Interpreting Thermal Analysis Data

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Differential Scanning Differential Scanning CalorimetryCalorimetry

Clare RawlinsonClare RawlinsonSchool of PharmacySchool of Pharmacy

University of BradfordUniversity of Bradford

““Cooking with ChemicalsCooking with Chemicals””

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OutlineOutline

Brief history of thermal analysisBrief history of thermal analysis

Theory of thermal analysis techniques Theory of thermal analysis techniques –– Thermal Gravimetric Analysis (TGA)Thermal Gravimetric Analysis (TGA)–– Differential Scanning Calorimetry (DSC)Differential Scanning Calorimetry (DSC)

Generating valid dataGenerating valid data–– CalibrationCalibration–– Sample preparationSample preparation

Interpreting data and ApplicationsInterpreting data and Applications–– Real eventsReal events–– ArtefactsArtefacts

Recent advancesRecent advances

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CalorimetryCalorimetryCalorimetryCalorimetry–– The study of heat transfer during The study of heat transfer during

physical and chemical processesphysical and chemical processes

CalorimeterCalorimeter–– A device for measuring the heat A device for measuring the heat

transferredtransferred

LavoisierLavoisier and and LaplaceLaplace (1782(1782--1784):1784):oil was burned in a lamp (oil was burned in a lamp (Fig 9Fig 9) held in ) held in a bucket (Fig. 8) held in a wire mesh a bucket (Fig. 8) held in a wire mesh cage (cage (ff))surrounded by ice in spaces surrounded by ice in spaces bb and and aa of of the double walled container a foot in the double walled container a foot in diameterdiameterlid (lid (FF) was topped with ice, as was a ) was topped with ice, as was a mesh lid (not shown) beneath it that mesh lid (not shown) beneath it that covered the inner volume covered the inner volume bb

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Oil lamps to Guinea PigsOil lamps to Guinea Pigs……Measured heat production of Measured heat production of the metabolic processes in the metabolic processes in the ice bath calorimeterthe ice bath calorimeterOuter jacket prevented Outer jacket prevented conduction of heat from the conduction of heat from the external environment which external environment which would have also melted the would have also melted the iceiceFrom latent heat of fusion for From latent heat of fusion for ice (334 J/gram ice at 0 ice (334 J/gram ice at 0 ººC) C) LavoisierLavoisier converted the rate converted the rate of water formation to heat of water formation to heat production production In 10 hours 370 grams of ice In 10 hours 370 grams of ice meltedmelted

Guinea pig produced 12,358 J per hour of heat Guinea pig produced 12,358 J per hour of heat (12.4 kJ/hr)(12.4 kJ/hr)

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Basic Principles of Thermal Analysis

Modern instrumentation used for thermal analysis usually consists of four parts:

sample/sample holder

sensors to detect/measure a property of the sample and the temperature

an enclosure within which the experimental parameters may be controlled

a computer to control data collection and processing

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TGA and DSCTGA and DSCThermogravimetric Analysis (TGA) –– mass change of a substance measured as function of mass change of a substance measured as function of

temperature whilst the substance is subjected to a controlled temperature whilst the substance is subjected to a controlled temperature programmetemperature programme11

–– mass is lost if the substance contains a volatile fractionmass is lost if the substance contains a volatile fraction

Differential Scanning Calorimetry (DSC)–– provides information about thermal changes that do not involve aprovides information about thermal changes that do not involve a

change in sample masschange in sample mass11

–– more commonly used technique than TGA more commonly used technique than TGA – Two basic types of DSC instruments: heat-flux and power

compensation

1Haines, P. J. (2002) The Royal Society of Chemistry, Cambridge.

