Would cerium(III) in solution be oxidized to cerium(IV) in ESI mass spectroscopy?
I found one study that examined cerium(III) polyoxometalate salts and concluded that at least under the conditions of their study, $\ce{Ce(III)}$ did not oxidize under electrospray conditions.  The study is Bray et al. 2011 in the International Journal of Mass Spectrometry.

They say:


  The presence of the redox active $\ce{Ce}$ metal ion introduces additional complicating effects as it would be difficult to differentiate such ions as $\ce{HCe(IV)P2Mo22O75^3−}$ from $\ce{H2Ce(III)P2Mo22O75^3−}$. However, successive degradation of $\ce{H_{g}Li_{2−g}Ce(III)P2Mo22O75^3−}$ to eventually form $\ce{HCe(III)PMo7O26−}$ yields an ion that can be directly compared to $\ce{LiCe(III)PMo7O26−}$. In particular, the two ions $\ce{HCe(III)PMo7O26−}$ and $\ce{LiCe(III)PMo7O26−}$, are separated by ∼6 m/z (Fig. 6), supporting the proposition of substitution of H+ by Li+ and, therefore, the identity of $\ce{LiCe(III)PMo7O26−}$. These results also aid in assigning the proton count on $\ce{H2Ce(III)P2Mo22O75^3−}$, $\ce{HCe(III)P2Mo18O62^2−}$, and $\ce{HCe(III)PMo13O44−}$, thereby indicating the trivalent oxidation state of cerium throughout. Although Ce(III) would not necessarily expected to be oxidized to Ce(IV) under the conditions of these experiments, confirming that cerium remains in the Ce(III) oxidation state from solution throughout the electrospray and CID processes is a useful methodology for oxidation state determination of redox active metal-complexes with f-block elements such as Ce(IV) and Pu(III/IV).


So they didn't expect Ce to be oxidized by electrospray in general, although the basis for this expectation wasn't clear.  They also report hard data showing that at least for polyoxometalate salts, no oxidation was apparent under electrospray conditions.

In general, the thousands of volts applied during the electrospray process does not mean that individual ions are exposed to fields of thousands of volts, because electrosprayed solutions are conductive and field strengths in solution are lowered because of Debye shielding.