Ambient gas influence on electrospray potential as revealed by potential mapping within the electrospray capillary

The effect of ambient gas on potentials inside the electrospray (ES) capillary was investigated. Potential measurements and differential electrospray emitter potential (DEEP) maps were obtained with the help of a small, movable, disklike platinum wire electrode inserted into the ES capillary. Typical solvents used for electrospray mass spectrometry such as methanol and mixtures of methanol-water and chloroform/methanol have been tested. It was found that oxygen is readily adsorbed from the surrounding ambient gas into the spraying liquid. Following adsorption, it resides in, or near to, the Taylor cone, thereby affecting the electrochemical potential near the ES capillary exit, as well as the character of the inherent electrochemical reactions occurring during the ES process. The potentials measured in an air environment with reactive oxygen present are contrasted against those obtained in an inert nitrogen environment. The kinetics of oxygen admission have been found to be quite fast, i.e., occurring in a matter of seconds, but it takes far longer to purge the system of oxygen by changing the ambient atmosphere to nitrogen. The oxygen effect is present in negative and positive ion modes of ES, but the total ES current is not affected by the change of ambient gas. The magnitude of the oxygen effect owing to ambient air was compared to the effect caused by initially dissolving oxygen in the solution prior to the start of ES; it was found that the presence of oxygen in the ambient gas has a far greater consequence. These results indicate that the presence of reactive gases, such as molecular oxygen, in the region of the ES emitter may have unintended secondary effects on the ES process prior to mass spectrometric analysis.