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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?). 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 ESI-MS spectrum of methanol

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

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.

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  • $\begingroup$ They probably have used acetic acid erozol for electrospray so malecular peak would have been (.M + AcOH). $\endgroup$
    – Mathew Mahindaratne
    Mar 21 at 8:11
  • $\begingroup$ @MathewMahindaratne But still there are no isotope peaks for 3 Br's. $\endgroup$
    – Krang Lee
    Mar 21 at 9:59
  • $\begingroup$ Can you post your ESI-MS as well so we can look at it? $\endgroup$
    – Mathew Mahindaratne
    Mar 21 at 16:17
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    $\begingroup$ @MathewMahindaratne Added it to the post. $\endgroup$
    – Krang Lee
    Mar 22 at 5:30
  • $\begingroup$ Cannot see peaks around 80 m/z to find out bromine is eliminated. There is anothe peak next to 382 m/z peak. My geuss is two bormine atoms must have eliminated. $\endgroup$
    – Mathew Mahindaratne
    Mar 22 at 15:44

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