Differential Scanning Differential Scanning Calorimetry - - PowerPoint PPT Presentation

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Differential Scanning Differential Scanning Calorimetry Calorimetry

Clare Rawlinson Clare Rawlinson School of Pharmacy School of Pharmacy University of Bradford University of Bradford

“ “Cooking with Chemicals Cooking with Chemicals” ”

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Outline Outline

• Brief history of thermal analysis

Brief 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 data

Generating valid data

– – Calibration Calibration – – Sample preparation Sample preparation

• Interpreting data and Applications

Interpreting data and Applications

– – Real events Real events – – Artefacts Artefacts

• Recent advances

Recent advances

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Calorimetry Calorimetry

• Calorimetry

Calorimetry

– – The study of heat transfer during The study of heat transfer during physical and chemical processes physical and chemical processes

• Calorimeter

Calorimeter

– – A device for measuring the heat A device for measuring the heat transferred transferred Lavoisier Lavoisier and and Laplace Laplace (1782 (1782-

• 1784):

1784):

• il was burned in a lamp (
• il was burned in a lamp (Fig 9

Fig 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 (f f) )

• surrounded by ice in spaces

surrounded by ice in spaces b b and and a a of

• f

the double walled container a foot in the double walled container a foot in diameter diameter

• lid (

lid (F F) 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 b b

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Oil lamps to Guinea Pigs Oil lamps to Guinea Pigs… …

• Measured heat production of

Measured heat production of the metabolic processes in the metabolic processes in the ice bath calorimeter the ice bath calorimeter

• Outer

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 ice ice

• From 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 º ºC) C) Lavoisier Lavoisier converted the rate converted the rate

• f water formation to heat
• f water formation to heat

production production

• In 10 hours 370 grams of ice

In 10 hours 370 grams of ice melted melted 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 DSC TGA and DSC

Thermogravimetric 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 programme temperature programme1

1

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

Differential Scanning Calorimetry (DSC)

– – provides information about thermal changes that do not involve a provides information about thermal changes that do not involve a change in sample mass change in sample mass1

1

– – 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 path Aluminium, stainless, platinum sample pans

Sensors: Sensors:

• temperature sensors

temperature sensors

• usually thermocouples

usually thermocouples

Furnace:

• ne block for both sample

and reference cells

Temperature controller:

• temperature difference between the sample and reference is

measured

Heat Flux DSC

sample pan inert gas vacuum heating coil reference pan thermocouples

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Sample holder Sample holder :

:

• Aluminium

Aluminium, platinum, stainless steel pans , platinum, stainless steel pans

Sensors: Sensors:

• Pt resistance

Pt resistance thermocouples. thermocouples.

• Separate sensors

Separate sensors and heaters for the and heaters for the sample and reference sample and reference

Furnace: Furnace:

• separate blocks for sample and reference cells

separate blocks for sample and reference cells

Temperature controller: Temperature controller:

• differential thermal power is supplied to the heaters to mainta

differential thermal power is supplied to the heaters to maintain the in the temperature of the sample and reference at the program value temperature of the sample and reference at the program value

sample pan ΔT = 0 inert gas vacuum inert gas vacuum

individual heaters

reference pan thermocouple

Power Compensated DSC Power Compensated DSC

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Outline Outline

• Brief history of thermal analysis

Brief 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 data

Generating valid data

– – Calibration Calibration – – Sample preparation Sample preparation

• Interpreting data and Applications

Interpreting data and Applications

– – Real events Real events – – Artefacts Artefacts

• Recent advances

Recent advances

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DSC Calibration

Baseline Calibration

evaluation of the thermal resistance of the sample and reference sensors measurements

• ver the temperature

range

• f

interest

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DSC Calibration

Temperature

• 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/g

inorganics

• KNO3 128.7 °C
• KClO4 299.4 °C
• rganics
• polystyrene 105 °C
• benzoic acid 122.3 °C; 147.3 J/g

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Sample Preparation

accurately-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 pans same material and configuration should be used for the sample and the reference material should completely cover the bottom of the pan to ensure good thermal contact avoid 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

• f

sensitivity

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Purge Gases Purge Gases

• Sample may react with air

Sample may react with air -

• oxidising or burning
• xidising or burning
• Control moisture content of atmosphere

Control moisture content of atmosphere

• Use inert gas e.g. nitrogen or argon

Use inert gas e.g. nitrogen or argon

• Flowing purge gas

Flowing 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 metal hydrogen to reduce an oxide to metal – – carbon dioxide which affects decomposition of metal carbonate carbon dioxide which affects decomposition of metal carbonate

