Bab 7 Analisa Termal 2

Analisa Termal

Teknik Analisa Termal

Differential Thermal Analysis (DTA)

Perbedaan suhu antara sampel dengan material standar yang inert, T = TS - TR,diukur saat keduanya diberi perlakuan panas tertentu.

Differential Scanning Calorimetry (DSC)

Sampel dan standar dijaga pada suhu yang sama, bahkan selama terjadi perubahan-perubahan termal tertentu pada sampel.

Variabel yang diukur adalah besarnya energi yang diperlukan untuk menjagaperbedaan suhu sampel dan standar sama dengan nol, d q/dt.

Thermogravimetric Analysis (TGA)

Pengukuran dilakukan pada perubahan massa sampel akibat pemanasan.

Sejumlah teknik pengukuran dimana sifat-sifat fisik diukur sebagai fungsi darisuhu, dimana sampel dikenakan proses pemanasan atau pendinginan tertentu.

Prinsip-prinsip dasar analisa termalInstrumentasi modern yang digunakan pada analisa termal biasanya terdiri dari empat bagian:

1)Sample/sample holder

2)Sensor untuk mendeteksi/mengukur sifat-sifat tertentu sampel dan suhu.

3)Pengaturan yang memungkinkan paremeter-parameter eksperimen dapat dikontrol.

4)Komputer yang memungkinkan pengumpulan dan pemrosesan data.

DTA power compensated DSC heat flux DSC

Differential Thermal Analysis

samplepan

inert gasvacuum

referencepan

heatingcoil

sample holder

sample and reference cells (Al)

sensors

Pt/Rh atau chromel/alumel thermocouples Satu untuk sampel dan satu untuk reference Dihubungkan dengan pengontrol suhu

diferensial

furnace

alumina block berisi sampel dan reference

temperature controller

Mengontrol program suhu dan atmosfer furnace

alumina block

Pt/Rh or chromel/alumelthermocouples

keuntungan:

Instrumen dapat digunakan pada suhu yangsangat tinggi

Instrumen sangat sensitif

Volume dan bentuk crucible fleksibel

Transisi atau suhu reaksi yang karakteristikdapat ditentukan dengan akurat,

kelemahan:

Ketidakpastian estimasi panas bagi reaksi,transisi dan fusi sekitar 20-50%

DTA

Differential Thermal Analysis

DSC differs fundamentally from DTA in that the sample and reference are bothmaintained at the temperature predetermined by the program.

during a thermal event in the sample, the system will transfer heat to or from thesample pan to maintain the same temperature in reference and sample pans

two basic types of DSC instruments: power compensation and heat-flux

Differential Scanning Calorimetry

power compensation DSC heat flux DSC

Power Compensation DSC

sample holder

Al or Pt pans

sensors

Pt resistance thermocouples separate sensors and heaters for the sample and reference

furnace

separate blocks for sample and reference cells


differential thermal power is supplied to the heaters to maintain the temperatureof the sample and reference at the program value

samplepan

T = 0

inert gasvacuum

inert gasvacuum

individualheaters

controller P

referencepan

thermocouple

Sample Preparation

accurately-weigh samples (~3-20 mg)

small sample pans (0.1 mL) of inert or treated metals (Al, Pt, Ni, etc.)

several pan configurations, e.g., open , pinhole, or hermetically-sealed pans

the same material and configuration should be used for the sample and thereference

material should completely cover the bottom of the pan to ensure goodthermal contact

avoid overfilling the pan to minimize thermal lag from the bulk of thematerial to the sensor

* small sample masses andlow heating rates increaseresolution, but at theexpense of sensitivity

Al Pt alumina Ni Cu quartz

sample holder

sample and reference are connected bya low-resistance heat flow path

Al or Pt pans placed on constantan disc

sensors

chromel-constantan area thermocouples (differential heat flow) chromel-alumel thermocouples (sample temperature)

furnace

one block for both sample and reference cells


the temperature difference between the sample and reference is converted todifferential thermal power, d q/dt, which is supplied to the heaters to maintain thetemperature of the sample and reference at the program value

