Determination of Photolysis Lamp Flux Using O-nitrobenzaldehyde

Actinometers, stable compounds with known photochemical properties that react predictably with light to produce stable products, are tools in determining the flux of photolysis lamps. O-nitrobenzaldehyde is an emerging actinometer, which can be analyzed using a variety of instrumental methods. This study attempted to determine the rate constant for the photochemical isomerization of o-nitrobenzaldehyde to o-nitrosobenzoic acid, and to derive the flux of the photolysis lamp. The flux of a photolysis lamp is a photochemical constant, which allows laboratory specific photochemical rate constants to be converted into a general quantum yield. The quantum yield is necessary for physical modeling of photochemical reactions and can be used to derive the quantum yield of other photochemical reactions. The rate constant for the photo-isomerization of o-nitrobenzaldehyde was obtained by irradiating 100 µM o-nitrobenzaldehyde samples in methylene chloride for set increments with a xenon/mercury lamp containing a dichroic mirror and cut off filter that isolates 320-400 nm light. The concentrations of o-nitrobendezaldehyde were quantified using mass spectrometry single ion monitoring scans, after gas chromatography separation. The concentrations of o-nitrobenzaldehyde were plotted in a first order rate law plot to yield the rate constant. The flux was determined by solving the quantum yield equation using the known quantum yield, cross-sectional area, and the rate constant. The rate constant was found to be .0220 s^-1 and the quantum yield was found to be 8.55*10¹⁴ photons cm^-2 s^-1. These values will be used to derive product quantum yields in polycyclic aromatic compound photo-oxidation studies in our laboratory.