• Removes waste products from sublimation or

Removes waste products from sublimation or decomposition decomposition

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Outline Outline

• Brief history of thermal analysis

Brief 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 data

Generating valid data

– – Calibration Calibration – – Sample preparation Sample preparation

• Interpreting data and Applications

Interpreting data and Applications

– – Real events Real events – – Artefacts Artefacts

• Recent advances

Recent advances

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Typical Features of a DSC Trace (Polymorphic System)

sulphapyridine

endothermic events melting sublimation solid-solid transitions desolvation chemical reactions exothermic events crystallization solid-solid transitions decomposition chemical reactions baseline shifts glass transition Exo

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Melting Processes by DSC

Pure substances

• linear melting curve
• melting

point defined by

• nset

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°C 28.87J/g

• 2.5
• 2.0
• 1.5
• 1.0
• 0.5

0.0 0.5

Heat Flow (W/g)

140 145 150 155 160 165 170 175

Temperature (°C)

Exo Up Universal V3.3B TA Instruments

extrapolated

• nset temperature

peak melting temperature

Exo

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Enthalpy of Fusion

157.81°C 156.50°C 28.87J/g

• 2.5
• 2.0
• 1.5
• 1.0
• 0.5

0.0 0.5

Heat Flow (W/g)

140 145 150 155 160 165 170 175

Temperature (°C)

Exo Up Universal V3.3B TA Instruments

Exo

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Enthalpy of Fusion by DSC

More difficult where multiple thermal events leading to stable melt

e.g. solid-solid transitions (A to B) before melt, or where melt / recrystallisation before melt Estimate from sum all areas

For a single (well-defined) melting endotherm area under peak minimal decomposition/sublimation readily measured for high melting polymorph can be measured for low melting polymorph Endo

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Purity by DSC

1-3 mg samples in hermetically- sealed pans are recommended Peak width a valuable measure

• f 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 Rate

many 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 /

• pening pan

hole, e.g. sublimation sensor contamination

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Ensuring correct interpretation of DSC Ensuring correct interpretation of DSC

• You can

You can’ ’t t

• Can minimise misinterpretation

Can minimise misinterpretation

• Essential to have valid data to interpret

Essential to have valid data to interpret

– – Calibration, reproducible data, appropriate sampling etc Calibration, reproducible data, appropriate sampling etc

• Kinetics / thermodynamics at elevated temps

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

• event would not happen at room temperature

event would not happen at room temperature

• DSC shows excipients interact at 120

DSC shows excipients interact at 120º ºC C

– – Does not necessarily show interaction at room temp Does 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 I Form II –––––– Form I –––––– Form II –––––– Variable Hydrate –––––– Dihydrate –––––– Acetic acid solvate Form III Form I Form II

• 2.5
• 2.0
• 1.5
• 1.0
• 0.5

0.0 0.5 1.0

Heat Flow (W/g)

50 100 150 200 250 300 350

Temperature (°C)

––––––– Form I ––––––– Form II ––––––– Variable Hydrate ––––––– Dihydrate ––––––– Acetic acid solvate Exo Up Form III Form I Form II

• 2.5
• 2.0
• 1.5
• 1.0
• 0.5

0.0 0.5 1.0

Heat Flow (W/g)

50 100 150 200 250 300 350

Temperature (°C)

––––––– Form I ––––––– Form II ––––––– Variable Hydrate ––––––– Dihydrate ––––––– Acetic acid solvate Exo Up Form III Form I Form II

Exo

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Effect of Phase Impurities

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

2046742 FILE# 022511DSC.1 2046742 FILE# 022458 DSC.1 Form II ?

• 5
• 4
• 3
• 2
• 1

Heat Flow (W/g)

80 130 180 230 280

Temperature (°C)

Exo Up Universal V3.3B TA Instruments

Lot A - pure Lot B - seeds

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

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Outline Outline

• Brief history of thermal analysis

Brief 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 data

Generating valid data

– – Calibration Calibration – – Sample preparation Sample preparation

• Interpreting data and Applications

Interpreting data and Applications

– – Real events Real events – – Artefacts Artefacts

• Recent advances

Recent advances

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Microcalorimetry Microcalorimetry