Heat Flux DSC

samplepan

inert gasvacuum

heatingcoil

referencepan

thermocouples

chromel wafer

constantan

chromel/alumelwires

Modulated DSC Heating Profile

Modulated DSC (MDSC)

introduced in 1993; heat flux design

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

total heat flow signal contains all ofthe thermal transitions of standardDSC

Fourier Transformation analysis is usedto separate the total heat flow into itstwo components:

heat capacity (reversing heat flow) kinetic (non-reversing heat flow)

glass transition crystallizationmelting decomposition

evaporationenthalpic relaxation

cure

Analysis of Heat-Flow in Heat Flux DSC

temperature difference may be deduced by considering the heat flow paths in the DSC system

thermal resistances of a heat-flux system change with temperature

the measured temperature difference is not equal to the difference in temperature between the sample and the reference

Texp TS TR

tem

pera

ture

Tfurnace

TRP

TRTS

TSP

heating block

TR TS

reference

sample

TLthermocouple is not in physical contact with sample

DSC Calibration

baseline

evaluation of the thermal resistance of thesample and reference sensors

measurements over the temperature rangeof interest

2-step process

the temperature difference of twoempty crucibles is measured

the thermal response is then acquiredfor a standard material, usuallysapphire, on both the sample andreference platforms

amplified DSC signal is automatically varied with temperature to maintain a constantcalorimetric sensitivity with temperature

use of calibration standards of known heat capacity, such as sapphire, slow accurateheating rates (0.52.0 C/min), and similar sample and reference pan weights

DSC Calibrationtemperature

goal is to match the melting onset temperatures indicated by the furnacethermocouple readouts to the known melting points of standards analyzed by DSC

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

heat flow

calibrants

high purity accurately known enthalpies thermally stable light stable (h ) nonhygroscopic unreactive (pan, atmosphere)

metals In 156.6 C; 28.45 J/g Sn 231.9 C Al 660.4 Cinorganics KNO3 128.7 C KClO4 299.4 Corganics polystyrene 105 C benzoic acid 122.3 C; 147.3 J/g anthracene 216 C; 161.9 J/g

Thermogravimetric Analysis (TGA)

thermobalance allows for monitoringsample weight as a function oftemperature

two most common instrument types

reflection

null

weight calibration using calibratedweights

temperature calibration based onferromagnetic transition of Curie pointstandards (e.g., Ni)

larger sample masses, lower temperaturegradients, and higher purge ratesminimize undesirable buoyancy effects

TG curve of calcium oxalate

12.15%

19.32%

29.99%

20

40

60

80

100

120

We

igh

t (%

)

0 20 40 60 80 100 120 140 160

Time (min)

Sample: Calcium OxalateSize: 7.9730 mg TGA

File: Y:\Data\TGA\Calcium oxalate\032304.001Operator: SLTRun Date: 23-Mar-04 14:57Instrument: 2950 TGA HR V5.4A

Universal V3.7A TA Instruments

Typical Features of a DSC Trace for a Polymorphic System

sulphapyridine

endothermic events

meltingsublimation

solid-solid transitionsdesolvation

chemical reactions

exothermic events

crystallizationsolid-solid transitions

decompositionchemical reactions

baseline shifts

glass transition

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

He

at

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

He

at

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

Thermal Methods in the Study of Polymorphs and Solvates

polymorph screening/identification

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

heat flow heat of fusion heat of transition heat capacity

mixture analysis chemical purity physical purity (crystal forms, crystallinity)

phase diagrams eutectic formation (interactions with other molecules)

Definition of Transition Temperature

157.81C

156.50C28.87J/g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

He

at

Flo

w (

W/g

)

140 145 150 155 160 165 170 175

Temperature (C)

