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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
temperature controller
• 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
temperature controller
• 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
TR
TS
TSP
heating block
TR TS
reference
sample
TL
thermocouple 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 (0.5–2.0 °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
157.81°C
156.50°C28.87J/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
157.81°C
156.50°C28.87J/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
Burger’s 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 = Hf
B - 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 onVan’t 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 1
f
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., 1–5 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 “variable”melting 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
Exo Up Universal V3.3B TA Instruments
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)
DSC010622b.1 483518 HCL (POLYMORPH 1)DSC010622d.1 483518 HCL
Exo Up Universal V3.3B TA Instruments
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)
DSC010622b.1 483518 HCL (POLYMORPH 1)DSC010622d.1 483518 HCL
Exo Up Universal V3.3B TA Instruments
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)
24.80°C100.0%
179.95°C84.45%
-1.8
-0.8
0.2
1.2
2.2
3.2
4.2
Te
mp
era
ture
Diffe
ren
ce
(µ
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
Exo Up Universal V3.3B TA Instruments
“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