Reactive Laser Ablation Electrospray Ionization Time-Resolved Mass Spectrometry of Click Reactions

Reactions in confined compartments like charged microdroplets are of increasing interest, notably because of their substantially increased reaction rates. When combined with ambient ionization mass spectrometry (MS), reactions in charged microdroplets can be used to improve the detection of analytes or to study the molecular details of the reactions in real time. Here, we introduce a reactive laser ablation electrospray ionization (reactive LAESI) time-resolved mass spectrometry (TRMS) method to perform and study reactions in charged microdroplets. We demonstrate this approach with a class of reactions new to reactive ambient ionization MS: so-called click chemistry reactions. Click reactions are high-yielding reactions with a high atom efficiency, and are currently drawing significant attention from fields ranging from bioconjugation to polymer modification. Although click reactions are typically at least moderately fast (time scale of minutes to a few hours), in a reactive LAESI approach a substantial increase of reaction time is required for these reactions to occur. This increase was achieved using microdroplet chemistry and followed by MS using the insertion of a reaction tube - up to 1 m in length - between the LAESI source and the MS inlet, leading to near complete conversions due to significantly extended microdroplet lifetime. This novel approach allowed for the collection of kinetic data for a model (strain-promoted) click reaction between a substituted tetrazine and a strained alkyne and showed in addition excellent instrument stability, improved sensitivity, and applicability to other click reactions. Finally, the methodology was also demonstrated in a mass spectrometry imaging setting to show its feasibility in future imaging experiments