The method described in this section allows a thorough investigation of enzymatic reactions. On-line monitoring using NANOES mass spectrometry thereby is a versatile tool which can be used to track the fate of each reactant during the reaction, thus allowing the elucidation of unknown reaction mechanisms. Even though this technique normally only provides time-intensity data, these data are sufficient for an general survey of the mechanism lying underneath an enzymatic reaction giving evidence for further, more detailed investigations. The main advantage of this technique is its capability to monitor a reaction for a long time (theoretically for several hours) with a very low sample amount (five to twenty microlitres). Furthermore, due to the mass spectrometric detector, there is almost no limitation with respect to the reactants under investigation. The choice of reactants is only limited by the fact, that they must occur as ions in the reaction solution (and therefore have at least one functional group which is ionised at the pH of the buffer solution) to be detectable by mass spectrometry. With tandem MS it is even possible, to obtain further information on possible reaction products and intermediates, using MS/MS experiments; this characteristic also can be used for the investigation of other reactions, e.g. the decay of compounds in aqueous solutions[45].
The NANOES source developed and presented proved to be a reliable enhancement for the instrument used, the API365, with multiple applications apart from on-line reaction monitoring. The source has been used successfully for the structure elucidation of several biological compounds, e.g. catechin, epicatechin and resveratrol [46,47,77], and to identify phenolic compounds [68].