Substituted ureas correspond to a class of organic compounds commonly used in agricultural and chemical fields. However, distinguishing between differently substituted ureas and differentiating substituted ureas from other compounds with similar structures, such as amides, N-oxides and carbamates, is challenging. In this paper, a four-stage tandem mass spectrometry method (MS4) coupled with positive-ion mode atmospheric pressure chemical ionization (APCI) is introduced for this purpose. This method is based on gas-phase ion-molecule reactions of isolated, protonated analytes ([M + H]+) with tris(dimethylamino)borane (TDMAB) (MS2) followed by subjecting a diagnostic ion-molecule reaction product ion to two steps of collision-activated dissociation (CAD) (MS3 and MS4) in a linear quadrupole ion trap mass spectrometer. All the analyte ions studied reacted with TDMAB to form an addition product that had lost a dimethylamine molecule, [M + H + TDMAB – HN(CH3)2]+. The product ion formed for substituted ureas and amides eliminated another HN(CH3)2 molecule upon CAD to generate a fragment ion [M + H + TDMAB – 2 HN(CH3)2]+. This fragmentation does not occur for many other analytes, such as N-oxides, sulfoxides and pyridines, which have been studied previously. When the [M + H + TDMAB – 2 HN(CH3)2]+ fragment ion was subjected to CAD, different fragment ions were generated for ureas, amides and carbamates. Fragment ions diagnostic for the ureas were formed via elimination of R-N=C=O (R = hydrogen atom or a substituent). These ions were not observed for any other protonated analytes and therefore enable the differentiation of ureas from amides and carbamates as well as from N-oxides, sulfoxides and pyridines (studied previously). Furthermore, these fragment ions can be utilized to classify differently substituted ureas. Quantum chemical calculations were employed to explore the mechanisms of the ion-molecule and fragmentation reactions. The method was coupled with HPLC to prove its practicality.

Characterization of protonated substituted ureas by using diagnostic gas-phase ion-molecule reactions followed by collision-activated dissociation in tandem mass spectrometry experiments

ACS Fall 2021

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