Isothermal microcalorimeter

Table 1 Instrument specifications of selected currently available isothermal microcalorimeters... 

Recently there has been considerable interest on the subject of chemical test reactions for isothermal microcalorimeters. Chemical test reactions allow a user to check if an instrument is functioning correctly because they reflect more accurately the processes under study in a real experiment. Indeed, the ideal case would be to have a universally accepted chemical test reaction for each type of experiment (perfusion, titration, etc.) one may wish to investigate. Examples of systems that have been proposed as chemical test reactions include 18-crown-6/barium sulfate (2) for titration calorimetry... 

Calorimetric forms of the Ng equation were used by Willson et al. (7) to analyse the solid-state degradation of L-ascorbic acid. Known amounts (0.5 g) of dry L-ascorbic acid were placed in ampoules along with known quantities of water and the heat changes in the samples were recorded using an isothermal microcalorimeter. The power-time data obtained were analyzed using the calorimetric form of the Ng equation and the parameters obtained are shown in Table 7. It was shown that, at low added quantities of added water, the reaction could satisfactorily be described by solid-state kinetics, but at higher added quantities of water (more than 500 pL) the reaction was best described by solution phase kinetics. 

Willson RJ, Beezer AE, Hills AK, Mitchell JC. The imidazole catalysed hydrolysis of triacetin a medium term chemical calibrant for isothermal microcalorimeters. Thermochimica Acta 1999 325 125-132. 

Willson RJ, Beezer AE, Mitchell JC. A kinetic study of the oxidation of L-ascor-bic acid (vitamin C) in solution using an isothermal microcalorimeter. Thermochimica Acta 1995 264 27-40. 

Table 7.1 [39] shows the differential heats of ion exchange, QA [kJ/mol], measured by means of an LKB 2277 heat-flow isothermal microcalorimeter for the ion-exchange reaction... 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

The isothermal microcalorimeter can therefore yield two types of data heat flow (a kinetic term) and the time-independent reaction enthalpy change (a thermodynamic term). It is possible, in principle, then to derive thermodynamic and kinetic information from the raw calorimetric data. 

FIGURE 8.1 Schematic representation of a heat conduction isothermal microcalorimeter. (Reproduced from Thermometric Ltd. With permission.)... 

FIGURE 8.2 Schematic representation of a first-order and zero-order process as detected by an isothermal microcalorimeter. 

In the discussion that follows it will be shown that isothermal microcalorimeters can be used to detect many different types of processes. The combination of high sensitivity with the ability to detect almost any process makes the instrument a valuable tool. The potential problem, however, is the lack of specificity in many experiments. Consequently, it is quite possible to measure a process that is not the one that was the intended subject of the study. Even in the best experiments, it is extremely likely that one will measure more than just the one process that is the subject of the study. This Catch-22 aspect of microcalorimetry must always be at the forefront of the researcher s mind, i.e., the ability to measure all things is an advantage, but the disadvantage is that it becomes hard to prove that the measured response was in fact due to the process that was the intended subject of the study. 

The fact that isothermal microcalorimeters may well be able to detect the presence of slow reactions has caused considerable excitement in the pharmaceutical industry. There is a serious need for a rapid method by which the stability of drugs, and the compatibility of drug-excipient mixtures, can be assessed or estimated. To what extent can isothermal microcalorimetry meet that need ... 

Angberg et al. (10) studied the hydrolysis of acetyl saliclyic acid solutions, for which it was shown that elevated temperatures were needed to follow this rapid hydrolysis process. Thus, the isothermal microcalorimeter was neither more accurate, nor quicker or easier than using a conventional analytical approach, such as titration or chromatography, for this hydrolysis reaction. 

Wadso (11) has noted that for a reaction with an enthalpy change of 500 kJ mol1, the smallest detectable reaction would be 0.02% degradation per year. This is calculated on the basis that the molecular mass is 100 (any increase in the molecular mass would result in the detection limit decreasing by the same factor because of the lowering of the number of moles available to react), that 3 g of sample is used and that all of the material is available to react, that the detection limit for the calorimeter is 0.1 pW, and that the process is first order. These assumptions are reasonable as, for example, oxidation reactions can often have high enthalpy changes. It is clear that on this basis the isothermal microcalorimeter would easily be able to detect reactions, and should be able to predict the shelf life parameters for a product. 

