other temperature ranges, then further checks with
additional standards suitable for those temperature ranges are recommended.
The tolerances given in this example are standard values and can be individually adapted.
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2 TGA Calibration, Temperature
Samples
Indium and aluminum (calibration standards, purity > 99.999 %)
Application
Standards for temperature calibration
Conditions
Measuring cell:
Pan:
Sample preparation:
TGA measurement:
Atmosphere:
Interpretation
Page 18
TGA850
Alumina 70 µl
Two indium pellets, one piece of aluminum wire. The metals are
pressed flat. Weight of each sample approx. 12 mg. The pure metals
are put in the pan well separated from each other.
Heating from 100 °C to 200 °C at 10 K/min, from 200 °C to
600 °C at 50 K/min, from 600 °C to 700 °C at 10 K/min (indiumaluminium check).
Nitrogen, 20 cm3/min
The SDTA signal shows the melting of both metals. The automatic evaluation determines
the melting points (onsets) of both metals and compares them with the reference values. If
the deviations are too large then the appropriate message is displayed. In this example the
results lie within the specifications. The two peaks at 230 °C and 600 °C are caused by the
change of heating rate.
The weight curve is not shown because no effects would be observed.
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Evaluation
Determination of the melting points (onsets) of indium and aluminum using the
SDTA curve.
1
Melting point )
Melting point 2)
1
2
Conclusion
Indium
measured
156.5
156.7
Ref. value
156.6 ± 1
156.6 ± 2
Aluminum
measured
660.3
660.4
Ref. value
660.3 ± 1.5 ° C
660.3 ± 3
° C
) based on the thermocouple of the sample holder (sample temperature)
) based on the thermocouple of the furnace (abscissa unit)
The so called indium-aluminium check is a quick and easy method to check the temperature
calibration of the thermobalance. If the measured values lie outside the given tolerances,
then the settings can be adjusted. If the thermobalance (as in the case of the TGA850 or
TGA/SDTA851e) is equipped with a suitable sample temperature sensor, then the check is
performed using the melting of known standards. Otherwise the method using the Curie
point transition temperatures of different metals is used.
A special weight calibration is not usually be performed, because many balances already
have an automatic internal test procedure available. A certified weight can be weighed at
defined intervals. For GMP investigations the use of reference materials to check the weight
calibration is recommended e.g. calcium oxalate monohydrate (Pharma Eur. 1997).
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3 DSC Calibration, Heating Rate Independence
Sample
Zinc (calibration standard, purity > 99.999 %)
Application
Standard for calibration
Conditions
Measuring cell:
Pan:
Sample preparation:
DSC measurement:
Atmosphere:
Interpretation
Evaluation
The DSC curves show the melting of zinc at different heating rates. If displayed with respect
to temperature, the peak area increases with increasing heating rates. The heat of fusion is
however the integral of the heat flow with respect to time. This is just as independent of the
heating rate as the melting point.
The onset temperature and the heat of fusion of zinc are determined.
Heating rate
Melting point (onset)
Heat of fusion
Page 20
DSC821e
Alumina 40 µl, with pierced lid
Zinc pellet, pressed flat and premelted
Heating from 350 °C to 475 °C at 5, 10 and 20 K/min.All
measurements are performed with the same sample. Cooling rate
is 5 K/min.
Nitrogen, 50 cm3/min
PHARMACEUTICALS
Measured
5
419.5
107.0
Measured
10
419.6
107.0
Measured
20
419.8
107.1
Ref. value
419.7
107.0
K/min
° C
J/g
METTLER TOLEDO Collected Applications TA
Conclusion
The calibration data of an instrument are affected by many factors such as the heating rate,
the purge gas used, the sample pan material and the temperature range used. Only when
these effects can be taken into account in the calibration data, are results obtained that are
independent of measurement parameters, as is shown in this example of the melting point
and the heat of fusion.
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5 Influences on Crystallization Behavior, Saccharose
Solutions
Sample
Application
D(+) Saccharose solution,
20 weight % in water (= 1.05 mole %)
CH2OH
OH
Inactive ingredient (solution stabilizer)
OH
Measuring cell:
Pan:
Sample preparation:
DSC measurement:
Atmosphere:
Interpretation
Page 24
O
O
OH
Conditions
CH2OH
O
OH
OH
CH2OH
DSC820
Aluminum 40 µl, hermetically sealed
One drop of solution is weighed into the pan using a fine pipette,
sample weight 2.260 mg.
Cooling from 25 °C to -50 °C at -1, -2, -5, -10, -20 K/min. The
same sample is used for all the measurements. Heating from
-50 °C to 25 °C at 5 K/min.
Nitrogen, 80 cm3/min
The curves show that crystallization and melting processes can be measured with the DSC.
