Mass spectrometric approaches for the investigation of dynamic processes in condensed phase

I. Introduction 00 II. Off‐Line Sampling 00 A. Quantitative Approaches for Kinetics Determinations 00 B. Characterization of Intermediates in Chemical Reactions 00 C. Off‐Line Investigation of Catalysis in Biological Systems 00 III. Continuous On‐Line Monitoring 00 IV. Rapid Mixing 00 V. Conclusions 00 VI. Abbreviations and Acronyms 00 Acknowledgments 00 References 00 Mass spectrometry (MS) offers many advantages over other established spectroscopic techniques employed for the investigation of processes in condensed phase. The sensitivity, specificity, and speed afforded by MS‐based methods enable to obtain very valuable insights into the mechanism of complex dynamic processes. Off‐line methods rely on quenching to halt the progress of the reaction of interest and allow for the implementation of a broad range of analytical procedures for sample fractionation, isolation, or desalting. On the contrary, on‐line methods are designed to carry out the real‐time monitoring of dynamic processes through a continuous uninterrupted analysis of reaction mixtures, with the only caveat that the sample solutions be directly amenable to the available ionization technique. The utilization of rapid mixing devices in direct connection with a mass spectrometer or included in off‐line schemes provides access to the initial moments of a reaction, which can offer very important information about the reaction mechanism. This report summarizes the different off‐ and on‐line strategies developed to study chemical and biochemical reactions in solution and obtain kinetic/mechanistic information. The merits of the various experimental designs, the characteristics of the different instrumental setups, and the factors affecting time resolution are discussed with the aid of specific examples, which highlight the contributions of MS to the different facets of the investigation of dynamic processes in condensed phase.