1 2004 Training Seminars DSC Understanding DSC Agenda • • • • • • What does a DSC measure? How does a DSC make that measurement? How is a Tzero™ DSC different? Tzero Calibration Tzero Results Advanced Tzero What Does a DSC Measure? A DSC measures the difference in heat flow rate (mW = mJ/sec) between a sample and inert reference as a function of time and temperature Endothermic Heat Flow 0.1 • Heat Flow Endothermic: heat flows into the sample as a result of either heat capacity (heating) or some endothermic process (glass transition, melting, evaporation, etc.) 0.0 Heat Flow (W/g) -0.1 -0.2 -0.3 -0.4 0 Exo Up 25 50 75 Temperature (°C) 100 125 150 Exothermic Heat Flow Heat Flow (W/g) 0.1 • Heat Flow Exothermic: heat flows out of the sample as a result of either heat capacity (cooling) or some exothermic process (crystallization, cure, oxidation, etc.) 0.0 -0.1 0 Exo Up 20 40 60 80 Temperature (°C) 100 120 140 160 Temperature • What temperature is being measured and displayed by the DSC? Sensor Temp: used by most DSCs. It is measured at the sample platform with a thermocouple, thermopile or PRT. Sample Platform Chromel Area Detector Reference Platform Constantan Body Thin Wall Tube Base Surface Constantan Wire Chromel Wire Chromel Wire Temperature • What temperature is being measured and displayed by the DSC? Pan Temp: calculated by TA Q1000 based on pan material and shape Uses weight of pan, resistance of pan, & thermoconductivity of purge gas What about sample temperature? The actual temperature of the sample is never measured by DSC Temperature • What other temperatures are not typically being displayed. Program Temp: the set-point temperature is usually not recorded. It is used to control furnace temperature Furnace Temp: usually not recorded. It creates the temperature environment of the sample and reference Understanding DSC Signals Heat Flow • Relative Heat Flow: measured by all DSCs except TA Q1000. The absolute value of the signal is not relevant, only absolute changes are used. • Absolute Heat Flow: used by Q1000. Dividing the signal by the measured heating rate converts the heat flow signal into a heat capacity signal DSC Heat Flow dH DSC heat flow signal dt Cp Sample Heat Capacity Sample Specific Heat x Sample Weight dH dT Cp f (T, t) dt dt dT Heating Rate dt f (T, t) Heat flow that is function of time at an absolute temperatu re (kinetic) Tzero Heat Flow Equation Heat Flow Sensor Model qr qs Cs Cr Tr Ts Besides the three temperatures (Ts, Tr, T0); what other values do we need to calculate Heat Flow? Rr Rs How do we calculate these? T0 1 T 1 dTs dT q T0 Cr Cs Cr Rr d d Rs Rr Measuring the C’s & R’s • Tzero™ Calibration calculates the C’s & R’s • Calibration is a misnomer, THIS IS NOT A • CALIBRATION, but rather a measurement of the Capacitance (C) and Resistance (R) of each DSC cell After determination of these values, they can be used in the Four Term Heat Flow Equation showed previously Measuring the C’s & R’s • Preformed using Tzero™ Calibration Wizard 1. Run Empty Cell 2. Run Sapphire on both Sample & Reference side Measuring the C’s & R’s Empty DSC constant heating rate Assume: qs qr 0 Heat balance equations give sensor time constants T0 s Cs Rs dTs d T0 T r Cr Rr dTs dT d d Measuring the C’s & R’s Repeat first experiment with sapphire disks on sample and reference (no pans) dTs dTr Assume: qs ms csapph qr mr csapph d d Use time constants to calculate heat capacities ms csapph Cs T0 1 dTs s d mr csapph Cr T0 T 1 dTs dT r d d Measuring the C’s & R’s Use time constants and heat capacities to calculate thermal resistances Rs s Cs Rr r Cr A few words about the Cs and Rs • The curves should be smooth and continuous, • • • without evidence of noise or artifacts Capacitance values should increase with temperature (with a decreasing slope) Resistance values should decrease with temperature (also with a decreasing slope) It is not unusual for there to be a difference between the two sides, although often they are very close to identical Good Tzero™ Calibration Run Bad Tzero™ Calibration Run Can see that it is bad during Tzero™ cal run Before Running Tzero™ Calibration • System should be dry • Dry the cell and the cooler heat exchanger