Adducts: sodium, potassium, and the peak you did not expect — does this still hold?

Asking directly, because I could not find a straight answer: Adducts: sodium, potassium, and the peak you did not expect — does this still hold?

Posting the method first, because I know what the first three replies will otherwise be.

• Column: C18, 3.0 x 150 mm, 2.6 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 13% to 45% organic over 22 minutes
• Detection: 220 nm
• Injection: 11 uL
• Sample: tirzepatide, reconstituted to 0.5 mg/mL, injected within an hour

The main peak integrates at 98.3% of total area. There is a small feature on the trailing edge that I cannot decide is a shoulder or a baseline artefact, and that is what I am actually asking about.

Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.

post #2 answers the question as asked. The question underneath it is different.

Tandem mass spectrometry: MS/MS fragments the molecular ion and uses fragment masses to confirm identity and detect modifications. A simple identity confirmation by LC-MS does not address post-translational modifications or impurities with the same or very close mass.

On post #2 — agreed on the reasoning, with one qualification.

Desalting before analysis: some samples need desalting to remove salts that suppress the peptide signal. Report whether desalting was used, because it can affect the apparent ionization efficiency and the reported purity.

Mass accuracy is expressed in parts per million. It is the difference between observed and theoretical mass divided by theoretical mass, multiplied by a million. A high-resolution instrument in good calibration achieves low single-digit ppm on a peptide of this size.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

post #6 is right about the mechanism and I think understates the practical bit.

Charge states observed: for semaglutide (4113.6 Da) the doubly charged ion appears at m/z ≈ 2057, triply charged at ≈ 1371, quadruply at ≈ 1029. Those are the positions to look for; the heights depend on the ionization efficiency.

Coming back to post #8, because the follow-up matters more than the original answer.

Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy.

Two things before anyone answers the substance.

First, the context in the first post is clear and specific. Second, the question is framed so that an answer can actually address it. Both are the norm here and both matter more than they sound.

Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation.

Coming back to post #11, because the follow-up matters more than the original answer.

Sample matrix effects: if a sample is dissolved in a complex matrix, other compounds in the matrix can suppress the peptide signal. Clean samples give higher sensitivity than dirty samples.

Picking up post #11: that is the part I would want checked first.

Purity and identity are different questions: LC-MS establishes that the species at a retention time has the expected mass. It does not establish how much of the sample is that species (that is what LC-UV purity answers).