Solvent adducts in ESI

[Phytochem]

Hi everyone,

I am running ESI LC-MS on Shimadzu 2020 using ACN and H20 acidified with 0.1% FA, and was getting tall mass peaks at 101 and 142 m/z, which I found are (2ACN + H2O + H)+ and (3ACN + H2O + H)+, respectively. My concern however is that my presumably pure compound mass peak also comes as two peaks with a difference of 41 m/z.

I thought that changing solvent would prove whether this difference is from ACN, so I changed my solvent B to methanol (0.1% FA), and now I am getting two mass peaks of my compound, with a difference of 32 which is a loss of methanol.

Because I don't know what my purified compound is, I am wondering what would be the best approach to just get one mass peak, instead of two. Would increasing pH make any difference?

Thank you in advance!

(I wasn't sure if I should have posted this in sample prep or LC-MS)

[JMB]

You do not need to do any further chromatography!.
Take advantage of the solvent-adduct peaks to confirm the true MW.
MW from [M + H] & [M + H + MeCN] gives M1.
MW From [M + H] & [M + H + MeOH] gives M2.
M1 should = M2

You MIGHT have [M + H] & [M + H - MeOH] if you have a >C(O)OMe or >C(H)OMe in your structure.

Do you see any 32 amu losses in the MeCN/water analysis?

[Phytochem]

Thank you so much for your response, JMB.

I don't see any 32 amu losses, and yes, M1 is = M2

Actually, it seems that ACN adducts are common on -OH, and we confirmed that the compound has 5 OH groups.

Increasing the drying gas flow rate and the heat element temperature got rid of, or at least reduced the 101 and 142 peaks.

[lmh]

You are right to check the spray chamber conditions, drying gas temperature/flow. This is particularly relevant if your work is partially quantitative and not just qualitative. The issue is that if you have solvent adducts of analyte ions, whose abundance depends on how well the solvent is being dried in the spray chamber, then minor changes in gas flow or temperature through a batch of samples will influence the peak areas of any ion you chose to quantify. If you can find conditions which are at least stable, then your peak areas will be more reliable.