FIGURE 8.3 Typical crystallization response measured for an ampoule experiment in an isothermal microcalorimeter (75% RH 25°C) for amorphous lactose. 

They also obtained good agreement between heat of solution and thermal activity measured in an isothermal microcalorimeter over the range 0 to 100% crystallinity. Salvetti et al. (36) have demonstrated that different physical forms of carbohydrates can be differentiated by measuring heats of solution. Pikal et al. (1978) used heat of solution measurements to correlate the extent of crystallinity with the chemical stability of antibiotics. 

The titration cell for an isothermal microcalorimeter provides an excellent way of following complex interactions for biomaterials, polymers, and surfactants. Thus, this approach will see increasing use in the pharmaceutical sciences in the years to come. As with other calorimetric methods, there will often be parallel processes that will need to be corrected for. Furthermore, the more information that is known about a system from other methods, the easier it will be to understand the microcalorimetry data. 

Figure 1 A calorimetric signal showing the sensitivity of an isothermal microcalorimeter ( Thermometric, TAM ) fitted with n Wamplifiers. The experiment was performed at 298.15 K using 3 cm glass ampoules containing - Ig of dry talcum powder. A 50 nW electrical input was applied for 1 min and then switched off...

Figure 9 The calorimetric response for vapour sorption of an amorphous drug substance with a glass transition temperature of about 95 °C. The experiment was performed in an isothermal microcalorimeter coupled with a vapour perfusion device at 25°C. A linear RH ramp was imposed at 5% RH h. The resulting signal shows initial moisture adsorption, recrystallisation and subsequent moisture desorption...

The author would like to thank Dr M. Richardson for his many helpful suggestions and Dr M. Phipps for his advice and expertise on isothermal microcalorimeters. 

A study of the relaxational transitions and related heat capacity anomalies for galactose and fructose has been described which employs calorimetric methods. The kinetics of solution oxidation of L-ascorbic acid have been studied using an isothermal microcalorimeter. Differential scanning calorimetry (DSC) has been used to measure solid state co-crystallization of sugar alcohols (xylitol, o-sorbitol and D-mannitol), and the thermal behaviour of anticoagulant heparins. Thermal measurements indicate a role for the structural transition from hydrated P-CD to dehydrated P-CD. Calorimetry was used to establish thermodynamic parameters for (1 1) complexation equilibrium of citric acid and P-CD in water. Several thermal techniques were used to study the decomposition of p-CD inclusion complexes of ferrocene and derivatives. DSC and derivative thermogravimetric measurements have been reported for crystalline cytidine and deoxycytidine. Heats of formation have been determined for a-D-glucose esters and compared with semiempirical quantum mechanical calculations. ... 

For a lot of compounds, it is possible to induce crystallization of amorphous fractions by subjecting the sample to humidity or solvent vapors, since adsorption of water or solvent in amorphous sections will reduce the glass transition temperature. By exposing samples to a controlled atmosphere in an isothermal microcalorimeter, the heat of crystallization can be measured and the amorphous content may be calculated after having performed a calibration of the system (Figure 8.8). In addition to finding conditions that are suitable to induce crystallization of the... 

Pineiro, A. Olvera, A. Garcia-Miaja, G. Costas, M. Excess molar enthalpies of tetrahydrofuran or diisoptopyl ether + 1-alkanols at 298.15 K, using a newly designed flow mixing cell for an isothermal microcalorimeter J. Chem. Eng. Data 2001,46, 1274-1279... 

It is worth recalling that the entropy of adsorption may be obtained from calorimetric experiments only if the heat exchange is reversible. A formula for evaluating the standard adsorption entropy Aa5 °from a reversible adsorption volumetric-calorimetric data was proposed by Garrone et al. [91] and applied to a selection of quasi-ideal systems, [97] consisting of CO adsorbed on non d/d metal oxides, at the surface of which cus cations acting as Lewis acidic sites were exposed. An isothermal microcalorimeter with a discontinuous (stepwise) introduction of the adsorptive, as the one described here, was fruitfully employed. 

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