Thermal analysis

Thermal analyses compromise a family of techniques whereby a physical property (i.e.

heat flow, weight loss) is measured as a function of temperature or time. It is a general term covering a group of related techniques whereby the dependence of the parameters of any physical property of a substance on temperature is measured. The physical parameter is determined as a dynamic function of temperature. Measurements made at fixed or isothermal temperatures are normally not included. Using the definition, the principal thermal analysis techniques are thermogravimetry and differential thermal analysis (Sichina, 2001).

The development and manufacturing of drugs requires that close attention to be paid to purity, quality, stability and safety in order to assure that the drug performs as intended.

Pharmaceuticals or organic compounds have a propensity to exist in different structural or morphological forms and this gives rise to concerns over processing, long-term stability, aging and biodelivery. Due to the complexity of the formulation of pharmaceutical material, it becomes important to have a complete understanding of the properties of pharmaceutical materials. One of the best analytical techniques for characterisation of pharmaceutical material is thermal analysis. Two most widely used thermal analytical techniques for the characterisation of pharmaceuticals are differential scanning calorimetry and thermogravimetric analysis (Sichina, 2001).

4.2.1 Thermogravimetric analysis and derivative thermogravimetry 4.2.1.1 Introduction

The thermal analysis technique of thermogravimetry (TG) is one where the change in sample mass (loss or gain) in a controlled atmosphere, is determined as a function of temperature or time (Skoog et al., 2004). Derivative thermogravimetry (DTG) is a method of expressing the results of TG by giving the first derivative curve as a function of temperature or time (Dodd and Tonge, 1987). This provides quantitative information on weight change processes, and enables the stereochemistry of a reaction to be followed directly, e.g., reactant (solid) → product (solid) + gas (Leung and Grant, 1997;

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Dodd and Tonge, 1987). It is also easier to compare the shape or overlap of DTG curves with other differential measurements such as DSC. The use of DTG improves the resolution of complex or overlapping TG curves, thus providing additional information about decomposition or mass loss phenomena of compounds (Sichina, 2001).

In Thermogravimetric analysis (TGA) a sample is placed into a tared TGA sample pan which is attached to a sensitive microbalance assembly. The sample holder portion of the TGA balance is subsequently placed into a high temperature furnace. The balance assembly measures the initial sample weight at room temperature and then continuously monitors changes in sample weight as heat is applied to the sample. TGA tests may be run in a heating mode at a controlled heating rate, or isothermally.

Dynamic TG, in which the sample is heated in an environment whose temperature, is changing in a predetermined manner, preferably linear, is a common mode of TG (Brown, 1998; Wendlandt, 1986).

The resulting mass change versus temperature curve, also known as a thermogram, provide information concerning the thermal stability and composition of the initial sample, the thermal stability and composition of any intermediate compounds that may be formed, and the composition of the residue, if any ( Wendlandt, 1986). Typical weight loss profiles could therefore be analysed to determine the amount or percent of weight loss at any given temperature, the amount or percent of non-combusted residue at some final temperatures, and the temperatures of various sample degradation processes (Brown, 1998).

4.2.2 Differential scanning calorimetry 4.2.2.1 Introduction

DSC is a thermal analysis technique which has been used for more than two decades to measure the temperatures and heat flows associated with transitions in materials as a function of time and temperature. Such measurements provide quantitative and

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qualitative information about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity (TA211, 2002).

DSC is one of the most widely used thermal analysis techniques for the characterisation of pharmaceutical solids. Thermal events such as melting, recrystallisation, decomposition, and glass transitions can be measured with it. Additionally, quantitative mixture analysis (e.g., to establish the presence of different polymorphs) can be performed utilising DSC. The use of modulated DSC expands the capabilities of DSC and allows one to measure heat capacities and characterise reversible and non-reversible thermal transitions (Sichina, 2001).

DSC records the energy necessary to establish a zero temperature difference between a substance and a reference material against either time or temperature, as the two specimens are subject to identical temperature regimes in an environment heated or cooled at a controlled rate (Pope and Judd, 1977).

When a thermal transition occurs in a sample, thermal energy is added to either the sample or the reference containers in order to maintain both the sample and reference at the same temperature. The balancing of energy yields a calorimetric measurement of the transition energy, because the energy transferred is exactly equivalent to the energy absorbed or evolved in the transition (Dodd and Tonge, 1987).

Thermal transition events in the sample thus appear as deviations from the DSC baseline, in either a endothermic or exothermic direction, depending upon whether more or less energy has to be supplied to the sample relative to the reference material (Brown, 1998).

4.2.3 Methodology

The TGA and DSC thermograms for cyclo(Phe-4Cl-Pro) and cyclo(D-Phe-4Cl-Pro) were obtained by heating the samples in a TA Instruments Q600 SDT (simultaneous

DSC-60

TGA) V8.3 (TA Instruments, USA). The TA Instruments Q600 SDT was calibrated for temperature using the melting points of indium and tin. Cyclo(Phe-4Cl-Pro) (2.710 mg) and cyclo(D-Phe-4Cl-Pro) (2.472 mg) were heated in open aluminium crucibles at a heating rate of 5 °C per minute, over a temperature range of 50 to 400 °C, with nitrogen as the purging gas (flow rate of 50 ml per minute).

4.2.4 Results and discussion

In all TGA thermograms the region x (as shown in the thermograms below) indicates a loss of weight. This could be free water, waters of hydration, or loss of solvent. This is followed by a weight loss portion (region y), indicating a melt. The inflection (region Z) indicates a change in the rate of decomposition.

In the TGA thermogram (Figure 4.5) the plateau of constant weight (region x) indicated free water or waters of hydration, coming of above 100 ˚C. A small decrease in the TGA curve is seen at 100 ˚C. The corresponding DSC peak relates to this enthalpy of hydration. The onset of the narrow endotherm starts at 172.71 ˚C, corresponding to the melting point of cyclo(Phe-4Cl-Pro) . The dipeptide started to decompose soon after the melting point.

A loss of mass in the TGA thermogram (Figure 4.6) at around 50 ˚C is observed. This can be associated to the presence of acetone, an indication that the compound was not dried properly. The onset point of the narrow endotherm starts at 174.06 ˚C, corresponding to the melting point of cyclo(D-Phe-4Cl-Pro) . From the spectrum it is very clear that the dipeptide started to decompose soon after the melting point.

61 Figure 4.5 Simultaneous DSC-TGA thermogram of cyclo(Phe-4Cl-Pro)

177.47°C

62 Figure 4.6 Simultaneous DSC-TGA thermogram of cyclo(D-Phe-4Cl-Pro)