Chromatography Forum

Hello everyone

I'm analysing a synthesised sample using ESI-MS. A customer wants to know if it's starting material dimerised accidentally during the synthesis besides its product.
I've observed ions of starting material (M1) as [M1+H]+, [M1+Na]+ and sodium adduct of its dimer (M2) as [M2+Na]+. However, how can I distinguish [M2+Na]+ from [2M1+Na]+ due to an ion source dimerisation?
Other ions such as [M2+H]+ or [M2+K]+ are not observed and the intensity of [M2+Na]+ is relatively high. From total ion current seeing the sample might not be concentrated too much. The sample might be a mixture but at this moment I did not use any column.
I'm thankful if I can hear some ideas.

By MS alone it's extremely hard to be certain. A lot depends on how it is expected to dimerise chemically. Many chemical dimerisations will involve mass change, but not all. Non-covalent adduct dimers (2M+Na etc.) tend to be rather fragile, so if you can do gentle MS2 you may find they disappear. But it's very hard to distinguish a non-covalent adduct dimer from a very fragile covalent dimer. You may be able to reduce the amount of dimer by working at a lower concentration, or increasing source energies a bit, but you'll never be sure there is no dimer.
This is where chromatography can be very useful. Of course it's hard to prove a negative, but it's unlikely the dimer would have exactly the same retention time as the monomer, so if you find that the 2M+Na peak coelutes exactly with the M+H peak, it is probable that they are the same thing. Of course you'll need to convince yourself that the M+H peak isn't an in-source fragment from a pure dimer!
Really the easiest situation is where you do have both the dimer and the monomer and you can separate them with chromatography.

Hi, Imh

Thank you for your reply. It sounds very interesting to measure gentle MSMS! I will try next time that. And I am also relieved to hear that it is difficult for others too.

The proportion of [2M1+Na]+ will be higher the more concentrated the sample.
Assuming you are using flow injection (rather than constant infusion) see if the ratio of MNa+ to [2M1+Na]+ changes as the signal gets smaller. If it does then most likely what you are seeing is [2M1+Na]+
Agree that LC separation would be more definitive though.