At low cooling rates, the onset temperatures are almost constant, but are displaced to lower
values (supercooling) at higher cooling rates. At very high cooling rates it is even possible
that the solution does not crystallize but vitrifies i.e. is transformed to glassy state.
The melting point depression and the ‘purity’ of the water can be calculated from the melting
peak.
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Evaluation
Cooling rate (K/min)
-1
-1
-5
-10
-20
+5
Onset, °C
-15.0
-15.3
-15.8
-15.3
-23.0
-3.8
Purity calculated using the
Melting peak:
Melting point depression:
Conclusion
∆H, J/g
173.2
168.3
162.5
170.0
157.3
153.5
Effect
crystallization
‘
‘
‘
‘
melting
99.02 mole % (theoretical value: 98.95)
-1.76 °C
The onset temperatures of the melting and crystallization processes are different. Crystallization processes are controlled kinetically by nucleation and are dependent on the cooling
rate and the amount of sample (number of nuclei present). The onset temperatures of melting
peaks are not normally subject to disturbing influences.
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6 Influence of the Heating Rate on Moisture Content
Determination, an O/W Cream
Sample
O/W Cream sample 647-A
Application
Basic material for the manufacture of creams
Conditions
Measuring cell:
Pan:
Sample preparation:
TGA measurement:
Atmosphere:
Interpretation
Evaluation
The TGA curves show the evaporation of the volatile components (mainly water) in the
region between 40 °C and 140 °C. At higher heating rates the evaporation is diplaced to
higher temperatures. The first derivative of the TGA curve is helpful for the determination
of the final step of the TGA signal.
Heating rate
2 K/min
5 K/min
Conclusion
Page 26
TGA850
Aluminum 100 µl, with pierced lid. The lid was automatically
pierced shortly before the measurement (sample changer with
needle, 1 mm diameter)
As received, no preparation
Heating from 20 °C to 200 °C at 2 and 5 K/min. Both
measurements are blank curve corrected.
Nitrogen, 50 cm3/min
Step, %
59.4
58.5
Peak temperature DTG, °C
102.1
122.4
The influence of the mass and form of the sample, the heating rate and the type of pan have
to be considered when developing methods. The heating rate is of special importance when
investigating time and temperature dependent effects such as evaporation.
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8 Influence of the Pan on Dehydration,
Glucose Monohydrate
Sample
OH
α-D-Glucose monohydrate
Application
Inactive ingredient, filler for tablets and capsules
Conditions
Measuring cells:
Pan DSC:
Pan TGA:
Sample preparation:
DSC measurement:
TGA measurement:
Atmosphere:
Interpretation
Evaluation DSC
CH2OH
O
OH
• H2O
OH
OH
DSC820 or TGA850
Aluminum 40 µl, hermetically sealed or with pierced lid
Aluminum 100 µl, with pierced lid
As received, no preparation
Heating from 30 °C to 250 °C at 20 K/min
Heating from 30 °C to 300 °C at 20 K/min
Nitrogen; DSC: 50 cm3/min, TGA: 80 cm3/min
A comparison of the two DSC curves of α-D-Glucose monohydrate shows the changes that
arise when the sample is measured in a sealed pan or in a pan with a pierced lid. In a
hermetically sealed pan the sharp melting peak of the monohydrate can be observed. If a
pan with a pierced lid is used, the water of crystallization can escape. This is noticeable as a
shift of the DSC curve at the beginning of the measurement and as a broad evaporation
peak. At the same time, a transition to β-D-Glucose anhydrate occurs, the melting point of
which is at about 158 °C. Above 200 °C the glucose starts to caramelize.
Measuring conditions
Sealed pan
Pan with pierced lid
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Onset, °C
81.4
157.4
Effect
melting
melting
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Interpretation
Thermogravimetric measurements using a pan with a pierced lid confirm the interpretation
of the results obtained from the DSC curves, in particular the weight loss caused by the
evaporation of the water of crystallization as well as the melting of the β-D-Glucose anhydrate
afterwards. The weight loss step of 7% between 53 °C and 134 °C is somewhat less than
that expected stoichiometrically. It can be explained however by a loss of water of
crystallization during storage of the sample.
Evaluation TGA
TGA, step
SDTA, onset
SDTA, onset
Conclusion
Page 30
Temperature, °C
Effect
53-134
59
154.2
7.0% weight loss (water of crystallization)
endothermic peaks
melting peak
A substance that contains water of crystallization and its anhydrous form normally have
different melting points (pseudopolymorphism). The melting point of the form containing
the water of crystallization can be determined in a hermetically sealed pan, provided that no
decomposition occurs. In an open pan the water of crystallization can escape so that the
melting point of the anhydrous form is measured. The presence of a form with water of
crystallization should always be confirmed by measuring the weight loss.
