Letter: Observation of the 16O/18O Exchange during Electrospray Ionization

Isotopic exchange approach coupled to high-resolution mass spectrometry has become the power analytical approach for a wide range of analytical and bioanalytical applications. Considerable efforts have been dedicated to developing fast exchange techniques directly in the ionization source. But all such methods are limited to the hydrogen/deuterium exchange approaches. In this paper we demonstrate that certain types of oxygen atoms can also be exchanged for 18O on the time scale of the ionization process. Using HIO3 and NaIO4 and by infusing the heavy water H218O in the ESI source we have demonstrated that it is possible to obtain a high level of oxygen exchange. It was observed that the rate of this exchange depends to a large extent on the temperature of the desolvating capillary of the mass spectrometer. Several other species, such as peptides, oligonucleotides and low weight organic molecules, were subjected to in-ESI 16O/18O exchange but the exchange was not observed.