using the cell/cooler conditioning template and the default conditions (2 hrs at 75°C) with the cooler off Preferably enable the secondary purge Do not exceed 75°C cell temperature with the cooler off, although the time can be extended indefinitely Stabilization before Calibration • System must be stable before Tzero™ • • Calibration Stabilization is achieved by cycling the baseline over the same temperature range and using the same heating rate as will be used for the subsequent calibration Typical systems will stabilize after 3-4 cycles, 8 cycles recommended to ensure that the system has stabilized Tzero™ Calibration Conditions • Normally, Heat Only calibration is all that is • • necessary Heating Rate should be 20°C/min Temperature Limits based on cooler type RCS; –90 to 400°C LNCS; –180 to 300°C • Use Diagnostic signals to improve troubleshooting capability Enable and Select Diagnostic Signals Check this box! Enable and Select Diagnostic Signals Select 1-8 for an RCS or all of them for an LNCS More on Diagnostic Signals later Example of Typical Results 50 50 40 40 This cell is very well balanced. It is acceptable and usual to have larger differences between sample and reference. 30 -200 -100 0 100 Temperature (°C) 200 0.05 0.04 0.04 0.03 0.03 0.02 0.02 0.01 300 Sample Capacitance (Joule/°C) 60 Reference Capacitance (Joule/°C) 60 Reference Resistance (°C/Watt) Sample Resistance (°C/Watt) Characteristics of the thermal resistances and heat capacities: 70 Both curves should be smooth, with no steps, spikes or inflection points. Thermal resistances should always have negative slope that gradually decreases. Heat capacities should always have positive slope that gradually decreases. Tzero™ vs Conventional Baseline 0.6 Conventional Baseline T zero Baseline Heat Flow (mW) 0.4 0.2 0.0 -0.2 -0.4 -100 0 100 200 Temperature (°C) 300 400 Indium with Q Series Heat Flow Signals Q1000 Q100 Q10 2 2004 Training Seminars DSC How to Get Better DSC Results Agenda • • • • Keeping your DSC cell clean Calibration Sample Preparation Thermal Method Agenda • • • • Keeping your DSC cell clean Calibration Sample Preparation Thermal Method Keeping the DSC Cell Clean • One of the first steps to ensuring good data is • to keep the DSC cell clean How do DSC cells get dirty? Decomposing samples during DSC runs Samples spilling out of the pan Transfer from bottom of pan to sensor How do we keep DSC cells clean? • DO NOT DECOMPOSE SAMPLES IN THE DSC CELL!!! • Run TGA to determine the decomposition • • • temperature Stay below that temperature! Make sure bottom of pans stay clean Use lids Use hermetic pans if necessary TGA Gives Decomposition Temperature Cleaning Cell • If the cell gets dirty Clean w/ brush Brush gently both sensors and cell if necessary Be careful with the Tzero™ thermocouple Blow out any remaining particles Brushing the Sample Sensor Calibration • Heat Flow (Cell Constant) (All DSC’s) • Temperature Calibration (All DSC’s) • Direct Cp (Q1000) Heat Flow Calibration (Cell Constant) • Heat Flow Calibration of Differential Scanning • • • Calorimeters – ASTM E-968 Enthalpy Calibration Performed using Calibration Wizard One Run Indium metal Sample Weight 1-5mg Pre-melt sample the first time you run it Heating rate of 10°C/min Dependent upon purge gas Cell Constant • The cell constant is calculated as the ratio of the theoretical heat of fusion of a standard material, to the measured heat of fusion Cell Constant Hf (Theoretical) / Hf (Measured) • Cell Constant should be 0.95-1.20 in N2 Calibration • Heat Flow (Cell Constant) (All DSC’s) • Temperature Calibration (All DSC’s) • Direct Cp (Q1000) Temperature Calibration • Temperature Calibration of Differential • • • Scanning Calorimeters – ASTM E-967 Performed using Calibration Wizard Indium Cell constant run also performs temperature calibration Can do up to 5 standards Pure metals typically used - In, Sn, Zn, Pb We’ve found that on the Q series DSC’s one temperature calibration point is all that is usually needed Calibration • Heat Flow (Cell Constant) (All DSC’s) • Temperature Calibration (All DSC’s) • Direct Cp (Q1000) Direct Cp Calibration (Q1000 Only) • Required to measure the absolute value of • • Heat