M/z 165 was observed in LC/MS scan (ESI+) when pH7.4 phosphate buffer (with NaCl) was diluted with mobile phase and injceted. Mobile phases are MeOH and water (both with 0.1% formic acid).

Any thoughts would be highly appreciated.

three clues you may get are from the mass defect, isotope ratio peaks, and Na/K adducts (both M and 2M adducts)

That additional information may help you (and others in the forum) glean an elemental composition and some structural clues.

[Na3PO4+H+]+ = 165 ( possible?)

It sounds like you're running a gradient. How is the retention of this peak? Unretained or does it elute off later? Do you see it when you run a gradient with no injection?

Yes. It's a gradient method. The peak eluted early. I didn't try no injection.

BTW. I am wondering whether [Na3PO4+H+]+ is possible.

Thanks!

Hmm, since it's early eluting, it's probably not a gradient artifact. I've never attributed anything to a species like [Na3PO4+H+]+, but I don't know if that is possible. Usually with salts, if the ions were high enough in mass, I would see the cation in positive mode, and the anion in negative mode. With phosphate salts, I recall seeing the [H2PO4]- and [HPO4]2- anions, but never saw anything I could attribute to an alkali metal cation alone (not including adducts with other compounds). If it really is some ion formed by the sodium phosphate, it should elute in the void volume.

with buffer of 7.4 and mobile phase around ~3 pH, I doubt you will see all three hydrogens removed from the phosphate. Though [Na2HPO4+Na+]+ may be more feasible. However, I'd imagine you would see an array of Na and H+ adducts.

Na and P do not have any appreciable stable isotopes. This molecule would have almost negligible m/z 166 and a 167 that is ~0.8% height of the 165 signal.

Are you seeing 166, 167 signals? What are their relative abundances to the 165 signal?

By intensity 166>165>167. It seems 143 is also found in the spectrum.

Thanks for all interesting discussions!