Dear all,
depending on the molecular structure of an analyte different adducts during the electrospray process are formed.
Even when high (up to 5 mM) of ammonium salts are present in the LC solvent, some analytes show preferential adduct formation with potassium or sodium even if ammonium adduct formation is observed as well. I do not understand why this is happening.
This is not necessarily a bad effect, because if I can understand which molecular features (ketones, alcohols, peroxides, amines, epoxides, ...) are causative for which adduct formation, i can use this data to increase the confidence of compound identification.
Can anybody recommend any literature on why different counter ions are adducting to the analytes? And which molecular features an analyte needs to bear in order to preferentially form a specific adduct?
I hope someone can help here.
Best Regards,
Mike
I can't remember if there was a quantitative explanation to the formation of these adducts. It seems that adducts with sodium and potassium are formed because of a very high affinity, because they show up even if these cations are present in trace amounts.
I do remember that oxygen-rich compounds made more of these adducts, because they are good Lewis Bases (electron pair donating), -as are amines-, and the cations are good Lewis acids (electron pair accepting). If you don´t find the literature on quantitative explanations, it may be worth exploring.
Also, for neutral lipids they used Lithium+ to make the adducts, I think it was nicer to the spectrometer. And Silver+ to make adducts with double bonds in the acyl chains.
best,
Matias
Yes, it certainly has to do with the basicity of the compounds.
After some reading today i found that the observed adducts cannot be explained so easily. Analytes ionized in the ESI droplet in solution are not necessarily also the compounds being measured in the MS (gas phase). Several different equilibria exist in the droplet and the gas phase:
(l) = liquid; (g) = gaseous
M -> M+H (l) -> M+H (g)
M -> M+Na (l) -> M+Na (g)
probably also M+H (g) M+Na (g) (by charge transfer)
now depending on the molecular structure, the equilibria are shifted towards specific adducts in the gas phase.
There were quite some studies on the influence of additives to the mobile phase and different explanations have been provided. For example changing surface tension of the droplet and thus higher intensity, changing surface activity of analytes by differential adduction and thus higher intensity, gas phase charge transfer depending on which ions are present, type of ion optics, type of esi source,...
But i cannot find quantitative studies on molecular determinants for the adduct formation. It seems to be not as simple as i initially thought.
thanks either way for the help matias!
Best
Mike
I assume that this issue is influenced by the vapor pressure of the salts. Ammonium salts finally will evaporate and are lost in the gas phase. I don't think that an equilibrium is established at all. Sodium and potassium salts have essentially no vapor pressure and form the leftovers... In order to avoid sodium and potassium adducts, you should use ultra-pure solvents and reagents. BTW: Many chemicals have no statement about sodium or potassium impurities and if they have one, the limits are quite high.
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