Identity by LC-MS
What a mass measurement establishes, what resolution buys you, and the specific compound pairs where unit resolution is not enough.
Identity
Charge-state deconvolution
Recovering a neutral mass from an electrospray envelope, the arithmetic, and the failure modes that produce confident wrong answers.
Electrospray produces a distribution of multiply protonated ions. Each is observed at its own m/z and each carries the same neutral mass. Deconvolution is the process of recovering that neutral mass from two or more observed m/z values.
The arithmetic
For a species observed at m/z = x with charge z:
M = z × (x − 1.007276)
Two adjacent charge states let you determine z without assuming it:
observed: x₁ = 1 203.6106 (higher charge)
x₂ = 1 604.4794 (lower charge)
z₁ = (x₂ − 1.007276) / (x₂ − x₁)
= 1 603.4721 / 400.8688
= 3.9999 → z₁ = 4
M = 4 × (1 203.6106 − 1.007276) = 4 810.4132 Da
Cross-check with z₂ = 3:
M = 3 × (1 604.4794 − 1.007276) = 4 810.4163 Da
Agreement to 0.003 Da (0.6 ppm). The assignment is secure.The important property of that calculation is that it is self-checking. Two charge states give two independent estimates of the same neutral mass; if they agree, the charge assignment is right. A deconvolution based on a single peak is an assumption dressed as a measurement.
Failure modes
| Failure | Signature | Consequence |
|---|---|---|
| Charge state off by one | Two charge states give masses differing by ~M/z | Mass error of hundreds of daltons; obvious once checked |
| Adduct mistaken for the protonated species | Deconvolved mass is +22 or +38 above theory | Apparent mass error of 5–8 ppm on a 4 kDa peptide |
| Dimer at half the expected m/z spacing | Charge states appear at non-integer spacing | Apparent mass double the true value |
| Overlapping envelopes from two co-eluting species | Non-Gaussian intensity distribution; extra peaks between charge states | Deconvolution algorithm returns a weighted average of two masses — a mass that corresponds to nothing |
| Sodiated species dominating a salt-heavy sample | Protonated series weak or absent | Deconvolution to the sodiated mass, reported as the neutral |
Envelope shape as information
The charge-state distribution is not arbitrary. A compact, folded molecule presents fewer basic sites to the solvent and produces a narrower envelope at lower charge; an unfolded or denatured one produces a broader envelope at higher charge. For the recombinant compounds in this index a shift in envelope shape between two lots of the same product is a conformational signal, and it is the only folding information a routine LC-MS run provides.
Normative references
Cited because they are the documents the acceptance criteria above are taken from. The index applies them as written and states every deviation.
- Ph. Eur. 2.2.43 — Mass spectrometry
- ICH Q6B — Specifications for biotechnological products
Related methods
Mass accuracy and calibration
What ppm means at 340 Da and at 4,813 Da, how calibration drifts, and the checks that separate an instrument problem from a material problem.
Identity
Monoisotopic versus average mass
A 3 Da distinction that produces a 620 ppm error when confused. The most common single mistake in submitted identity data.
Identity