Dimer in ESI/MS

[shangyer]

Hi, I am not sure if this is discussed before or not.

We just synthesized a new compound, and run LC/MS (positive ESI) and found there are 2M+H and 2M+Na peaks, as well as M+H.
My question is how this happen? just because too high concentration or what? If anyone can show a paper, that will be great.

The mobile phase: A, 1mM CH3COONH4 buffer; B, ACN. flow rate 0.2 mL/min. The compound is an ester.

[alealbor]

Hi Shangyer,
I think that you must induce the formation of aduct of ammonium, increasing the salt of acetate (25 mM eg.). Then, you try to break this aduct of ammonium with C.I.D. in quadrupole or before to.


Alejandro

[MG]

Sorry I don't have a reference, but this certainly can happen in ESI. I've found that the cause is usually that the sample is too concentrated, or the CID voltage (fragmentor, DP, cone voltage, etc.) is too low.

[shangyer]

thanks you guys, I gonna dilute the samples, see if I can decrease the dimer intensity

[Roy]

Hi,

What you are calling a dimer is actually a di-adduct, anyway sometimes its present no matter what you do. I agree with MG, try to dilute the sample and increase the Probe Voltage (sometime it helps!).
Some one suggested CID; you dont have to worry about it unless you are conducing MS/MS experiment.

Let us know it helps or not.

Thanks.

[shangyer]

I diluted the sample, but nothing changed. The ratio of M+H and 2M+H did not change.

[Roy]

I know sometimes it just doesnt work. However you may try to prep your sample in a different solvent and see what happens, Otherwise I guess you just have to go with it.

Thanks,

[MG]

Clarification on CID voltage: I was referring to up-front CID, which is a consideration in any LC/MS instrument whether MS/MS is being done or not. This is usually controlled by changing the voltage difference between an entrance orifice (or capillary) and a skimmer, and affects the ions' travel in the first pumping stage (below 1 atmosphere, but not high-vacuum). If the value is too low, you get poor ion transmission and greater tendency to see clusters, hence poor M+H signal. If it's too high, the M+H ion will fragment, and you'll see a mixed spectrum containing M+H and fragments (or only fragments if it's high enough). In our lab, we call these fragments "bastard" ions, because unlike with MS/MS, you can never be certain which ion was the parent. Nonetheless, on an instrument lacking MS/MS capability, a person could do "poor man's MS/MS" by running a sample at low and high CID voltages, assuming they had good chromatographic separation. A similar experiment could be done on a triple-quad to give "poor man's MS^3". Some vendor-specific terms for up-front CID voltage:

Agilent: Fragmentor
Bruker: Capillary Exit & Skimmer 1, or Capillary Exit Offset
AB/Sciex: Declustering Potential (DP) and Focusing Potential (FP)

[Roy]

MG,

Thanks for the clarification.

[JM]

Suggest to try diluting your sample in TFA or Formic acid and use in your mobile phase. This might help!

JM