Photooxidation of 3-Substituted Pyrroles:  A Postcolumn Reaction Detection System for Singlet Molecular Oxygen in HPLC

A postcolumn photochemical reaction detection scheme, based on the reaction of 3-substituted pyrroles with singlet molecular oxygen (1O2), has been developed. The method is selective and sensitive for the determination of a class of organic compounds called 1O2-sensitizers and is readily coupled to HPLC. Following separation by HPLC, analytes (1O2-sensitizers) are excited by a Hg pen-ray lamp. Analytes that are efficient 1O2-sensitizers promote ground-state O2 (3Σg -) to an excited state (1Σg + or 1Δg), which reacts rapidly with tert-butyl-3,4,5-trimethylpyrrolecarboxylate (BTMPC) or N-benzyl-3-methoxypyrrole-2-tert-carboxylate (BMPC), which is added to the mobile phase. Detection is based on the loss of pyrrole (BTMPC or BMPC). The reaction is catalytic in nature since one analyte molecule may absorb light many times, producing large amounts of 1O2. Detection limits for several 1O2-sensitizers were improved by 1−2 orders of magnitude over optimized UV-absorbance detection. This paper discusses the optimization of the reaction conditions for this photochemical reaction detection scheme and its application to the detection of PCBs, nitrogen heterocycles, nitro and chloro aromatics, and other substituted aromatic compounds.