Bab 7 Analisa Termal

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Analisa Termal

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tentang analisa thermal

Transcript of Bab 7 Analisa Termal

  • Analisa Termal

  • Teknik Analisa TermalDifferential Thermal Analysis (DTA)

    Perbedaan suhu antara sampel dengan material standar yang inert, DT = 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 menjaga perbedaan suhu sampel dan standar sama dengan nol, dDq/dt.

    Thermogravimetric Analysis (TGA)

    Pengukuran dilakukan pada perubahan massa sampel akibat pemanasan. Sejumlah teknik pengukuran dimana sifat-sifat fisik diukur sebagai fungsi dari suhu, dimana sampel dikenakan proses pemanasan atau pendinginan tertentu.

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

    Sample/sample holder

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

    Pengaturan yang memungkinkan paremeter-parameter eksperimen dapat dikontrol.

    Komputer yang memungkinkan pengumpulan dan pemrosesan data. DTApower compensated DSCheat flux DSC

  • Differential Thermal Analysissamplepaninert gasvacuumreferencepanheatingcoilsample holder

    sample and reference cells (Al)

    sensors

    Pt/Rh atau chromel/alumel thermocouples Satu untuk sampel dan satu untuk referenceDihubungkan dengan pengontrol suhu diferensial

    furnace

    alumina block berisi sampel dan reference

    temperature controller

    Mengontrol program suhu dan atmosfer furnacealumina blockPt/Rh or chromel/alumelthermocouples

  • keuntungan:

    Instrumen dapat digunakan pada suhu yang sangat tinggi

    Instrumen sangat sensitif

    Volume dan bentuk crucible fleksibel

    Transisi atau suhu reaksi yang karakteristik dapat ditentukan dengan akurat,

    kelemahan:

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

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

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

    two basic types of DSC instruments: power compensation and heat-fluxDifferential Scanning Calorimetrypower compensation DSCheat flux DSC

  • Power Compensation DSCsample 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 temperature of the sample and reference at the program value

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

  • sample holder

    sample and reference are connected bya low-resistance heat flow pathAl 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 to differential thermal power, dDq/dt, which is supplied to the heaters to maintain the temperature of the sample and reference at the program valueHeat Flux DSC

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

    glass transitioncrystallizationmeltingdecompositionevaporationenthalpic relaxationcure

  • Analysis of Heat-Flow in Heat Flux DSCtemperature 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 referenceDTexp TS TR

  • DSC Calibrationbaseline

    evaluation of the thermal resistance of the sample and reference sensors

    measurements over the temperature range of interest

    2-step process

    the temperature difference of two empty crucibles is measured

    the thermal response is then acquired for a standard material, usually sapphire, on both the sample and reference platformsamplified DSC signal is automatically varied with temperature to maintain a constant calorimetric sensitivity with temperature

  • use of calibration standards of known heat capacity, such as sapphire, slow accurate heating rates (0.52.0 C/min), and similar sample and reference pan weightsDSC Calibrationtemperature

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

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

    heat flowcalibrants

    high purityaccurately known enthalpiesthermally stablelight stable (hn)nonhygroscopicunreactive (pan, atmosphere)metalsIn 156.6 C; 28.45 J/gSn 231.9 CAl 660.4 CinorganicsKNO3 128.7 CKClO4 299.4 Corganicspolystyrene 105 Cbenzoic acid 122.3 C; 147.3 J/ganthracene 216 C; 161.9 J/g

  • Thermogravimetric Analysis (TGA)thermobalance allows for monitoring sample weight as a function of temperature

    two most common instrument types

    reflection

    null

    weight calibration using calibrated 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 effectsTG curve of calcium oxalate

  • Typical Features of a DSC Trace for a Polymorphic Systemendothermic events

    meltingsublimationsolid-solid transitionsdesolvationchemical reactions

    exothermic events

    crystallizationsolid-solid transitionsdecompositionchemical reactions

    baseline shifts

    glass transition

  • Thermal Methods in the Study of Polymorphs and Solvatespolymorph screening/identification

    thermal stabilitymeltingcrystallizationsolid-state transformationsdesolvationglass transitionsublimationdecomposition

    heat flowheat of fusionheat of transitionheat capacity

    mixture analysischemical purityphysical purity (crystal forms, crystallinity)

    phase diagramseutectic formation (interactions with other molecules)

  • Definition of Transition Temperature

  • Melting Processes by DSCpure substances

    linear melting curve

    melting point defined by onset temperatureimpure substances

    concave melting curve

    melting characterized at peak maxima

    eutectic impurities may produce a second peakmelting with decomposition

    exothermic

    endothermiceutectic melt

  • Glass Transitionssecond-order transition characterized by change in heat capacity (no heat absorbed or evolved)

    transition from a disordered solid to a liquid

    appears as a step (endothermic direction) in the DSC curvea gradual volume or enthalpy change may occur, producing an endothermic peak superimposed on the glass transition