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Sample holder :sample and reference are connected by a low-resistance heat flow pathAluminium, stainless, platinum sample pans

Sensors:Sensors:temperature sensorstemperature sensorsusually thermocouplesusually thermocouples

Furnace:one block for both sample and reference cells

Temperature controller:• temperature difference between the sample and reference is

measured

Heat Flux DSC

samplepan

inert gasvacuum

heatingcoil

referencepan

thermocouples

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Sample holderSample holder : : AluminiumAluminium, platinum, stainless steel pans, platinum, stainless steel pans

Sensors:Sensors:Pt resistance Pt resistance

thermocouples. thermocouples. Separate sensors Separate sensors

and heaters for theand heaters for thesample and referencesample and reference

Furnace:Furnace:separate blocks for sample and reference cellsseparate blocks for sample and reference cells

Temperature controller:Temperature controller:differential thermal power is supplied to the heaters to maintadifferential thermal power is supplied to the heaters to maintain the in the

temperature of the sample and reference at the program valuetemperature of the sample and reference at the program value

samplepan

ΔT = 0

inert gasvacuum

inert gasvacuum

individualheaters

referencepan

thermocouple

Power Compensated DSCPower Compensated DSC

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OutlineOutline






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DSC Calibration

BaselineCalibration

evaluation of the thermal resistance of the sample and reference sensors

measurements over the temperature range of interest

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DSC CalibrationTemperature

• match melting onset temperatures to the known melting points of standards analyzed by DSC

• should be calibrated as close to desired temperature range as possible

Heat flow• use calibration standards of known heat capacity, slow accurate heating

rates (0.5–2.0 °C/min), and similar sample and reference pan weights

calibrants• high purity• accurately known enthalpies• thermally stable• light stable• not hygroscopic• do not react (pan, atmosphere)

metals• Indium 156.6 °C; 28.45 J/g• Zinc 419.47°C, 108.17 J/ginorganics• KNO3 128.7 °C• KClO4 299.4 °Corganics• polystyrene 105 °C• benzoic acid 122.3 °C; 147.3 J/g

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Sample Preparationaccurately-weighed samples (~3-20 mg, usually 3-5 mg for simple powders)small sample pans (0.1 mL) of inert or treated metals (Al, Pt, stainless)several pan configurations, e.g., open , pinhole, or hermetically-sealed panssame material and configuration should be used for the sample and the referencematerial should completely cover the bottom of the pan to ensure good thermal contactavoid overfilling the pan to minimize thermal lag from the bulk of the material to the sensor

Al Pt alumina Ni Cu quartz

* small sample masses and low heating rates increase resolution, but at the expense of sensitivity

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Purge GasesPurge GasesSample may react with air Sample may react with air -- oxidising or burningoxidising or burning

Control moisture content of atmosphereControl moisture content of atmosphere

Use inert gas e.g. nitrogen or argonUse inert gas e.g. nitrogen or argon

Flowing purge gasFlowing purge gas

In some cases deliberately choose reactive gas, e.g. In some cases deliberately choose reactive gas, e.g. –– hydrogen to reduce an oxide to metalhydrogen to reduce an oxide to metal–– carbon dioxide which affects decomposition of metal carbonatecarbon dioxide which affects decomposition of metal carbonate

Removes waste products from sublimation or Removes waste products from sublimation or decompositiondecomposition

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OutlineOutline






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Typical Features of a DSC Trace (Polymorphic System)

sulphapyridine

endothermic eventsmelting

sublimationsolid-solid transitions

desolvationchemical reactions

exothermic eventscrystallization

solid-solid transitionsdecomposition

chemical reactions

baseline shiftsglass transition

Exo

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Melting Processes by DSC

Pure substances

• linear melting curve

• melting point defined by onset temperature

eutectic melt

Melting with decomposition

• exothermic

• endothermic

Impure substances

• Broad, asymmetric melting peak

• melting characterized at peak maxima

• eutectic impurities may produce a second peak

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Definition of Transition Temperature

157.81°C

156.50°C28.87J/g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Hea

t Flo

w (W

/g)

140 145 150 155 160 165 170 175

Temperature (°C)Exo Up Universal V3.3B TA Instruments

extrapolatedonset temperature

peak meltingtemperature

Exo

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Enthalpy of Fusion

157.81°C

156.50°C28.87J/g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Hea

t Flo

w (W

/g)

140 145 150 155 160 165 170 175


Exo

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Enthalpy of Fusion by DSC

More difficult where multiple thermal events leading to stable melte.g. solid-solid transitions (A to B) before melt, or where melt / recrystallisation before meltEstimate from sum all areas

