ONLINE MONITORING OF THE PHOTOLYSIS OF BENZYL ACETATE AND 3,5-DIMETHOXYBENZYL ACETATE BY MEMBRANE INTRODUCTION MASS-SPECTROMETRY

Membrane introduction mass spectrometry (MIMS) allows on-line monitori ng of the products of photolysis (254 nm) of benzyl acetate in aqueous methanol and 3,5-dimethoxybenzyl acetate in water. The reaction mixtu re is continuously exposed to a silicone membrane through which analyt e molecules permeate into a triple quadrupole mass spectrometer for qu alitative and quantitative analysis. Ionization is achieved by either isobutane or ammonia chemical ionization, and ions characteristic of t he reactant ester and its products are monitored simultaneously and co ntinuously. Three products, benzyl methyl ether, ethylbenzene, and bib enzyl are observed in the benzyl acetate photolysis. Two products, 3,5 -dimethoxybenzyl alcohol and 3, 5-dimethoxyethylbenzene, are formed in the photolysis of 3,5-dimethoxybenzyl acetate. Quantitation is achiev ed through calibration using external standard solutions and, in the c ase of benzyl methyl ether, tandem mass spectrometry is used to verify product identification. During the photolysis of benzyl acetate, benz yl methyl ether and ethylbenzene are produced at onset with similar ef ficiencies. For the 3,5-dimethoxy ester photolysis, performed in aqueo us solution, the efficiency of formation of the polar product 3,5-dime thoxybenzyl alcohol is about 300 times greater than that of the nonpol ar product 3,5-dimethoxyethylbenzene. The results show that the relati ve reaction rates are dependent on the solvent and on the photon inten sity and are consistent with earlier off-line experiments by Pincock e t al. which showed that the photolysis proceeds through both ion and r adical pair intermediates. To the best of our knowledge, the work repo rted here describes the first analysis of the photochemistry of an ara lkyl ester in water and the first use of on-line mass spectrometry in a mechanistic study.