  • Enthalpy of Fusion

  • Burgers Rules for Polymorphic TransitionsenantiotropyHeat of Transition Ruleendo-/exothermic solid-solid transition

    Heat of Fusion Rulehigher melting form; lower DHf

    exothermic solid-solid transition

    higher melting form; higher DHfmonotropy

  • Enthalpy of Fusion by DSCsingle (well-defined) melting endotherm

    area under peakminimal decomposition/sublimationreadily measured for high melting polymorphcan be measured for low melting polymorph

    multiple thermal events leading to stable melt

    solid-solid transitions (A to B) from which the transition enthalpy (DHTR) can be measured*

    crystallization of stable form (B) from melt of (A)DHfA = DHfB - DHTR* assumes negligible heat capacity difference between polymorphs over temperatures of interestDHfA = area under all peaks from B to the stable melt

  • Purity by DSCeutectic impurities lower the melting point of a eutectic system

    purity determination by DSC based on Vant Hoff equation

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

    1-3 mg samples in hermetically-sealed pans are recommended

    polymorphism interferes with purity determination, especially when a transition occurs in the middle of the melting peakPlato, C.; Glasgow, Jr., A.R. Anal. Chem., 1969, 41(2), 330-336.

  • Effect of Heating Ratemany transitions (evaporation, crystallization, decomposition, etc.) are kinetic events

    they will shift to higher temperature when heated at a higher rate

    the total heat flow increases linearly with heating rate due to the heat capacity of the sample

    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., 15 K/min) should be usedDSC traces of a low melting polymorph collected at four different heating rates. (Burger, 1975)

  • Effect of Phase ImpuritiesLot 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 polymorphLot A - pureLot B - seedslots A and B of lower melting polymorph (identical by XRD) are different by DSC

  • Polymorph Characterization: Variable Melting Pointlots A and B of lower melting polymorph (identical by XRD) appear to have a variable melting pointLot ALot Balthough melting usually happens at a fixed temperature, solid-solid transition temperatures can vary greatly owing to the sluggishness of solid-state processes

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

    small reversing component discernable on close inspection of endothermic conversions occurring at the higher temperatures, i.e., near the melting pointPolymorph Characterization: Variable Melting Pointthe variable melting point was related to the large stability difference between the two polymorphs; the system was driven to undergo both melting and solid-state conversion to the higher melting form

  • Polymorph Stability from Melting and Eutectic Melting Datapolymorph stability predicted from pure melting data near the melting temperatures(G1-G2)(Te1) = DHme2(Te2-Te1)/(xe2Te2)

    (G1-G2)(Te2) = DHme1(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) = DHm2(Tm2-Tm1)/Tm2

    (G1-G2)(Tm2) = DHm1(Tm2-Tm1)/Tm1eutectic melting method developed to establish thermodynamic stability of polymorph pairs over larger temperature range

  • development of hyphenated techniques for simultaneous analysis

    TG-DTA

    TG-DSC

    TG-FTIR

    TG-MSHyphenated Techniquesthermal techniques alone are insufficient to prove the existence of polymorphs and solvates

    other techniques should be used, e.g., microscopy, diffraction, and spectroscopyevolved gas analysis(EGA)TG-DTA trace of sodium tartrate

  • Best Practices of Thermal Analysissmall sample size

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

    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 at the time scale of the measurement, melting has a moderately strong reversing componentdQ/dt = Cp . dT/dt + f(t,T)

    reversing signal heat flow resulting fromsinusoidal temperature modulation(heat capacity component)non-reversing signal(kinetic component)total heat flow resulting from average heating rate

  • Recognizing Artifacts

    **Power compensated DSC and heat flux DSC provide the same information but are fundamentally different.*A modern thermal analysis instrument is made up of a furnace for heating (or cooling) the sample at a controlled rate and a selective transducer (a thermocouple to measure heat flow (DSC or DTA)or a balance to monitor weight changes (TG)) to monitor changes in the substance. *Chromel-alumel system (150 500 C) is well suited for pharmaceutical materials.

    Thermocouples are joined so that the differential temperature between the sample and reference, and the actual sample temperature, can be monitored.