For a single (well-defined) melting endotherm

area under peakminimal

decomposition/sublimationreadily measured for high

melting polymorphcan be measured for low

melting polymorph Endo

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Purity by DSC1-3 mg samples in hermetically-sealed pans are recommended

Peak width a valuable measure of purity:

impurities lower the melting point

Less pure (non-perfect) crystals melt first followed by purer larger crystals

polymorphism interferes with purity determination, especially when a transition occurs in the middle of the melting peak

Accurate measurement of ΔHf needs pure samples of polymorphs

benzoic acid

Plato, C.; Glasgow, Jr., A.R. Anal. Chem., 1969, 41(2), 330-336.

Exo

97%

99%

99.9%

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Glass Transitions

transition from a disordered solid to a liquid

appears as a step (endothermic direction) in the DSC curve

gradual enthalpy change may occur, producing an endothermic peak superimposed on the glass transition

characterized by change in heat capacity (no heat absorbed or evolved)

Exo

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Effect of Heating Ratemany transitions (evaporation, crystallization, decomposition, etc.) are kinetic so shift to higher temp. when heated at a higher rate

increasing the scanning rate increases sensitivity, while decreasing the scanning rate increases resolution

to obtain thermal event temperatures close to the true thermodynamic value, slow scanning rates (e.g., 1–5 K/min) should be used

Rapid scanning can obscure thermal events

Advantageous in fast scan DSC, e.g. 500K/min

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Recognizing Artefacts

Sample movement in

pan

cool air entry into cell

sample pan

distortion

Pan moves in furnace

mechanical shock / knock

bench

electrical effects, power spikes, etc.

atmosphere changes

burst of pan lid

Closing / opening pan

hole, e.g. sublimation

sensor contamination

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Ensuring correct interpretation of DSCEnsuring correct interpretation of DSC

You canYou can’’t t Can minimise misinterpretationCan minimise misinterpretationEssential to have valid data to interpretEssential to have valid data to interpret–– Calibration, reproducible data, appropriate sampling etcCalibration, reproducible data, appropriate sampling etc

Kinetics / thermodynamics at elevated tempsKinetics / thermodynamics at elevated temps–– High temp can speed kinetics High temp can speed kinetics –– event would happen at room event would happen at room

temperature but slowlytemperature but slowly–– Effect activated by increased temp (overcome activation energy) Effect activated by increased temp (overcome activation energy)

-- event would not happen at room temperatureevent would not happen at room temperature

DSC shows excipients interact at 120DSC shows excipients interact at 120ººCC–– Does not necessarily show interaction at room tempDoes not necessarily show interaction at room temp

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Polymorph Screening and Indentification

thermal stability– melting– crystallization– solid-state transformations– desolvation– glass transition– sublimation– decomposition

heat flow– heat of fusion– heat of transition– heat capacity

mixture analysis– physical purity (crystal

forms, crystallinity)– chemical purity

phase diagrams / interactions

–––––– Form I–––––– Form II–––––– Variable Hydrate–––––– Dihydrate–––––– Acetic acid solvate

Form III

Form IForm II

–––––– Form I–––––– Form II–––––– Variable Hydrate–––––– Dihydrate–––––– Acetic acid solvate

Form III

Form IForm II

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Hea

t Flo

w (W

/g)

0 50 100 150 200 250 300 350

Temperature (°C)

––––––– Form I––––––– Form II––––––– Variable Hydrate––––––– Dihydrate––––––– Acetic acid solvate

Exo Up

Form III

Form IForm II

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Hea

t Flo

w (W

/g)

0 50 100 150 200 250 300 350

Temperature (°C)

––––––– Form I––––––– Form II––––––– Variable Hydrate––––––– Dihydrate––––––– Acetic acid solvate

Exo Up

Form III

Form IForm II

Exo

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Effect of Phase Impurities

Lot A: pure low melting polymorph – melting observedLot B: seeds of high melting polymorph induce solid-state transition below the melting temperature of the low melting polymorph

2046742FILE# 022511DSC.1

2046742FILE# 022458 DSC.1 Form II ?

