The word “confirmed” is doing an enormous amount of work on this certificate
An intact mass measurement establishes elemental composition, at best. The number of distinct sequences consistent with a given composition is astronomically large.
Mass spectrometry
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At 214 nanometres you see every peptide bond. At 280 you see three residues.
Area per cent silently assumes that every species in the sample absorbs as strongly per unit mass as the parent. For peptide impurities that assumption is usually wrong, and…
The change that weighs nothing at all
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.
The arithmetic behind every identity confirmation, worked in full
A peptide has a monoisotopic mass and an average mass, they differ by several daltons at this molecular size, and a certificate that does not say which it quotes cannot be…
The specification sheet says 500,000. The report says conforms.
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
One method is one opinion
An orthogonal method separates on a different physical principle, so that species co-eluting in the first are likely to resolve in the second. Two runs of the same method at…
Disregard limits: what a regulated laboratory does with a tiny peak
Chromatographic software does not integrate every fluctuation in the baseline. It applies a threshold, and the threshold changes the reported purity by amounts that matter…
The reproducible spectrum: the single most useful thing a report can carry
What the Journal asks for when it writes to a supplier about an identity claim, and how often it gets it.
Deamidation, oxidation, and the two shifts worth memorising
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.
The reproducible spectrum: the single most useful thing a report can carry
None of what a checkable identity statement requires is commercially sensitive, and all of it is known to whoever produced the document.
Resolving power and mass accuracy are two different specifications
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
What a mass spectrometer does to a peptide before it measures anything
A brief and unromantic tour of the interface between a liquid sample and a vacuum.
A part per million, in context
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
What “LC-MS: conforms” tells a reader, which is almost nothing
What the Journal asks for when it writes to a supplier about an identity claim, and how often it gets it.
Twelve minutes or forty: what gradient slope buys
A shallow gradient resolves impurities that a steep one runs into the parent peak. Both methods are legitimate; only one of them can see the small stuff.
Before anything is weighed, the peptide has to be charged
A brief and unromantic tour of the interface between a liquid sample and a vacuum.
Two ways to lift a peptide into a vacuum, and why the answer differs
The ionisation method determines the charge states you see, the adducts you must account for, and the modifications you might destroy in the process.
A peak with no chromophore is a peak with no peak
Non-chromophoric components — counter-ions, salts, some excipients — contribute mass to the vial and nothing at all to the chromatogram.
The arithmetic behind every identity confirmation, worked in full
A peptide has a monoisotopic mass and an average mass, they differ by several daltons at this molecular size, and a certificate that does not say which it quotes cannot be…
Pore size, particle size, and why peptides need the wide pores
A shallow gradient resolves impurities that a steep one runs into the parent peak. Both methods are legitimate; only one of them can see the small stuff.