Journal of Pharmaceutical and Biomedical Analysis 24 (2000) 167–178 2-Nitrobenzaldehyde: a convenient UV-A and UV-B chemical actinometer for drug photostability testing John M. Allen a, *, Sandra K. Allen a , Steven W. Baertschi b a Department of Chemistry, Indiana State Uniersity, Terre Haute, IN 47809, USA b Lilly Research Laboratories, Eli Lilly & Co., Lilly Corporate Center, Indianapolis, IN 46285, USA Received 1 March 2000; accepted 22 May 2000 Abstract We report the development of a new ‘photochemical titration’ actinometric method for measurement of UV-B (290–320 nm) and UV-A (320–400 nm) light dose during drug photostability testing. It is based upon photolysis of aqueous 2-nitrobenzaldehyde solution, a well-characterized reaction that has been previously demonstrated to be useful as an accurate and reliable actinometric method. Our new method can be performed by a chemical technician using only common reagents. It has been developed for use with xenon arc lamp illumination chambers which are commonly employed for drug photostability testing, but can be readily modified for use with the other illumination sources. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Drug photostability; Actinometer; 2-Nitrobenzaldehyde www.elsevier.com/locate/jpba 1. Introduction Photostability testing of drug substances and products is conducted in order to determine if and to what degree they are likely to undergo photodegradation under the illumination condi- tions encountered during their manufacture, stor- age, and handling. A number of important issues such as loss of efficacy and formation of toxic photoproducts are associated with drug photodegradation [1]. The results obtained from photostability testing are highly dependent upon such variables as choice of illumination source, sample placement, temperature, and humidity. In order to standardize such testing, a monograph describing the guidelines for photostability testing of drug substances and products has been pub- lished by the International Committee on Harmo- nization (ICH) [2]. Actinometric measurements are made during photostability testing in order to characterize the irradiance (light dose) to which samples are ex- posed. Several options exist for making actino- metric measurements. Some laboratories employ an instrumental approach such as radiometry or spectroradiometry. In order to be used for mea- suring irradiance, instrumental methods must be calibrated by the use of standard illumination * Corresponding author. Tel.: +1-812-2372234; fax: +1- 812-2372232. E-mail address: challen@scifac.indstate.edu (J.M. Allen). 0731-7085/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0731-7085(00)00423-4