A Critical Reappraisal of the Evidence for Spontaneous Generation of Hydroxyl Radicals in Aqueous Nanodroplets

Small aqueous droplets are ubiquitous in the natural world and are often generated by human intervention for various purposes. Therefore, recent claims that micromolar levels of hydroxyl radicals spontaneously form in such droplets have drawn considerable attention and criticism. If true, the simple formation of aqueous droplets would spontaneously create highly reactive species that could be leveraged for interesting chemistry, but abundant hydroxyl radicals would also pose a significant danger to all living organisms, making the issue of clarity of broad importance. Previous criticisms, although compelling, have not been able to reproduce the original results used to support the spontaneous formation theory. Herein, we faithfully reproduce the original data. However, when the species previously identified as a hydroxyl radical is examined by collisional activation or isotopic labeling, the results are consistent with assignment as contaminant ammonia, which has the same nominal mass. Furthermore, we used theta capillaries, hydrogen peroxide, and ultraviolet activation to create de facto hydroxyl radicals within droplets as they are sprayed into a mass spectrometer. As expected, analytes subjected to true hydroxyl radicals undergo extensive oxidative damage. Additionally, previous results obtained with caffeine and melatonin are reproduced but are shown to be ammonia adducts or artifacts caused by narrow isolation windows. In summary, reappraisal of the evidence leads to the conclusion that no appreciable amount of hydroxyl radicals is spontaneously formed in aqueous droplets. We conclude by suggesting criteria that can be used to assess mass spectrometry data and should help prevent similar misinterpretations in the future.