Chromatography Forum

In LC mass spectra, one unknown impurity showing the mass of 83, parent peak showing mass of 83 and 234. The parent peak mass is 234 theoretical. Does it mean that the impurity peak mass is 83 or is it a fragmentation ?

Can anybody explain ?

Theoretically it can be both, but more information is needed.
Do you know your compound?
Can you do MSMS on 83 m/z?
If MSMS is not available, may be ion source fragmentation at higher voltage?
I would try different techniques to identify that 83 m/z peak.
You may need even to do preparative isolation.

do 83 and 234 coelute? If not, then 83 is an impurity. Do they coelute if you use a different column?
If you have MS-MS, is 83 a fragment of 234 in "proper" MS-MS? If so, it's highly likely to be a source-fragment anyway.

It would be good to see how the ion traces look. Even if the peaks are "far enough" apart, it is possible that the trace for the impurity peak extends into the main compund. (Without additional information, the first question I have is: Are we sure that 83 is even part of the spectrum of the main compund, as raised by lmh - and we have to note that the peaks do not have to be right on top of each other to get confused signals.)

Is 83 in the impurity a fragment or does it represent a whole molecule? While LC/MS is typically a soft ionization thechnique, there are molecules that will fragment even in the LC/MS source, so you don't know.

So, some thoughts: What buffer are you using and what ionization mode? Are there synthetic intermediates that might account for the m/z 83 or are there possible degredation products? And be sure that this is not background picked up in perpartion of the sample or from the chromatographic system.

Have you tried varying the fragmentation voltage (also known as declustering potential or various other names depending on manufacturer)? A lower frag voltage will produce less in-source fragments but if you go too low, a lot of molecular ions are still caught up in solvent clusters.

And yes, you may also want to see if the 83 precursor has any fragments of its own. Then you can try the larger precursor at a range of collision energies and see if those same fragments appear.

It has been my experience that in-source fragments are usually very similar to those that can be produced by MS/MS. In fact on a triple-quad this is a nice way to do "MS^3" for extra structural information - set the fragmentation voltage high enough to produce the first fragment in-source, and then do a second fragmentation in the collision cell.