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9 Sample Preparation, Butylated Hydroxyanisole
Sample
Butylated hydroxyanisole
H3CO
Application
OH
Inactive ingredient (antioxidant)
C(CH3)3
Conditions
Interpretation
Evaluation
Measuring cell:
Pan:
Sample preparation:
DSC measurement:
Atmosphere:
The two curves show the effects that sample preparation can have on the results. In both
cases, two melting peaks can be observed that differ noticeably in temperature range and in
the heats of fusion. The explanation lies in the polymorphic behavior of butylated
hydroxyanisole. The two peaks correspond to the possible crystal modifications.
Sample preparation
as received
ground in a mortar
Conclusion
DSC820
Aluminum 40 µl, hermetically sealed
As received (1) or crystals ground in a mortar (2)
Heating from 30 °C to 70 °C at 2.5 K/min
Nitrogen, 50 cm3/min
Onset 1, ° C
59.3
55.1
∆H, J/g
78.2
96.8
Onset 2, ° C
∆H, J/g
63.3
61.7
27.6
1.7
A difference in sample preparation (especially mechanical treatment) can lead to different
results. This is particularly the case with substances that exhibit polymorphism.
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10 Influence of the Sample Weight,
Butylated Hydroxytoluene
Sample
Butylated hydroxytoluene
H3C
Application
Inactive ingredient (antioxidant)
Conditions
Measuring cell:
Pan:
Sample preparation:
DSC measurement:
Atmosphere:
DSC821e with IntraCooler
Aluminum 40 µl, hermetically sealed
As received, no preparation
Heating from 50 °C to 80 °C at 2.5 K/min
Air, stationary environment, no flow
OH
C(CH3)3
Interpretation
The curves show the melting peaks as a function of sample weight. As expected, the peaks
in the original presentation (ordinate in mW) increase in height but also in width with
increasing weight. Because of this the resolution decreases. In contrast, the normalized
presentation shows that the lowest sample weight gives the highest peaks.
Evaluation
The onset temperature and heat of fusion of the peaks are determined. The mean values of
a number of measurements are presented in the table.
Sample weight
18 ± 0.3 mg
8.5 ± 0.3 mg
4.0 ± 0.4 mg
Conclusion
Page 32
C(CH3)3
Onset, ° C
69.4 ± 0.1
69.6 ± 0.1
69.5 ± 0.1
Heat of fusion ∆H, J/g
85.6, 84.7, 85.6
83.9, 84.5
82.6, 84.1, 83.6
The sample weight influences the shape of the melting peak. The time required for melting
is longer for larger samples because a greater amount of heat has to be transferred. As a
result of this, the peaks are shifted to higher temperature. For comparison purposes, the
measurement of samples of similar weight is recommended. Samples that are too large are
disadvantageous: the peaks become broad (lower resolution) and non-uniform melting leads
to irregularly shaped peaks.
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11 Influence of the Pan on the Determination of Moisture
Content, Cellulose
HOH2C
OH
HO
HO
O
O
HO
HOH2C
HOH2C
OH
O
OH
HO
O
O
O
OH
HO
O
HOH2C
OH
n
Sample
Application
Conditions
Microcrystalline Cellulose (Avicel)
Inactive ingredient (gel binder, adsorption agent, flow improver)
Measuring cell:
Pan:
Sample preparation:
TGA measurement:
Atmosphere:
Interpretation
TGA850 with sample robot
Aluminum 100 µl, without a lid or with a pierced lid. The lid was
pierced automatically immediately before the measurement (needle
diameter 1 mm).
As received, no preparation
Heating from 30 °C to 300 °C at 20 K/min, all measurements are
blank curve corrected.
Nitrogen, 80 cm3/min
Cellulose and its derivatives easily take up water from the surroundings or release water
depending on the humidity in the laboratory.
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At the same heating rate, the release of moisture during the measurement occurs more
rapidly if no lid is used than with a pierced lid. This has to be taken into account when
comparing methods using open pans to methods in which pierced lids are used (sample
changer operation). Samples which are in open pans on the sample changer turntable can
lose moisture because of the relatively low humidity of the surroundings. The true moisture
content of samples can be determined using sealed pans whose lids are pierced immediately
before the measurement.
Evaluation
Pan
Step, %
Sealed, lid pierced immediately -4.8
before the measurement
Without lid
-4.1
Initial value of the curve
sample weight *, %
99.9
99.1
Corrected
step, %
-4.9
-5.0
* Original sample weight = 100%
Conclusion
Page 34
The possible effect of the delay between the weighing out and the actual measurement of
the sample must be investigated when determining the moisture content with TGA. When
using pans with pierced lids, the smaller the hole in the lid the more the weight loss step is
shifted to higher temperature.
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