Real time detection of photoreactivity in pharmaceutical solids and solutions with isothermal microcalorimetry

Purpose. In this study an irradiation cell made as an accessory for an isot hermal microcalorimeter is introduced, and its suitability for detection ph otoreactivity in pharmaceutical solutions and solids is demonstrated. The p harmaceuticals employed are chosen as sample materials to evaluate the usef ulness and stability of the irradiation cell. Methods. An irradiation cell has been constructed and tested in an isotherm al microcalorimeter with pharmaceutical solutions and solids known to be se nsitive to daylight or UV light. Light is produced with an Xe-arc lamp, spl it into two parts and introduced into calorimetric vessels with optical lig ht cables. One of the vessels containing the reference sample gives the res ponse to the heat absorbed by the material (radiant power), and the other v essel containing the sample material gives the response also to the photore action. The two irradiation cells are positioned in the sample sides of two separate twin microcalorimetric units. Results. Nifedipine and L-ascorbic acid were found to be photosensitive in solutions and solid states, the extent of the degradation depending on the irradiation intensity and wavelength. The threshold values of the wavelengt h for the photoreactions, as well as the wavelengths for the maximum reacti on rates, were estimated via the scanning irradiation measurements. The abi lity of photons with different energies to produce heat in the photosensiti ve reaction of nifedipine was calculated using constant lambda measurements . Conclusions. The technique introduced offers a rapid and versatile method t o study the photosensitivity of materials in any state. In the measurements , various conditions can be simulated and thus provide information on the r eal behavior of materials.