A mass on a WWB certificate is a beginning, not a conclusion
What a single-stage mass measurement supports, what it excludes, and the gap between the two.
Mass spectrometry
Page 2 of 6 of this archive, newest first.
Two decimal places, one injection, no procedure
This piece takes the measurement apart into the decisions it is made of, because each decision moves the answer.
Baseline construction, and the area that depends on where you draw the line
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…
Why a 0.036-dalton difference is the hardest number in peptide identity
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
Pore size, particle size, and why peptides need the wide pores
Ion-pairing additives sharpen peptide peaks and suppress ionisation in the mass spectrometer, which is why the purity method and the identity method are often not the same…
Size exclusion sees the thing reversed-phase dissolved
Where two methods disagree, the conservative convention is to report the lower figure. It is not universal, and whether a laboratory follows it belongs on the report.
Why purity is the cheapest number to flatter
Peptide content, identity, aggregate burden, moisture and endotoxin are all more informative for some purposes. All of them cost more.
Tandem mass spectrometry, and the difference between weighing and reading
The Journal’s standing position: a mass that matches is necessary evidence of identity and nowhere near sufficient.
Leucine, isoleucine, and a difference no balance can measure
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.
Two analysts, one chromatogram, two answers
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 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.
Why Medutest can tell you the mass is right and not that the sequence is
The Journal’s standing position: a mass that matches is necessary evidence of identity and nowhere near sufficient.
Two decimal places, one injection, no procedure
Chromatographic purity is cheap, fast and comparable-looking. Those three properties, and not its usefulness, explain why it became the industry’s single figure of merit.
Where the solvent front ends is a decision, not a fact
The denominator excludes solvent-front features and injection artefacts by convention. Conventions differ on where the front ends, and a fragment eluting early exists in one…
The software decides what counts as a peak
Baseline placement under a tailing peak is a judgement call with a direct effect on the area. It is one of the few places where two competent analysts genuinely disagree.
What a diode array adds, and why the certificate never shows it
A diode-array detector records the whole spectrum at every time point and can confirm that a peak is spectrally homogeneous. Almost nobody prints that information.
Electrospray, MALDI, and the choice that shapes every spectrum after it
The two ionisation techniques in general use produce different charge distributions, different adducts and different failure modes. Certificates almost never say which was…
One method is one opinion
Where two methods disagree, the conservative convention is to report the lower figure. It is not universal, and whether a laboratory follows it belongs on the report.
Assay against a standard, or area per cent: two different questions
We asked the four independent testing services what standards they run against and what suitability criteria they apply. The answers are printed.
The ionisation line on a report is the most informative line on it
The ionisation method determines the charge states you see, the adducts you must account for, and the modifications you might destroy in the process.