Bromine isotope peaks were observed in EI-MS, but not in ESI-MS

I have been conducting a synthesis involving 2,3,5-tribromothiophene. Because I thought that desired product doesn't have so small molecular weight, I decided to use ESI instead of EI, and so I also requested ESI-MS experiment of 2,3,5-tribromothiohpene as a standard. In that time I got somewhat weird spectrum ([Why is the base peak of my MS spectra of 2,3,5-tribromothiophene at m/z=382?](https://chemistry.stackexchange.com/questions/155648/why-is-the-base-peak-of-my-ms-spectra-of-2-3-5-tribromothiophene-at-m-z-382)). Below are the ESI-MS spectra of 2,3,5-tribromothiophene, and methanol which was used as solvent to insert it to the MS instrument, respectively.


[![ESI-MS spectrum of 2,3,5-tribromothiophene](https://i.stack.imgur.com/wGgTV.png)](https://i.stack.imgur.com/wGgTV.png)
[![ESI-MS spectrum of methanol](https://i.stack.imgur.com/Luq1U.png)](https://i.stack.imgur.com/Luq1U.png)


Recently, I made a hypothesis that since 2,3,5-tribromothiohpene was purchased at around 2019, it may decomposed in long-term storage and produced strange spectrum. To verify this, I requested a EI-MS experiment and in this time I got a reasonable spectrum that has clear isotope peaks.


[![EI-MS spectrum of 2,3,5-tribromothiophene](https://i.stack.imgur.com/5gr4F.png)](https://i.stack.imgur.com/5gr4F.png)


What is the problem with ESI-MS? May the operator miss some settings of the instrument? In fact, I have never observed any isotope peaks whenever I requested ESI-MS experiments of my reaction mixture. As a client, are there any instrument settings that I can request to the analyzer? For what it's worth, the model of the instrument is Xevo TQ-S micro.


