Thermal Methods in the Study of Polymorphs and Solvates Susan M. Reutzel-Edens, Ph.D. Research Advisor Lilly Research Laboratories Eli Lilly & Company Indianapolis, IN Presented at: “Diversity Amidst Similarity: A Multidisciplinary Approach to Polymorphs, Solvates and Phase Relationships” (The 35 th Crystallographic Course at the Ettore Majorana Centre) Erice, Sicily June 9-20, 2004

Thermal Analysis Techniques Differential Thermal Analysis (DTA) the temperature difference between a sample and an inert reference material,  T = T S - T R, is measured as both are subjected to identical heat treatments Differential Scanning Calorimetry (DSC) the sample and reference are maintained at the same temperature, even during a thermal event (in the sample) the energy required to maintain zero temperature differential between the sample and the reference, d  q/dt, is measured Thermogravimetric Analysis (TGA) the change in mass of a sample on heating is measured A group of techniques in which a physical property is measured as a function of temperature, while the sample is subjected to a predefined heating or cooling program.

Basic Principles of Thermal Analysis Modern instrumentation used for thermal analysis usually consists of four parts: 1)sample/sample holder 2)sensors to detect/measure a property of the sample and the temperature 3)an enclosure within which the experimental parameters may be controlled 4)a computer to control data collection and processing DTApower compensated DSCheat flux DSC

Differential Thermal Analysis sample pan inert gas vacuum reference pan heating coil sample holder sample and reference cells (Al) sensors Pt/Rh or chromel/alumel thermocouples one for the sample and one for the reference joined to differential temperature controller furnace alumina block containing sample and reference cells temperature controller controls for temperature program and furnace atmosphere alumina block Pt/Rh or chromel/alumel thermocouples

Differential Thermal Analysis advantages: instruments can be used at very high temperatures instruments are highly sensitive flexibility in crucible volume/form characteristic transition or reaction temperatures can be accurately determined disadvantages: uncertainty of heats of fusion, transition, or reaction estimations is 20-50% DTA

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-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 temperature of the sample and reference at the program value sample pan  T = 0 inert gas vacuum inert gas vacuum individual heaters controller PP reference pan thermocouple

sample holder sample and reference are connected by a 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 to differential thermal power, d  q/dt, which is supplied to the heaters to maintain the temperature of the sample and reference at the program value Heat Flux DSC sample pan inert gas vacuum heating coil reference pan thermocouples chromel wafer constantan chromel/alumel wires

Modulated DSC Heating Profile 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 transitioncrystallization meltingdecomposition evaporation enthalpic 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  T exp ≠ T S – T R temperature T furnace T RP TRTR TSTS T SP heating block TRTR TSTS reference sample TLTL thermocouple is not in physical contact with sample

DSC Calibration baseline 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 platforms amplified 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 (0.5–2.0 °C/min), and similar sample and reference pan weights DSC Calibration temperature 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 flow calibrants high purity accurately known enthalpies thermally stable light stable (h ) nonhygroscopic unreactive (pan, atmosphere) metals In °C; J/g Sn °C Al °C inorganics KNO °C KClO °C organics polystyrene 105 °C benzoic acid °C; J/g anthracene 216 °C; J/g

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 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 Al Ptalumina NiCuquartz

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 effects TG curve of calcium oxalate

Typical Features of a DSC Trace for a Polymorphic System sulphapyridine endothermic events melting sublimation solid-solid transitions desolvation chemical reactions exothermic events crystallization solid-solid transitions decomposition chemical reactions baseline shifts glass transition

Recognizing Artifacts mechanical shock of measuring cell sample 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

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 extrapolated onset temperature peak melting temperature

Melting Processes by DSC pure substances linear melting curve melting point defined by onset temperature impure substances concave melting curve melting characterized at peak maxima eutectic impurities may produce a second peak melting with decomposition exothermic endothermic eutectic melt

Glass Transitions second-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 curve a gradual volume or enthalpy change may occur, producing an endothermic peak superimposed on the glass transition

Enthalpy of Fusion

Burger’s Rules for Polymorphic Transitions enantiotropy endothermic Heat of Transition Rule endo-/exothermic solid-solid transition Heat of Fusion Rule higher melting form; lower  H f exothermic solid-solid transition higher melting form; higher  H f monotropy endothermic

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 (  H TR ) can be measured* crystallization of stable form (B) from melt of (A)  H f A =  H f B -  H TR * assumes negligible heat capacity difference between polymorphs over temperatures of interest  H f A = area under all peaks from B to the stable melt

Purity by DSC eutectic impurities lower the melting point of a eutectic system purity determination by DSC based on Van’t 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 peak T m = T o -. HoHo RTo2RTo2  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),

Effect of Heating Rate many 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., 1–5 K/min) should be used 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 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 Lot A Lot B although melting usually happens at a fixed temperature, solid-solid transition temperatures can vary greatly owing to the sluggishness of solid-state processes

Reversing and Non-Reversing Contributions to 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 component dQ/dt = C p. dT/dt + f(t,T) reversing signal heat flow resulting from sinusoidal temperature modulation (heat capacity component) non-reversing signal (kinetic component) total heat flow resulting from average heating rate

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 point Polymorph Characterization: Variable Melting Point Lot A Lot B Lot A Lot B reversing heat flownon-reversing heat flow the “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 Data polymorph stability predicted from pure melting data near the melting temperatures (G 1 -G 2 )(T e1 ) =  H me2 (T e2 -T e1 )/( x e2 T e2 ) (G 1 -G 2 )(T e2 ) =  H me1 (T e2 -T e1 )/( x e1 T e1 ) Yu, L. J. Am. Chem. Soc, 2000, 122, Yu, L. J. Pharm. Sci., 1995, 84(8), (G 1 -G 2 )(T m1 ) =  H m2 (T m2 -T m1 )/T m2 (G 1 -G 2 )(T m2 ) =  H m1 (T m2 -T m1 )/T m1 eutectic 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-MS “Hyphenated” Techniques thermal techniques alone are insufficient to prove the existence of polymorphs and solvates other techniques should be used, e.g., microscopy, diffraction, and spectroscopy 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-sensing device proper sample encapsulation starting temperature well below expected transition temperature slow scanning speeds proper instrument calibration use purge gas (N 2 or He) to remove corrosive off-gases avoid decomposition in the DSC