Why mass spectrometry cannot tell you about an epimer

Adducts — sodium and potassium especially — shift the observed mass in predictable increments. A report showing an unexplained plus twenty-two is usually showing you a sodium adduct.

Two people can read the same figure differently here and both be reasonable.

Picking up post #10: that is the part I would want checked first.

The thing about mass spectrometry that took me longest to accept is that a plausible mechanism is not evidence of an effect. It is a reason to look, not a result.

Sample preparation for mass spectrometry can itself introduce modifications, particularly oxidation. A finding at trace level may be telling you about the preparation rather than the material.

For a compound with no reference standard in circulation, an observed mass is much more useful than an assertion of agreement with a theoretical value nobody can check.

Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.

This follows post #49 rather than contradicting it.

Sample preparation for mass spectrometry can itself introduce modifications, particularly oxidation. A finding at trace level may be telling you about the preparation rather than the material.

The rule of thumb is fine; the edge cases are where it earns its keep.

What mass accuracy establishes: the measured mass is consistent with a specific composition. What it does not establish: purity, sequence order, stereochemistry, or the absence of an isobaric species. Every one of those requires something else.

A weak preference rather than a position.

Common adducts: sodium adds ≈22, potassium adds ≈38 compared to hydrogen. A [M+Na]+ peak is common and its mass is predictable from the base mass.

It is a small point and it changes the answer, which is an awkward combination.

The most useful reply I ever got about mass spectrometry was a request to state my units. It sounds like pedantry and it has saved me twice.