Chromatography Forum

A coworker keeps seeing half his expected mass for a series of compounds using electrospray positive. There is no way it would fragment that way. My only thought is if the major charge was + 2H+ instead of 1H+. I've never actually heard of that happening before, and we've never seen that before, but I'm curious if it ever happens.

A coworker keeps seeing half his expected mass for a series of compounds using electrospray positive. There is no way it would fragment that way. My only thought is if the major charge was + 2H+ instead of 1H+. I've never actually heard of that happening before, and we've never seen that before, but I'm curious if it ever happens.

Of course it happens. The question is whether you can prove it.

One piece of proof: Are there isotopic peaks spaced at expected intervals, or do they appear to be packed closer together than unit mass?

Another piece: Can you fragment the molecule into a mass with higher m/z? This can only happen if the precursor ion was multiply charged and the product is less charged. For example, cyanocobalamin (a form of vitamin B12) is visible on MS/MS at 678.3>997.5. The exact lowest-isotope mass is 1354.6, so this is [M+2H]+ fragmenting into a singly charged ion. If you don't have MS/MS you could try cranking up the fragmentor voltage (or declustering potential or whatever it's called on your MS) to promote in-source fragmentation.

Thanks for the reply. Later in the synthesis (this was an intermediate) it started ionizing at M+1 again, so fairly sure that is what was happening. The M+2/2 ion was most likely real since it was a very strong signal under the compound UV peak and the compound couldn't fragment in half that way (it was symmetric, but the center was a piperizine which would require breaking two carbon-carbon bonds). There were many other ways it would have preferred fragmenting, but we didn't see that.