Sample: INDIUM CRIMPED PAN CHECKSize: 7.6300 mgMethod: indiumComment: P/N 56S-107

DSCFile: C:...\10C per min crimped\DSC010920A.3Operator: Ron VansickleRun Date: 20-Sep-01 09:13Instrument: 2920 MDSC V2.6A

Exo Up Universal V3.3B TA Instruments

extrapolatedonset temperature

peak meltingtemperature

Melting Processes by DSC

pure substances

linear melting curve

melting point definedby onset temperature

impure substances

concave melting curve

melting characterizedat peak maxima

eutectic impuritiesmay produce a secondpeakmelting with decomposition

exothermic

endothermic

eutectic melt

Glass Transitions

second-order transition characterized bychange in heat capacity (no heat absorbedor evolved)

transition from a disordered solid to aliquid

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

a gradual volume or enthalpy change may occur, producing an endothermic peaksuperimposed on the glass transition

Enthalpy of Fusion

157.81C

156.50C28.87J/g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

He

at

Flo

w (

W/g

)

140 145 150 155 160 165 170 175

Temperature (C)

Sample: INDIUM CRIMPED PAN CHECKSize: 7.6300 mgMethod: indiumComment: P/N 56S-107

DSCFile: C:...\10C per min crimped\DSC010920A.3Operator: Ron VansickleRun Date: 20-Sep-01 09:13Instrument: 2920 MDSC V2.6A


Burgers Rules for Polymorphic Transitions

enantiotropy

end

otherm

ic

Heat of Transition Rule endo-/exothermic solid-solid transition

Heat of Fusion Rule higher melting form; lower Hf

exothermic solid-solid transition

higher melting form; higher Hf

monotropy

end

otherm

ic

Enthalpy of Fusion by DSC

single (well-defined) melting endotherm

area under peak minimal decomposition/sublimation readily measured for high melting polymorph can be measured for low melting polymorph

multiple thermal events leading to stable melt

solid-solid transitions (A to B) from which the transition enthalpy ( HTR) can bemeasured*

crystallization of stable form (B) from melt of (A)

HfA = HfB - HTR

* assumes negligible heat capacity difference between polymorphs over temperatures of interest

HfA = area under all peaks from B to the stable melt

Purity by DSC

eutectic impurities lower the meltingpoint of a eutectic system

purity determination by DSC based onVant Hoff equation

applies to dilute solutions, i.e., nearlypure substances (purity 98%)

1-3 mg samples in hermetically-sealedpans are recommended

polymorphism interferes with puritydetermination, especially when atransition occurs in the middle of themelting peak

Tm = To -.

Ho

RTo2 1f

melting endotherms as a function of purity.

benzoic acid

97%

99%

99.9%

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

Effect of Heating Rate

many transitions (evaporation, crystallization,decomposition, etc.) are kinetic events

they will shift to higher temperature whenheated at a higher rate

the total heat flow increases linearly withheating rate due to the heat capacity of thesample

increasing the scanning rate increasessensitivity, while decreasing the scanningrate increases resolution

to obtain thermal event temperatures closeto the true thermodynamic value, slowscanning rates (e.g., 15 K/min) should beused DSC traces of a low melting polymorph collected

at four different heating rates. (Burger, 1975)

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

2046742FILE# 022511DSC.1

2046742FILE# 022458 DSC.1 Form II ?

-5

-4

-3

-2

-1

0

He

at

Flo

w (

W/g

)

80 130 180 230 280

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

Lot A - pure

Lot B - seeds

lots A and B of lower melting polymorph (identical by XRD) are different by DSC

Polymorph Characterization: Variable Melting Point

lots A and B of lower melting polymorph (identical by XRD) appear to have a variablemelting point

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

0.1

He

at

Flo

w (

W/g

)

110 120 130 140 150 160 170 180

Temperature (C)

