I’d like to add a few additional considerations that may be helpful for the next stages of method development.
1) On the mechanism behind the mass shift:
I agree that the 0.5 Da pattern clearly supports the presence of the doubly charged ion, [M+2H]²⁺. One aspect that may complement your interpretation is understanding why this behavior differs between the standard and the sample.
The literature on solvent effects in ESI (Effect of eluent on the ionization process in liquid chromatography–mass spectrometry) shows that more polar and alkaline media stabilize multiply charged species in solution more effectively, shifting the charge-state distribution toward higher charge states prior to gas-phase ion emission. While acidic conditions generally favor [M+H]⁺ formation in positive ESI mode, the alkaline environment created by sodium carbonate in pure water may instead favor [M+2H]²⁺, particularly for a compound with multiple ionizable sites such as a penem. This would explain why the standard, if prepared in a neutral or slightly acidic diluent, behaves differently from the sample, a mechanistically expected outcome rather than a random artifact.
The same work also reports that electrolyte concentrations above 10 mM can already produce measurable ion suppression in ESI. The sodium carbonate present in the formulation may well exceed this threshold in the sample solution, introducing an additional ion suppression effect on top of the charge-state issue itself.
2) A point to consider regarding the daughter ion as the quantifier:
Before establishing the product ion as the quantifier, it is worth confirming an assumption that is not yet entirely clear: are the standard and the sample actually generating different charge states, or do they both exhibit the same 0.5 Da isotopic spacing?
If the standard also ionizes predominantly as [M+2H]²⁺, then the likelihood of differential fragmentation under CID is minimal, if not nonexistent. On the other hand, if the standard is observed as [M+H]⁺ with a 1.0 Da isotopic spacing, then this point certainly deserves verification: precursor ions with different charge states experience different effective kinetic energies at the same nominal collision energy, which can lead to distinct fragmentation profiles and potentially compromise the consistency of the transition pair between standard and sample.
Therefore, the first practical step is simply to inspect the spectrum of the standard using the same level of zoom that was applied to the sample cluster and confirm the isotopic spacing. The answer to that question will determine whether this risk is real or not.
3) A practical path forward:
One approach that could simultaneously address both the charge-state distribution and ion suppression would be to prepare the samples in a diluent with controlled pH.
For example, 10 mM ammonium formate adjusted to approximately pH 3.5–4.0 instead of pure water. This would likely promote a more uniform ionization behavior between standard and sample, favor the formation of [M+H]⁺, and reduce the impact of carbonate species at the ESI droplet surface.
Following this adjustment, a formal matrix factor assessment would also be advisable to confirm, quantitatively, that the observed effects have indeed been mitigated.
I hope these thoughts are useful as the method development progresses. I’d be happy to discuss any of these points further if that would be helpful.