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Design, Testing and Pharmaceutical Applications of a Gas PressureController Device for Solid-Gas Microcalorimetric Titration A. Bakri University Joseph Fourier Faculty of Pharmacy PharmaceuticalEngineering Avenue de Verdun 38240 Meylan, France INTRODUCTION Interactions between water and component materials take place in all steps of dos-age from manufacture. These interactions can affect the mechanical, physical and chemicalproperties and consequently also the behaviour of the material of interest. The use of the Thermal Activity Monitor (TAM) is already well established in thefield of compatibility and stability testing of pharmaceutical preparations and other chemi-cals in general. The sample is loaded into a sealed ampoule which is placed in the measur-ing position of the TAM. The recorded heat flux (P) can be related to the reaction rate andthe reaction enthalpy (refs. 1-4). However, such a closed system does not allow any controlor change of experimental factors such as oxygen pressure or water vapour pressure in theampoule during an ongoing experiment. In order to overcome this limitation, the Gas Pres-sure Controller device has been specifically developed to control, in particular the relativehumidity (RH) within the sample ampoule (fig.1). During a calorimetric experiment. drygas (1) is delivered by a flow rate controller (2) to a switching valve (3). In one position,a, the valve delivers dry gas directly to the sample ampoule. In the other position. b, thegas is passed through two humidifier reservoirs (4) in thermal contact with the calorimetric bath (5), and the gas it is thensaturated with water (100 %RH) before reaching the sample(6). Both dry (0 % RH) and wet(100%RH) gas pass separatelythrough heat exchangers (7) forthermal equilibration. The in-coming RH to the sample canbe controlled from the valveand varied from 0 to 100 % bymixing various ratios of dry andwet gas. The device operates witha precision better than ±0.1%RH within the range of 0% and100% RH throughout the oper-ating temperature of the TAM.It is possible to program the RHthrough the Digitam software Figure 1 Schematic view oithe Gas Pressure Con-troller units fitting in TAM. either asa series of discretesteps or as an upward or down-ward ramp. The very rapid responseto changes in RH makes thisinstrument system a powerfultool for determining sorptionisotherms and surface energieswithin a few hours. Only smallamcunts of material are needed.Typical sample loadings forpowders are on the order of 50to 200 mg. EXPERIMENTAL A liquid or solid sampleis placed in the measuring am-poule as shown in figure 2. Theexchange between the incom-ing RH-controlled gas may takeplace at the upper geometricalsample surface Oeft) or at thetotal exchange surface when apowder is analyzed. If the latteroption is preferred. the outletis lowered down and the gas ispassed through the powder bed(right). Liquids:: The effect ofRH uponwater evaporationthermograms has been studiedat various flow rates using airor N, as carrier. A typical ther-mogram is shown is shown infigure 3. This relationship wasused to calorimetrically deter-mine the RH-values of standardsalt solutions of known RH. Thestandard solutions were placedin. a thermostat ted bath at 25°Coutside the calorimeter. Air wasthen pumped through the stan-dard solution directly to themeasuring ampoule (via the dry Figure 2. Sample disposition in the ampoule Figure 3. Effect of RH on water evaporation ther-mograms (f=100 ml/h, T=25℃) Figure 4 shows the effect ofthe gas flow rate onwater evaporation heat flux at 25℃: ◇(50 ml/h)口(110 ml/h):)◆(160 ml/h) 口(220 ml/h) inlet) containing water. The thermograms and their interpretation are shown in figures 5and6.The physical modelling and the mathematics behind these results will be publishedsoon. Similar experimentswere carried out at other tem-peratures (40°C and 50 ℃).Figure 8 shows a plot of the to-tal adsorption heat at three tem-peratures as a function of theimposed vapour pressure. tal mass can then be obtained. Powders: As an exampleof a solid material, the results ofcalorimetric sorption isothermsof a pharmaceutical polymericmaterial are presented here. 150mg of the material was placedin the measuring ampoule anddried under 0 % RH (f=100 ml/h) until zero heat flux was ob-served. The incoming RH wasthen set to 10 % and a positiveadsorption response was record-ed. When the equilibrium wasFigure 5. Water evaporation thennograms; Incom-reached (P equals zero) the in-ing air from standard salt solutions (f=l(}()ml/h.coming RH was set to zero and aT=25“C)desorption isotherm (P
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