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


Lot A

Lot B

although melting usually happens at a fixed temperature, solid-solid transitiontemperatures can vary greatly owing to the sluggishness of solid-state processes

the low temperature endotherm was predominantly non-reversing, suggestive of asolid-solid transition

small reversing component discernable on close inspection of endothermic conversionsoccurring at the higher temperatures, i.e., near the melting point

Polymorph Characterization: Variable Melting Point

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

Re

v H

ea

t F

low

(W

/g)

110 120 130 140 150 160 170 180

Temperature (C)



Non-reversing (heat flow component)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

No

nre

v H

ea

t F

low

(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

the variable melting point was related to the large stability difference between thetwo polymorphs; the system was driven to undergo both melting and solid-stateconversion to the higher melting form

T1

x0 1

TmA

TmB

xe

Te

x0 1

Tm1

xe1

Te1

Tm2

xe2

Te2

TmRC

A

B RC

P1

P2

(a) (b)

Polymorph Stability from Melting and Eutectic Melting Data

40 60 80 100 120

DS

C S

ign

al

+thymol +azobenzene+benzil

+acetanilidepure forms

YYON

YY

ON

ONY

ONON

meltingeutectic melting

T, oC

-0.4

-0.2

0

0.2

0.4

sd

f

GON-GY, kJ/mole

TtON

Y

polymorph stability predicted from pure melting data near the melting temperatures

(G1-G2)(Te1) = Hme2(Te2-Te1)/(xe2Te2)

(G1-G2)(Te2) = Hme1(Te2-Te1)/(xe1Te1)

Yu, L. J. Am. Chem. Soc, 2000, 122, 585-591.

Yu, L. J. Pharm. Sci., 1995, 84(8), 966-974.

(G1-G2)(Tm1) = Hm2(Tm2-Tm1)/Tm2

(G1-G2)(Tm2) = Hm1(Tm2-Tm1)/Tm1

eutectic melting method developedto establish thermodynamic stabilityof polymorph pairs over largertemperature range

development of hyphenated techniques for simultaneous analysis

TG-DTA

TG-DSC

TG-FTIR

TG-MS

15.55%(0.9513mg)

24.80C100.0%

179.95C84.45%

-1.8

-0.8

0.2

1.2

2.2

3.2

4.2

Te

mp

era

ture

Diffe

ren

ce

(

V/m

g)

-40

0

40

80

120

We

igh

t (%

)

20 70 120 170 220 270

Temperature (C)

Sample: SODIUM TARTRATE (ALDRICH)Size: 6.1176 mgMethod: 25C TO 300Comment: LOT# 22411A0

TGA-DTAFile: C:\TA\Data\Sdtcal\2004\TGA040105A.5Operator: Ron VansickleRun Date: 6-Jan-04 12:09Instrument: 2960 SDT V3.0F


Hyphenated Techniques

thermal techniques alone are insufficient to prove the existence ofpolymorphs and solvates

other techniques should be used, e.g., microscopy, diffraction, andspectroscopy

evolved gas analysis(EGA)

TG-DTA trace of sodium tartrate

Best Practices of Thermal Analysis

small sample size

good thermal contact between the sample and the temperature-sensingdevice

proper sample encapsulation

starting temperature well below expected transition temperature

slow scanning speeds

proper instrument calibration

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

avoid decomposition in the DSC

Reversing and Non-Reversing Contributionsto Total DSC Heat Flow

* whereas solid-solid transitions are generally too sluggish to be reversing atthe time scale of the measurement, melting has a moderately strongreversing component

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

reversing signalheat flow resulting from

sinusoidal temperature modulation(heat capacity component)

non-reversing signal(kinetic component)

total heat flow resulting from

average heating rate

Recognizing Artifacts

mechanical shock of

measuring cellsample topples

over in pan

sample pan distortion shifting

of Al pan

cool air entry into cell

electrical effects, power spikes, etc.

RT changes

burst of pan lid

intermittant closing of hole

in pan lid

sensor contamination

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Bab 7 Analisa Termal 2