title: Real time detection of photoreactivity in pharmaceutical solids and solutions with isothermal microcalorimetry.
abstract: OBJECTIVE
  In this study an irradiation cell made as an accessory for an isothermal microcalorimeter is introduced, and its suitability for detection photoreactivity in pharmaceutical solutions and solids is demonstrated. The pharmaceuticals employed are chosen as sample materials to evaluate the usefulness and stability of the irradiation cell.
  
  
  METHODS
  An irradiation cell has been constructed and tested in an isothermal microcalorimeter with pharmaceutical solutions and solids known to be sensitive to daylight or UV light. Light is produced with an Xe-arc lamp, split into two parts and introduced into calorimetric vessels with optical light cables. One of the vessels containing the reference sample gives the response to the heat absorbed by the material (radiant power), and the other vessel containing the sample material gives the response also to the photoreaction. 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 wavelength for the photoreactions, as well as the wavelengths for the maximum reaction rates, were estimated via the scanning irradiation measurements. The ability of photons with different energies to produce heat in the photosensitive reaction of nifedipine was calculated using constant lambda measurements.
  
  
  CONCLUSIONS
  The technique introduced offers a rapid and versatile method to study the photosensitivity of materials in any state. In the measurements, various conditions can be simulated and thus provide information on the real behavior of materials.
authors.affiliation: Department of Physics, University of Turku, Finland.
authors.forename: V P
authors.initials: VP
authors.lastname: Lehto
authors.forename: J
authors.initials: J
authors.lastname: Salonen
authors.forename: E
authors.initials: E
authors.lastname: Laine
mesh_terms: D000975:Antioxidants / Q000737:chemistry / Q000528:radiation effects; D001205:Ascorbic Acid / Q000737:chemistry / Q000528:radiation effects; D002121:Calcium Channel Blockers / Q000737:chemistry / Q000528:radiation effects; D002151:Calorimetry / Q000379:methods*; D002626:Chemistry, Pharmaceutical; D003199:Computer Systems; D005069:Evaluation Studies as Topic; D009543:Nifedipine / Q000737:chemistry / Q000528:radiation effects; D010777:Photochemistry
chemical_list: D000975:Antioxidants; D002121:Calcium Channel Blockers; D009543:Nifedipine; D001205:Ascorbic Acid