    Choice between DTA and DSC is less clear than in the past:results from early DTA instruments could not be converted to calorimetric valuesWhen the thermocouples are in thermal (but not physical) contact with the sample and reference materials, the area under the exotherms or endotherms can be related to the enthalpy change during the phase transition*endothermic processes will lower the sample temperature relative to that of reference, so the sample must be heated more in order to maintain equal T in both pans

    *Power-compensated DSC was introduced in the early 1960s.*Because DSC measures the difference in heat flow between a sample and reference, the baseline stabilizes faster if the difference in heat capacity between the sample and reference is kept small by adding weight (same material as pan) to the reference pan so that it is similar in total weight to the sample pan.

    Aluminum pans can be used in most experiments, unless the sample reacts with aluminum or the temperature is to exceed 600 C.

    Purity determination 1-3 mg; melting 5-10 mg; Tg or weak transitions up to 20 mg; highly endo/exothermic responses less than 5 mg*Ag heating block dissipates heat to the sample and reference via the constantan disc.*Typical reversing events are glass transitions and melting; non-reversing events are crystallization, evaporation, decomposition and solid-solid transitions.

    underlying heating rate: 0-100 C/minutemodulation period: 10-100 secondsmodulation temperature amplitude: +/- 0.01 10 C

    The net effect of imposing the complex heating profile is the same as if two experiments were run simultaneously one at the linear (average) heating rate and the other at a sinusoidal (instantaneous) rate.*Thermal resistances of the heat flux system changes with temperature. DSC instruments are therefore used in the calibrated mode, whereby the amplified DSC signal is automatically varied with temperature to maintain a constant calorimetric sensitivity with temperature.

    Temperature difference may develop between the samples (S and R) and the thermocouples, since these are not in direct physical contact with the samples.*organic compounds have been recommended as standards when studying organic material to minimize differences in thermal conductivity, heat capacity, and heat of fusion and may be used predominantly at temperatures below 300 K.Standard procedures can be obtained from the American Society for Testing of Materials (ASTM).

    *For pharmaceutical samples, temperatures up to 350 C and sample sizes of 5-20 mg are generally adequate.

    Reflection-type TGA (like Perkin-Elmer TGA7) depends on balance beam deflection about its fulcrum due to mass changes change in restoring force, which is proportional to the change in the sample mass, is monitored

    The sample hangs from the balance inside the furnace; the balance is thermally isolated from the furnace.**The melting peak temperature in DSC corresponds to the maximum melting rate.

    The simplest approximation to the baseline is a straight line connecting the star and finish of the transformation, which is valid for sharp DSC peaks, but becomes more difficult for broader DSC peaks. Using the extrapolated onset of melting as the melting point often accounts for the thermal lag.

    Impure samples, whose melting curves are concave in shape, are characterized by the peak melting temperatures.*structural relaxation can occur due to the restricted but finite mobility of the molecules below the glass transition.*The enthalpy of fusion, Hf, is obtained from the area of the endothermic transition.

    The area of the transition is affected by the selection of the baseline.

    The baseline is generally obtained by connecting the point at which the transition deviates from the baseline of the scan to where it rejoins the baseline after melting is completed.

    For some materials that undergo a significant change in heat capacity change on melting, other baseline approximations (such as a sigmoidal baseline) are used.*Monotropy transition is reversible; enantiotropy transition is irreversible.*Accurate measurement of heats of fusion require pure samples of polymorphs.*The broadness of the peak defines the purity of the crystalline compound undergoing melting, with the less pure and less perfect smaller crystals melting first followed by melting of the purer larger crystals.

    If there is no interaction between two compounds in the solid state, but their liquids are miscible, eutectic behavior will be observed. This is the basis of purity analysis by DSC.

    The first thing that has to be checked when validating a method for purity determination is whether the substance forms a eutectic system with the impurity.**Contrast the heating rate effects with phase contaminants same effect, different cause.

    ** Note: The width of the melting transition is significantly narrower than that of the solid-solid transition.

    As a result of its much larger free energy (in excess of the higher melting polymorph), the system was driven to undergo both melting and direct solid-state conversion from Form I to Form II. Which transition occurs is sample specific, and is likely related to crystal defects, particle size, chemical impurities, etc.**He improves the resolution of the DSC, but the DSC must be recalibrated for temperature and heat flow when using a helium flow due to its higher thermal conductivity.*This component is a function of absolute time and temperature and will shift to higher temperatures as the heating rate is increased.*Sample toppling over in pan causes abrupt change of the heat transfer between the sample and the pan.

    Shifting of Al pan, caused by different coefficients of expansion (of Al vs DSC sensor), and distortion of sample pan, due to sample vapor pressure, cause abrupt change in heat transfer between the pan and the DSC sensor.

    Mechanical shock of the measuring cell can cause the pans to jump around on the sensor.

    If the cell lid is poorly adjusted, cool air entry into the measuring cell leads to temperature fluctuations (noisy signal).