Chromatography Forum

I am a new user of LCMS. I run a pure compound in LCMS but got the multiple peaks (looks like proton NMR). This problem arise last week and I could not figure out the problem. My compound is pure and a month ago I had nice single peak. Retention time is same in both run. I check the purity of the compound, it is pure. In my thermo LCMS, there is nice peak for UV and PDA just MS is ugly. I checked other compounds the problem is same. Do anyone know the solution for the problem?

Can you post the masses that you see now?

This UV-PDA detector is part of the LC-MS where you get this dobule peak ? or its another machine ?

How does the ms do with other compunds? And, can you post the traces for the various detector for your problem peak.

Thank you for your interest for help.
I am monitoring the compound with mass 180.0 and the mass of all peaks (multipletes) are 180.0.

UV-PDA detector is the part of the LC/MS system.

I tried with the other compound that have the same problem. I believe something wrong in MS part, as I have good separation of compounds and consistent retention time compared to previous run which do not have multiple peak for same compound.

It sounds indeed like a MS problem.

Is your LC-MS a single quadrupole? If yes maybe the peak shape is not great at the apex of the spectrum and the software picks multiple masses.

Have you tried to retune the instrument?


If you decrease the concentration, is there a change?

Could you tell us the name of the compound?

Could you be fragmenting your compound in-source and creating something else in addition to the analyte of interest?

You get something like that ?
http://www.voila.pl/173/4rsri/?1

Red one from UV-PDA and black double peak from MS with the same 180 mass ?

I come from an LCMS background but I'll take a stab at this.

Sometimes when I saturate my detector I get split peaks. If I attenuate the signal down the split disappears and I get one peak.
Is your sample concentration relatively high? Try diluting it.

Also

If you are analyzing a compound with Br or Cl or other atom with stable isotopes, you will see multiple peaks depending on the number of such atoms in the analyte.


Alp

Alp, the different isotope peaks have different mass.

DOH!

You DID mention that you were looking only at m/z 180 in a later post.
My mistake.

alp, if you find very concentrated samples lead to splits in peaks, it might mean that you are using a system whose mass accuracy goes a bit off at high concentrations. This is a common problem in older ion-traps, for example.

If so, if you make an extracted ion chromatogram, you'll find that the middle section of a peak may wander in mass far enough to be outside the range of the EIC, so it disappears... This can also happen if you are looking at SRM, because the trapping efficiency has gone wrong in the middle section due to excessive space charging effects.