• High sensitivity DSC

High sensitivity DSC

• Solutions

Solutions

• Scan range typically

Scan range typically 0-

• 120

120 ° °C C

• Scanning rate of 0

Scanning rate of 0-

• 120

120 ° °C/hr C/hr

• Reverse scan rate 0

Reverse scan rate 0-

• 45

45 ° °C/hr C/hr (depending on efficiency (depending on efficiency

• f cooling tank)
• f cooling tank)
• Useful for looking at low

Useful for looking at low energy modifications energy modifications

• e.g. protein relaxation and

e.g. protein relaxation and refolding, polymer refolding, polymer characterisation characterisation

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

• f 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 transition melting

MDSC for Polymorph Characterization

• 0.50
• 0.45
• 0.40
• 0.35
• 0.30
• 0.25
• 0.20
• 0.15
• 0.10
• 0.05

0.00

Rev Heat Flow (W/g)

110 120 130 140 150 160 170 180

Temperature (°C)

• 0.8
• 0.6
• 0.4
• 0.2

0.0 0.2

Nonrev Heat Flow (W/g)

110 120 130 140 150 160 170 180

Temperature (°C)

Lot A Lot B Lot A Lot B reversing heat flow non-reversing heat flow

Kinetic (non-reversing heat flow) crystallization decomposition evaporation

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‘ ‘Hyper Hyper’ ’ DSC DSC

• Fast scanning DSC

Fast scanning DSC

• Only possible with power compensated

Only possible with power compensated

• Normal equipment

Normal equipment ≈ ≈ 100 100 º ºC/min C/min

• Specialised up to 500

Specialised up to 500 º ºC/min C/min

• Increased sensitivity, loss of resolution

Increased sensitivity, loss of resolution

• e.g. amorphous content in mainly crystalline sample

e.g. amorphous content in mainly crystalline sample

– change of specific heat at Tg Tg is linear relationship to the amorphous content – – Conventional DSC 10% amorphous limit of detection Conventional DSC 10% amorphous limit of detection – – Hyper DSC Hyper DSC <1% amorphous easily detected <1% amorphous easily detected Lappalainen Lappalainen, M., I. , M., I. Pitkanen Pitkanen, et al. (2006). , et al. (2006). International Journal of International Journal of Pharmaceutics Pharmaceutics 307 307(2): 150 (2): 150-

• 155.

155.

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Best Practices for Thermal Analysis

proper 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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Caution Caution… …

• It is a bulk tool

It 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 studies decomposition studies

• Instrument error in DSC typically

Instrument error in DSC typically ± ± 0.5 0.5 -

• 1

1º ºC C

• In Scanning modes, thermal events may be

In Scanning modes, thermal events may be “ “smeared smeared” ” by a thermal lag by a thermal lag – – Sample temperature not keeping up with Sample temperature not keeping up with instrument instrument – – Bigger effect at higher heating rates Bigger effect at higher heating rates – – Typically 1 Typically 1º ºC at 10 C at 10º ºC/min C/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 / interaction stability / interaction

• Don

Don’ ’t over t over-

• interpret data

interpret data

• Care when using thermal analysis in isolation

Care when using thermal analysis in isolation

• Artefacts / heating rate effects etc

Artefacts / heating rate effects etc

• Couple with other analytical tools

Couple with other analytical tools – – Heated X Heated X-

• ray, heated vibrational

ray, heated vibrational spectroscopy, hot stage microscope spectroscopy, hot stage microscope

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Acknowledgements Acknowledgements

Professor Adrian Williams, University of Reading Professor Adrian Williams, University of Reading Dr Ian Grimsey, University of Bradford Dr Ian Grimsey, University of Bradford Dr Peter Timmins, Bristol Myers Squibb Dr Peter Timmins, Bristol Myers Squibb Dr Wendy Dr Wendy Hulse Hulse, University of Bradford , University of Bradford Luciana Luciana DeMatos DeMatos, University of Sheffield , University of Sheffield

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Questions Questions

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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 from sinusoidal 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

• f temperature
• weight

calibration using known weights

• temperature

calibration based

• n

ferromagnetic transition

• f

Curie point standards (e.g., Ni)

• larger sample masses, lower

temperature gradients, and higher purge rates minimize undesirable buoyancy effects

Weight (%)

Time (min)

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Differential Thermal Analysis

Sample 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

temperatures

• instruments are highly sensitive
• flexibility in sample volume/form
• characteristic 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°C 100.0% 179.95°C 84.45%

• 1.8
• 0.8

0.2 1.2 2.2 3.2 4.2

Temperature Difference (µV/mg)

• 40

40 80 120

Weight (%)

20 70 120 170 220 270

Temperature (°C)

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