Dynamic Additive Scanning for Precise Control in Electrospray Ionization Mass Spectrometry

In order to achieve optimum conditions of electrospray ionization (ESI) mass spectrometry (MS) methods, samples and mobile phases are often supplemented with acids, bases, supercharging reagents, and electrospray-friendly solvents. Typically, one pH or concentration of these additives is used in a given method, which is selected based on iterative optimization or literature. However, different pH values and concentrations of additives can be suitable for the analysis of different species, and subtle changes can bring different analytical information. Therefore, here we demonstrate a precise MS optimization system enabling dynamic scans of acid–base and additive concentrations in ESI-MS. In the case of low-molecular-weight analytes, MS signals can be enhanced by selecting the optimum conditions in single scans. The online acid–base scan showed enhancement factors of ∼5.4–7.9 for amino acids and related compounds, ∼44.7 for glutathione, and ∼4.5–10.3 for some tested phospholipids at 25%, 75%, and 90% base (stock solution volume ratio), respectively. For proteins, charge state distributions (CSDs) can be manipulated, bringing information on vulnerability of the protein tertiary structures to the changing environment. Multiple charging of cytochrome c and myoglobin was enhanced to varying degrees upon increasing concentrations of sulfolane and dimethyl sulfoxide, while increasing concentrations of organic solvents shifted CSDs to lower charge states. The setup for such measurements was constructed by using off-the-shelf components and by taking advantage of a simple Python code. Coupling online additive scans with ESI-MS streamlines optimization by eliminating the need for multiple sequential analyses of additives used to enhance signal intensity or induce supercharging.