-5

-4

-3

-2

-1

0

Hea

t Flo

w (W

/g)

80 130 180 230 280


Lot A - pure

Lot B - seeds

lots A & B of polymorph (identical by XRD) are different by DSC:

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OutlineOutline






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MicrocalorimetryMicrocalorimetryHigh sensitivity DSCHigh sensitivity DSCSolutionsSolutionsScan range typically Scan range typically 00--120 120 °°CCScanning rate of 0Scanning rate of 0--120 120 °°C/hrC/hrReverse scan rate 0Reverse scan rate 0--45 45 °°C/hrC/hr

(depending on efficiency(depending on efficiencyof cooling tank)of cooling tank)

Useful for looking at low Useful for looking at low energy modificationsenergy modificationse.g. protein relaxation and e.g. protein relaxation and refolding, polymer refolding, polymer characterisationcharacterisation

trehlose

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Modulated DSC Heating Profile

Modulated DSC (MDSC)

introduced in 1993; “heat flux”design

sinusoidal (or square-wave or sawtooth) modulation is superimposed on the underlying heating ramp

total heat flow signal contains all of the thermal transitions of standard DSC

Fourier Transformation analysis is used to separate the total heat flow into its two components: reversing and non-reversing heat flow

increased sensitivity, resolution and the ability to separate multiple thermal events

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Heat capacity (reversing heat flow)

glass transitionmelting

MDSC for Polymorph Characterization

Reversing (heat flow component)

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Rev

Hea

t Flo

w (W

/g)

110 120 130 140 150 160 170 180

Temperature (°C)

DSC010622b.1 483518 HCL (POLYMORPH 1)DSC010622d.1 483518 HCL

Exo Up Universal V3.3B TA

Non-reversing (heat flow component)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Non

rev

Hea

t Flo

w (W

/g)

110 120 130 140 150 160 170 180

Temperature (°C)

DSC010622b.1 483518 HCL (POLYMORPH 1)DSC010622d.1 483518 HCL

Exo Up Universal V3.3B TA

Lot A

Lot B

Lot A

Lot B

reversing heat flow non-reversing heat flow

Kinetic(non-reversing heat flow)

crystallizationdecomposition

evaporation

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‘‘HyperHyper’’ DSCDSCFast scanning DSCFast scanning DSCOnly possible with power compensatedOnly possible with power compensatedNormal equipment Normal equipment ≈≈ 100 100 ººC/minC/minSpecialised up to 500 Specialised up to 500 ººC/minC/minIncreased sensitivity, loss of resolutionIncreased sensitivity, loss of resolutione.g. amorphous content in mainly crystalline samplee.g. amorphous content in mainly crystalline sample– change of specific heat at TgTg is linear relationship to the

amorphous content–– Conventional DSC 10% amorphous limit of detectionConventional DSC 10% amorphous limit of detection–– Hyper DSC Hyper DSC <1% amorphous easily detected<1% amorphous easily detected

LappalainenLappalainen, M., I. , M., I. PitkanenPitkanen, et al. (2006). , et al. (2006). International Journal of International Journal of PharmaceuticsPharmaceutics 307307(2): 150(2): 150--155.155.

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Best Practices for Thermal Analysisproper instrument calibration

use purge gas (N2 or He) to remove corrosive off-gases

small sample size

good thermal contact between the sample and the temperature-sensing device

proper sample encapsulation

start temperature well below expected transition temperature

slow scanning speeds(Unless aiming to obscure thermal transitions, e.g fast scan DSC)

avoid decomposition in the DSC(Run TGA first – its easier to clean up!)

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CautionCaution……It is a bulk toolIt is a bulk tool–– Analysing the gross average of events in a sample Analysing the gross average of events in a sample –– Conversely, small powder sample in DSC may not Conversely, small powder sample in DSC may not

represent packing of powder bulk in represent packing of powder bulk in decomposition studiesdecomposition studies

Instrument error in DSC typically Instrument error in DSC typically ±± 0.5 0.5 -- 11ººCC

In Scanning modes, thermal events may be In Scanning modes, thermal events may be ““smearedsmeared”” by a thermal lagby a thermal lag–– Sample temperature not keeping up with Sample temperature not keeping up with

instrumentinstrument–– Bigger effect at higher heating ratesBigger effect at higher heating rates–– Typically 1Typically 1ººC at 10C at 10ººC/minC/min

