I was told and have observed that sometimes, without any real input of Na in mobile phase, Na adducts of compounds of interest are formed, while performing LC-MS. Are there any clear explaination about the source of Na?

Hi Jie,

when using a degasser with lc/ms, you should use one channel only for buffered mobile phases.
Cations and TFA are known to adsorb to the degasser chambers and traces will be flushed from the degasser into ms for a very long time.
Further the column itself can be the source for Na, if it was used recently with buffered mobile phase.
The glass of the solvent bottles contains alkali metals and acidic mobile phase can solve them from the glass (normally this effect is of no interest). Brown bottles contains more metals then clear ones.
When your compound tends to show Na adducts you can add very low(!) concentrated Na salts to your mobile phase to analyse this compound as Na adduct.

Thank Andreas for your comments. I would like to futher know your suggestion about the low concentrated Na salts. Which salt (anion) do you recommend? And which overall (final) concentration in the mobile phase?

Now you catched me with my trousers down.

I guess the concentration is less then 0.1 mmol/l or even lower (you don't need much Na ions).
Use formiat or acetat.