Capacity (Cp) with a single run Reset previous calibration value to 1.0 Run standard material (sapphire) in standard mode, @ 10-20°C/min Set to 1.0 Direct Cp Calibration • The heat capacity calibration constant, K, is calculated as the ratio of the theoretical heat capacity of a standard material, to the measured heat capacity of the material Theo. Cp K Meas. Cp Use sapphire encapsulated in pan Setup to do Cp Constant (Direct) Direct Cp Calibration 1.2 Theoretical Values Heat Capacity (J/g/°C) 1.0 56.85°C 0.8380J/g/°C 106.85°C 0.9168J/g/°C 0.8 56.85°C 0.7932J/g/°C 0.6 106.85°C 0.8567J/g/°C 156.85°C 0.9775J/g/°C 156.85°C 0.9054J/g/°C 206.85°C 1.025J/g/°C 206.85°C 0.9467J/g/°C Measured Values 0.4 0.2 0.0 0 50 100 150 Temperature (°C) 200 250 Cp Constant (Direct) • Get data table from UA • Calculate Cp constant @ a single point or average values (see below) Temperature Heat Capacity °C J/g/°C 56.85 0.7932 106.85 0.8567 156.85 0.9054 206.85 0.9467 Theoretical kCp 0.8380 0.9168 0.9775 1.0250 1.0565 1.0702 1.0796 1.0827 Avg 1.0722 Cp Constant (Direct) • Type new value into calibration table Agenda • • • • Keeping your DSC cell clean Calibration Sample Preparation Thermal Method Sample Pans • Type of pan depends onSample form Volatilization Temperature range • Use lightest, flattest pan possible • Always use reference pan of the same type as sample pan Standard DSC Pans (Crimped) • Pan & lid weighs ~23mg, bottom of pan is flat • Used for solid non-volatile samples • Always use lid (see exceptions) • Lid improves thermal contact Keeps sample from moving Exceptions to using a lid Running oxidative experiment Running PCA experiment Standard DSC Pans (Crimped) • Crimped pans are available in: • Aluminum - up to 600°C Copper - up to 725°C (in N2) Gold - up to 725°C Standard Pans without lids Graphite - up to 725°C (in N2) Platinum - up to 725°C Hermetic Pans (Sealed) • Pan & Lid weigh ~55mg, bottom of pan is not • • • as flat as std pans Used for liquid samples and samples with volatiles Always use lid (same exceptions as before) After sealing pans, should form dome Hermetic Pans (Sealed) • Hermetic Pans are available in: Aluminum – <600°C; <3 atm (300 kPa gage) Alodined Aluminum - <600°C; <3 atm (300 kPa gage) • (For aqueous samples) Gold – <725°C, <6 atm (600 kPa gage) Specialized Sealed Pans High Volume - 100µL; <250°C; 600 psig(4.1 MPa) High Pressure - 35µL; <300°C; 1450 psig(10 MPa) • Note: 3 atm is approximately 44 psig It Does Matter What Pan you use Monohydrate Pharmaceutical sample Sample Shape • Keep sample thin • Cover as much as the bottom of pan as possible Sample Shape • Cut sample to make thin, don’t crush • If pellet, cut cross section Sample Shape • Cut sample to make thin, don’t crush • If pellet, cut cross section • If powder, spread evenly over the bottom of the pan Using Sample Press • When using crimped pans, don’t over crimp • Bottom of pan should remain flat after crimping Crimped Pans Hermetic Pans Good Not Sealed Sealed Bad • When using Hermetic pans, a little more • pressure is needed Hermetic pans are sealed by forming a cold wield on the Aluminum pans Sample Size • Larger samples will increase sensitivity • but……………. Larger samples will decrease resolution • Goal is to have heat flow of 0.1-10mW going through a transition Sample Size • Sample size depends on what you are measuring If running an extremely reactive sample (like an explosive) run very small samples (<1mg) Pure organic materials, pharmaceuticals (1-5mg) Polymers - ~10mg Composites – 15-20mg Effect of Sample Size on Indium Melt 0 Size: 1.2100 mg Heat Flow (mW) -5 Size: 0.4900 mg Size: 5.7010 mg -10 -15 -20 -25 150 Weight (mg) 0.49 1.21 5.70 152 154 156 158 Temperature (°C) Onset (°C) 156.41 156.45 156.61 160 Peak (°C) 156.56 156.76 157.17 162 Width (°C) 0.17 0.29 0.55 164 Agenda • • • • Keeping your DSC cell clean Calibration Sample Preparation Thermal Method Purge Gas • Purge gas should always be used during DSC • experiments Provides dry,inert atmosphere Ensures even heating Helps sweep away any off gases that might be released Nitrogen Most common Increases Sensitivity Typical flow rate of 50ml/min Purge Gas • Helium • Must be used with LNCS High Thermo-conductivity