Development of a new on-line mass spectrometer to study the reactivity of soluble organic compounds in the aqueous phase under tropospheric conditions: Application to OH-oxidation of N-methylpyrrolidone

A new on-line analyser was developed to study OH-oxidation of soluble organic compounds in the aqueous phase. The system consisted in the coupling of an aqueous phase photo-reactor with a triple quadrupole mass spectrometer equipped with an electrospray ionisation source (ESI). The system was operated in both positive (ESI-MS+) and negative (ESI-MS−) modes, and in (ESI-MS–MS+) mode for several parent ions. The system was validated by comparison with results obtained by HPLC-UV, during the OH-oxidation of N  -methylpyrrolidone (NMP) in the aqueous phase. The rate constant of the reaction was determined at four temperatures (ln(kOH)=(24.4±1.1)−(760±320)/Tln(kOH)=(24.4±1.1)−(760±320)/T), showing that NMP is highly reactive in the aqueous phase, thus its reaction products should significantly influence the water droplet's composition. The results of the reaction products’ study proved that the on-line analyser was highly promising, as it allowed us to detect the formation of 66 different ions, among which 24 correspond to identify reaction products. The main reaction products identified were succinimide, N-methylsuccinimide, formyl-pyrrolidone, N-hydroxymethylpyrrolidone, 5-hydroxy-N-methylpyrrolidone, 2-pyrrolidone, methylamine, formamide, acetamide, N-methylformamide, N-ethylacetamide and dimethylacetamide. The time profiles of these compounds are presented and a chemical mechanism is suggested for the reaction of OH radicals with NMP in the aqueous phase. The new on-line system also detected the formation of high molecular compounds (compared to the parent compound NMP), up to 300 amu, with mass intervals ranging from 13 to 18 amu. The processes leading to such compounds are discussed.