Analytical Tools Integrated in Continuous-Flow Reactors: Which One for What?

The concept and practices of chemical synthesis are being profoundly transformed toward the development of fully autonomous continuous processes. Critical to the development of autonomous continuous processes is the efficient monitoring of the reaction composition and product quality by in-line and online analyses. The in-line/online acquisition of analytical data allows one to monitor at regular intervals the reaction composition, including hazardous or air-sensitive intermediates with the possibility of adapting reaction parameters or interrupting the flow process when a chemical or technical failure is detected. This review presents the main in-line/online analytical tools that can be integrated into flow reactors for the monitoring of chemical reactions. This contribution is more a guide at the service of synthetic chemists illustrated by selected published examples from leading research laboratories than an exhaustive list of published articles. Ultimately, we would like this review to be an answer to the following recurrent, yet complex, question: "Which is/are the most suitable analytical solution(s) to monitor my chemical reaction?".