Increases Resolution Upper temp limited to 350°C Typical flow rate of 25ml/min Air or Oxygen Used to view oxidative effects Typical flow rate of 50ml/min Sample Temperature Range • Rule of Thumb Have 2-3 minutes of baseline before and after transitions of interest - if possible DO NOT DECOMPOSE SAMPLES IN DSC CELL Temperature range can affect choice of pans Just because the instrument has a temperature range of –90°C to 550°C (with RCS) doesn’t mean you need to heat every sample to 550°! Start-up Hook 12 9.56mg PET @ 10°C/min 10 Do not attempt to interpret transitions before Heating rate has stabilized Heat Flow (W/g) 8 6 -0.15 4 2 -0.25 0 -5 Exo Up 5 15 Temperature (°C) 25 35 Deriv. Temperature (°C/min) -0.05 Heating Rate • Faster heating rates increase sensitivity • but……………. Faster heating rates decrease resolution • Good starting point is 10°C/min Effect of Heating Rate PMMA 10.04mg Thermal History • The thermal history of a sample can and will • affect the results The cooling rate that the sample undergoes can affect : Crystallinity of semi-crystalline materials Enthalpic recovery at the glass transition • Run Heat-Cool Heat experiments to see effect of & eliminate thermal history Heat at 10°C/min Cool at 10°C/min Heat at 10°C/min Heat-Cool-Heat of PET 1.5 1.0 Cool Heat Flow (W/g) 0.5 Second Heat 0.0 First Heat -0.5 -1.0 -1.5 20 60 100 140 180 Temperature (°C) 220 260 6 2004 Training Seminars DSC Calculating % Crystallinity from the Latent Heat of Fusion Calculating % Crystallinity Background • DSC or MDSC® measure the energy required (latent heat of fusion) to convert crystalline structure to amorphous structure at that temperature. • The heat of fusion is temperature dependent; i.e., it takes more heat to melt crystalline structure at higher temperatures. • The increase in the heat of fusion with temperature is due to the difference in the heat capacity curves for amorphous and crystalline material. • In order to correctly calculate % crystallinity from the heat of fusion requires a knowledge of how the heat of fusion changes with temperature. Calculating % Crystallinity (cont.) 1- If 100% crystalline and amorphous material is available, create Cp and enthalpy plots from measured heat flow Enthalpy plots are obtained by measuring the absolute integral of a Cp plot over some temperature range All enthalpy plots are normalized to the same value (J/g) at a convenient temperature above the melting point where all samples should theoretically be the same because all have the same liquid structure (mobile amorphous) See Figures 1-3 Enthalpy Plots Are Integrals of Heat Capacity Plots Integrals of 100% Crystalline and 100% Amorphous Heat Capacity Curves Can Be Used to Create an Enthalpy Plot Figure 1 Drug 3.75mg MDSC® .159/60/1 Figure 2 Effect of the Temperature-Dependence of the Heat of Fusion on Crystallization and Melting Peak Areas for a Drug The Enthalpy Plot Can Be Used to Calculate % Crystallinity Illustrating the Temperature Dependence of the Heat of Fusion on the Monohydrate Form of the Drug Figure 3 Calculating % Crystallinity (cont.) 2. For polymers, enthalpy values for 100% crystalline and 3. 100% amorphous structure are available from ATHAS Databank as a function of temperature See ATHAS Figures Calculate % Crystallinity by dividing measured heat of fusion by the difference (*) in enthalpy for 100% crystalline and 100% amorphous structure at that temperature (Fig. 4) % Crystallinity = 37/113* 33b % @ 160ºC *113 J/g = Heat of Fusion = (88323a – 66589a) J/mole 192a g/mole a = values obtained from ATHAS Databank note b; use of 140J/g for H = 26% Crystallinity ATHAS – Page 1 ATHAS - Page 2 These are the Major Polymer Categories for ATHAS ATHAS – Page 3 Each Polymer Category has a Sub-listing ATHAS – Page 4 ATHAS – Page 5 ---------------------- Calculate g/mole from molecular structure which equals 192 g/mole for PET Convert ºK to ºC Specific Heat Capacity Absolute Enthalpy ATHAS – Page 6 ---------------------- Figure 4; % Crystallinity of PET @160 °C Use of ATHAS Databank to Calculate % Crystallinity on 12.64mg Sample of Quench Cooled PET after Cold Crystallization 20°C/min
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