Ion/molecule reactions of dimethylamine with protonated analytes facilitate the identification of tertiary N-oxide functionalities in a linear quadrupole ion trap mass spectrometer

Abstract

A method is presented for the rapid identification of tertiary aliphatic and aromatic N-oxide functionalities in protonated analytes via ion-molecule reactions with dimethylamine (DMA) in a linear quadrupole ion trap mass spectrometer (LQIT). DMA was leaked into the trapping region of the mass spectrometer and allowed to react with protonated analytes (ionized by electrospray ionization) for 500ms, after which all ions were detected. Protonated analytes with the tertiary N-oxide functionality react with DMA to yield exclusively product ions with m/z-values that are 45 units greater than that of the protonated analyte, corresponding to a stable DMA adduct. Collision-activated dissociation of the adduct ions suggests that formation of a hydrogen bond between DMA and the protonated analytes is responsible for the formation of the stable adduct. Hydrogen bond energies were calculated for several adducts at the B3LYP/6-31++G(d,p) level of theory (including correction for basis-set superposition error) to define the potential energy wells for the formation of [M+H+DMA]+ adduct ions. The relative enthalpies calculated for the [M+H+DMA]+ adduct ions were found to be lower than those of the products resulting from proton transfer from protonated tertiary N-oxides and one aromatic ketone, ketoprofen, to DMA. On the other hand, a protonated primary N-oxide, nitrosobenzene, exclusively reacts via proton transfer with DMA, which was calculated to be more exothermic than formation of an adduct ion. Collisional cooling was found to be crucial to the formation of an adduct ion for analytes with proton affinities lower than that of DMA. Ion-molecule reactions of protonated 4-nitro-2-picoline N-oxide (PA=206kcalmol−1) with DMA (PA=222kcalmol−1) in an FT-ICR at ultra-high vacuum with no buffer gas resulted solely in proton transfer to DMA. Finally, examination of the reactions of DMA with protonated clozapine and clozapine-4′-N-oxide demonstrated that DMA can be used to identify the N-oxide functionality in the presence of other basic functionalities. Compared to other reagents reported for the identification of tertiary N-oxide functionalities via ion-molecule reactions in a mass spectrometer, DMA is the only one volatile enough to be used in a multi-ported pulsed valve system designed for rapid introduction of several reagents, each diagnostic for a different functionality, for characterization of analytes as they elute from an HPLC.