Differential Scanning Differential Scanning Calorimetry Calorimetry “Cooking with Chemicals Cooking with Chemicals” ” “ Clare Rawlinson Clare Rawlinson School of Pharmacy School of Pharmacy University of Bradford University of Bradford EYP 2006 EYP 2006
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 � EYP 2006 EYP 2006
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 and and Laplace Laplace (1782 (1782- -1784): 1784): Lavoisier � oil was burned in a lamp ( oil was burned in a lamp ( Fig 9 ) held in Fig 9 ) held in � a bucket (Fig. 8) held in a wire mesh a bucket (Fig. 8) held in a wire mesh cage ( f ) cage ( f ) � surrounded by ice in spaces surrounded by ice in spaces b b and and a a of of � the double walled container a foot in the double walled container a foot in diameter diameter � lid ( lid ( F ) was topped with ice, as was a F ) 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 b covered the inner volume b EYP 2006 EYP 2006
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 Outer jacket prevented 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 º ice (334 J/gram ice at 0 ºC) C) Lavoisier converted the rate converted the rate Lavoisier of water formation to heat of 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) EYP 2006 EYP 2006
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 EYP 2006 EYP 2006
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 1 1 temperature programme – 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 1 1 change in sample mass – more commonly used technique than TGA – more commonly used technique than TGA – Two basic types of DSC instruments: heat-flux and power compensation 1 Haines, P. J. (2002) The Royal Society of Chemistry, Cambridge. EYP 2006 EYP 2006
Heat Flux DSC Sample holder : � sample and reference are connected by a low-resistance heat flow path � Aluminium, stainless, platinum sample pans heating coil Sensors: Sensors: � temperature sensors temperature sensors � sample reference � usually thermocouples usually thermocouples pan pan � Furnace: � one block for both sample and reference cells inert gas thermocouples vacuum Temperature controller : • temperature difference between the sample and reference is measured EYP 2006 EYP 2006
Power Compensated DSC Power Compensated DSC Sample holder : Sample holder : � Aluminium Aluminium, platinum, stainless steel pans , platinum, stainless steel pans � individual heaters Sensors: Sensors: � Pt resistance Pt resistance � sample reference thermocouples. thermocouples. pan pan � Separate sensors Separate sensors � and heaters for the and heaters for the sample and reference sample and reference inert gas inert gas vacuum vacuum thermocouple Δ T = 0 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 EYP 2006 EYP 2006
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 � EYP 2006 EYP 2006
DSC Calibration Baseline Calibration � evaluation of the thermal resistance of the sample and reference sensors � measurements over the temperature range of interest EYP 2006 EYP 2006
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 metals • Indium 156.6 °C; 28.45 J/g calibrants • Zinc 419.47 ° C, 108.17 J/g • high purity inorganics • accurately known enthalpies • KNO 3 128.7 °C • thermally stable • KClO 4 299.4 °C • light stable organics • not hygroscopic • polystyrene 105 °C • do not react (pan, atmosphere) • benzoic acid 122.3 °C; 147.3 J/g EYP 2006 EYP 2006
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 * small sample masses and low heating rates increase resolution, but at the expense of Pt alumina Al Ni Cu quartz sensitivity EYP 2006 EYP 2006
Purge Gases Purge Gases � Sample may react with air Sample may react with air - - oxidising or burning oxidising 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 EYP 2006 EYP 2006
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 � EYP 2006 EYP 2006
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