The proportion of the oxidized product ions relative to the pro-tonated TEMPO decreases with increased analyte neutral sprayflow rate and is also lower in non-polar chloroform than in polarwater/methanol solvent. We postulate that the oxidized productions are produced by static charge build-up on the neutral ana-lyte sprayer due to its placement in a strong electric field formedbetween the ESI sprayer and the instrument interface. In our desireto be able to rapidly switch between the ESI and EESI techniques wehave electrically isolated both sprayers from the rest of the instru-ment, apart from the high voltage line that is connected to theESI sprayer (see Fig. 1). This means that, while the neutral sprayeris electrically isolated, it was not actively grounded, allowing thebuild up of static charge. This is similar to a number of inlet ioniza-tion techniques, where surface charging has been shown to play arole in creating ions [15]. Charging effects are more pronouncedwith a polar solvent, such as water/methanol, than a non-polarone, like chloroform. The magnitude of charge, and therefore theamount of oxidation, should be also dependent on the neutral sprayflow rate. If the rate of the charge build-up is constant, then thehigher flow rate would be expected to lower the charge on theneutral sprayer as more of it would be carried away by the solu-tion. Also, the charge density in the solution and the droplet wouldbe lower the higher the flow rate, lowering the rate of any possibleelectrochemical reactions.