An instrumentation perspective on reaction monitoring by ambient mass spectrometry

The elucidation of reaction mechanisms in inorganic and organic chemistry, and more recently in supramolecular chemistry, has attracted great interest. Better understanding of these mechanisms would enable people to work more wisely and innovatively in designing new reactions and synthesizing novel chemicals and materials. In addition, monitoring both substrates and reaction products during the reaction process aids the study of reaction dynamics, which will be helpful in optimizing reaction conditions. In comparison with other reaction-monitoring techniques [e.g., electron-spin resonance spectroscopy, ultraviolet spectroscopy and chemically-induced dynamic nuclear polarization], mass spectrometry is widely used due to its high sensitivity, specificity and scanning speed. In this review, we focus on ambient ion sources that have undergone rapid development in the past decade and greatly facilitate direct, rapid analysis of samples with little or no sample preparation. These ion sources, including but not limited to desorption electrospray ionization, extractive electrospray ionization, low-temperature plasma probe, electrospray-assisted laser desorption/ionization, and desorption/ionization on porous silicon, offer varied reaction times (several milliseconds to several minutes) and are controlled by corresponding experimental parameters. We discuss examples of different ion sources coupled with MS for reaction monitoring and highlight some of their unique applications in bioorganic and organic chemical reactions. We believe that, with this abundant toolbox of ion sources as sampling methods, we can be more confident in choosing the most effective technique to address problems of interest in reaction monitoring. (C) 2012 Elsevier Ltd. All rights reserved.