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And more caution!And more caution!Thermal analysis tells you what is happening at Thermal analysis tells you what is happening at the temperature it happens at!the temperature it happens at!–– Care when extrapolating to room temperature Care when extrapolating to room temperature stability / interactionstability / interaction

DonDon’’t overt over--interpret datainterpret data

Care when using thermal analysis in isolationCare when using thermal analysis in isolationArtefacts / heating rate effects etc Artefacts / heating rate effects etc Couple with other analytical toolsCouple with other analytical tools

–– Heated XHeated X--ray, heated vibrational ray, heated vibrational spectroscopy, hot stage microscopespectroscopy, hot stage microscope

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AcknowledgementsAcknowledgements

Professor Adrian Williams, University of ReadingProfessor Adrian Williams, University of ReadingDr Ian Grimsey, University of BradfordDr Ian Grimsey, University of BradfordDr Peter Timmins, Bristol Myers SquibbDr Peter Timmins, Bristol Myers Squibb

Dr Wendy Dr Wendy HulseHulse, University of Bradford, University of BradfordLuciana Luciana DeMatosDeMatos, University of Sheffield, University of Sheffield

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QuestionsQuestions

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Reversing and Non-Reversing Contributions

to Total DSC Heat Flow

dQ/dt = Cp. dT/dt + f(t,T)total heat flow

resulting from average heating

rate reversing signal

heat flow resulting fromsinusoidal temperature

modulation(heat capacity component)

non-reversing signal(kinetic

component)

e.g. see Pharmaceutical Research: 17 (6): 696-700, June 2000 Craig, DQM et al.

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Some Common Thermal Analysis Techniques

Differential Thermal Analysis (DTA)• the temperature difference between a sample and an inert reference material, ΔT = TS -

TR, is measured as both are subjected to identical heat treatments

Differential Scanning Calorimetry (DSC)• the sample and reference are maintained at the same temperature, even during a

thermal event (in the sample)

• the energy required to maintain zero temperature differential between the sample and the reference, dΔq/dt, is measured

Isothermal titration calorimetry (ITC)• The temperature of a “reaction” is kept constant whilst the energy change is measured

Thermogravimetric Analysis (TGA)• the change in mass of a sample on heating is measured

There are various techniques in which a physical property is measured as a function of temperature, while the sample is subjected to a predefined heating or cooling program.

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Thermogravimetric Analysis (TGA)• thermobalance to monitor

sample weight as a function of temperature

• weight calibration using known weights

• temperature calibration based on ferromagnetic transition of Curie point standards (e.g., Ni)

• larger sample masses, lower temperature gradients, and higher purge rates minimize undesirable buoyancy effects

12.15%

19.32%

29.99%

20

40

60

80

100

120

Wei

ght (

%)

0 20 40 60 80 100 120 140 160

Time (min) Universal V3.7A TA Instruments

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Differential Thermal AnalysisSample holder: Sample and reference cells

Sensors: Thermocouples, one for the sample and one for the reference

Furnace: Block containing sample and reference cells

Temperature controller: Controls temperature program

Advantages: instruments can be used at very high temperaturesinstruments are highly sensitiveflexibility in sample volume/formcharacteristic transition or reaction temperatures can be determined

Disadvantages:uncertainty of heats of fusion and transition temperatures

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• development of “hyphenated” techniques for simultaneous analysis

TG-DTA

TG-DSC

TG-FTIR

TG-MS

15.55%(0.9513mg)

24.80°C100.0%

179.95°C84.45%

-1.8

-0.8

0.2

1.2

2.2

3.2

4.2

Tem

pera

ture

Diff

eren

ce (µ

V/m

g)

-40

0

40

80

120

Wei

ght (

%)

20 70 120 170 220 270


TG-DTA trace of sodium tartrate

“Hyphenated” Techniques• thermal techniques alone are insufficient to prove the existence of polymorphs

and solvates• other techniques should be used, e.g., microscopy, diffraction, and spectroscopy

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Interpreting Thermal Analysis Data