May be it is possible that you isolated the anomer alpha and beta and, once re-dissolved in solution, they reached again the eqilibrium

When relatively pure or pure compounds are run in ESI LC/MS/MS (or a component has high concentration) it often shows not only the molecular ion M.H+ but also an adduct M.M.H+ which seems like dimer. Many compounds perform like this. It is very common in ESI at high component concentrations. It appears like performing ESI MS or LC/MS/MS in absence of sufficient amount of retainable charges (protons). Changing the pH or even may be the composition of the mobile phase would change the ratio of the two components. A simpler test involves injecting 1/10 or 1/50 of the amount of the analyte. The dimer-like adduct will decrease or disaapear at these condition.

Are you doing LC and seeing two major ions for each peak? Or are you redissolving your purified compounds and injecting directly into the MS? You might try changing ionization parameters and see if that makes a difference, or switch from APCI to ESI (or vice versa). You might look at the positive ion instead and see if the phenomena persists. If the rate of equilibration between monomer and dimer is rapid this will be a challenging problem. It would not be that surprising to find a monomer negative ion also present in your dimer, dimer ion in the monomer would I think be unusual as a probe reaction.

The answer is no

Another easy way to differentiate adducts from dimers is changing the polarity mode of your instrument. Some adducts are formed in the cationic mode but not in the anionic and vice-versa. A real dimer should prevail in both modes.

Nikolay gave the correct answer. if you swtich between positive and negative ion you wll most likely see ti disappear.

Alternatively collect each fraction and re-inject - if you get the same profile then it may be a simple case of picking up different counter ions or ion-paring type species.

Checking the exact mass and/or the isotope pattern of the 'dimer' should also do the trick....

Rafaela Guimarães et al. separated cis- and trans-ferulic acid hexoside and a dimer. You can find their paper, containing LC reparation and MS data, on the Internet:

Infusion and decoction of wild German chamomile: bioactivity and

characterization of organic acids and phenolic compound

Dimerisation is a fairly common occurrence with an ESI ion source, even with commercial standards. As ESI is a soft ionisation technique, the most abundant ion is likely the molecular ion. Unless you have two ions of comparable abundance (e.g. fraction 2), it is likely that only one ion is present in the sample.

I agree with the answer of Nikolay. Changing the pH and lowering the concentration would lead to dimer-like suppression

Thanks all of you for your suggestions, they were really useful. I think I am getting adducts of both compounds. In the case of the monomer I got [2M-H]- and in the dimer [M/2-2H]-. This explains why I am getting the 2 m/z in both isolated compounds. In addition I will check the exact mas of each purified fraction.

Thanks again

Please, could I know whether you have a difference of 0.5 between the M and M+1 spectrum peaks of [M/2-2H]-?

My instinctive answer is that you're observing M+H and 2M+H, as already suggested. Running in an instrument which can give you an accurate mass should help. Using deuterated solvents can sometimes help in this sort of puzzle, as all exchangeable protons will be deuterated, increasing the apparent M proportionally

Dear Jose

Everything it has probably already been said in accurate way. LC-MS is a very information dense technique !!! As a general rule just keep in mind that the mass spectra of a peak you observe it is the outcome not only of the compound (assuming it is only one in the peak) and its concentration (as already noted) but also the specific sample matrix (it is a salt ? what type ? what solvent you used to soulbilize or dilute the sample? ect ) and eluent compositions (even organic solvents can produce clusters with analytes).

For example if you use ammonium formate in the acqueous eluent phase than you should observe also [M+18]+ and [M+45]- peaks besides all the possible others already mentioned.

Best Regards