Identification of Protonated Primary Carbamates by Using Gas-Phase Ion–Molecule Reactions Followed by Collision-Activated Dissociation in Tandem Mass Spectrometry Experiments

Differentiation of Protonated Sulfonate Esters from Isomeric Sulfite Esters and Sulfones by Gas-Phase Ion-Molecule Reactions Followed by Diagnostic Collision-Activated Dissociation in Tandem Mass Spectrometry Experiments

Sulfonate esters, a class of potentially mutagenic drug impurities, are strictly regulated in pharmaceuticals. On the other hand, sulfite esters and sulfones, analogs of sulfonate esters, have limited safety concerns. However, previously developed analytical methods for sulfonate ester identification cannot be used to differentiate sulfonate esters from the isomeric sulfite esters and sulfones. A tandem mass spectrometric method is introduced here for the differentiation of these compounds. Diisopropoxymethylborane (DIMB) reacts with protonated sulfonate esters, sulfite esters, and sulfones (and many other compounds) in the gas phase to form the product ion [M + H + DIMB - CH₃CH(OH)CH₃]⁺. Upon collision-activated dissociation (CAD), these product ions generate diagnostic fragment ions that enable the differentiation of sulfonate esters, sulfite esters, and sulfones from each other. For example, SO₂ elimination enabled the unambiguous identification of sulfite esters. On the other hand, elimination of CH₃B═O followed by elimination of (CH₃)₂C═O was only observed for sulfonate esters. Neither type of diagnostic fragment ions was detected for the products of sulfones. However, the product ions formed for sulfones with an additional hydroxyl substituent underwent the elimination of another CH₃CH(OH)CH₃ molecule, which enabled their identification. Finally, ion-molecule reactions of DIMB with various other functionalities were also examined. Some of them yielded the product ions [M + H + DIMB - CH₃CH(OH)CH₃]⁺ but none of these product ions underwent the diagnostic CAD reactions discussed above. Quantum chemical calculations were employed to explore the mechanisms of the reactions. The limits of detection for the diagnostic ion-molecule reaction product ions in high-performance liquid chromatography (HPLC)/mass spectrometry (MS²) experiments were found to range from 0.075 to 1.25 nmol.

Determination of the compound class and functional groups in protonated analytes via diagnostic gas-phase ion-molecule reactions

Diagnostic gas-phase ion-molecule reactions serve as a powerful alternative to collision-activated dissociation for the structural elucidation of analytes when using tandem mass spectrometry. The use of such ion-molecule reactions has been demonstrated to provide a robust tool for the identification of specific functional groups in unknown ionized analytes, differentiation of isomeric ions, and classification of unknown ions into different compound classes. During the past several years, considerable efforts have been dedicated to exploring various reagents and reagent inlet systems for functional-group selective ion-molecule reactions with protonated analytes. This review provides a comprehensive coverage of literature since 2006 on general and predictable functional-group selective ion-molecule reactions of protonated analytes, including simple monofunctional and complex polyfunctional analytes, whose mechanisms have been explored computationally. Detection limits for experiments involving high-performance liquid chromatography coupled with tandem mass spectrometry based on ion-molecule reactions and the application of machine learning to predict diagnostic ion-molecule reactions are also discussed.

Characterization of Protonated Substituted Ureas by Using Diagnostic Gas-Phase Ion-Molecule Reactions Followed by Collision-Activated Dissociation in Tandem Mass Spectrometry Experiments

Substituted ureas correspond to a class of organic compounds commonly used in agricultural and chemical fields. However, distinguishing between different ureas and differentiating substituted ureas from other compounds with similar structures, such as amides, N-oxides, and carbamates, are challenging. In this paper, a four-stage tandem mass spectrometry method (MS⁴) 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 product ion to two steps of collision-activated dissociation (CAD) (MS³ and MS⁴). All the analyte ions reacted with TDMAB to form a product ion [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 - 2HN(CH₃)₂]⁺, which was not observed for many other analytes, such as N-oxides, sulfoxides, and pyridines (studied previously). When the [M + H + TDMAB - 2HN(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), which enabled the differentiation of ureas from amides and carbamates. Furthermore, these fragment ions can be utilized to classify differently substituted ureas. Quantum chemical calculations were employed to explore the mechanisms of the reactions. The limit of detection for the diagnostic ion-molecule reaction product ion in HPLC/MS² experiments was found to range from 20 to 100 nM.