Accuracy, precision and the tolerance nobody states
Parts per million sound impressive until they are converted into daltons at the molecular weight of the thing being measured.
Mass spectrometry
Identity confirmation by mass, and the difference between a mass that matches and a sequence that is confirmed.
Isobaric substitutions: identical masses, different molecules
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.
The word “confirmed” is doing an enormous amount of work on this certificate
The Journal’s standing position: a mass that matches is necessary evidence of identity and nowhere near sufficient.
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.
When a better instrument changes the answer rather than the confidence
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
What orthogonal actually means, and what it does not
Aggregates dissociate in the mobile phase and are recorded as monomer. Only a size-based separation reports them.
Five things a purity figure cannot tell you
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…
Carbon-13, and the reason a peptide has more than one mass
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…
Five things a purity figure cannot tell you
Aggregates dissociate in the mobile phase and are recorded as monomer. Only a size-based separation reports them.
System suitability: the five checks that make a run interpretable
Accreditation to the international competence standard covers the scope a laboratory has been assessed for, which is not necessarily the test you commissioned.
Nought point nine eight of a dalton: the modification that hides in plain sight
What would be needed to catch each of these, and what it would cost.
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.
Why VendorInvestigate 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.
What a reference standard is, and what an in-house standard is not
Matching a retention time against a standard is evidence of consistency, not proof of identity. Two different species can elute at the same time on one method.
What orthogonal actually means, and what it does not
The most useful single addition to any purity determination is a second separation under a different pH or on a different stationary phase, and the second-most useful is a…
Counter-ion, water, salt: three masses with no chromatogram
Aggregates dissociate in the mobile phase and are recorded as monomer. Only a size-based separation reports them.
Quadrupole, time-of-flight, orbital trap: what each class can and cannot see
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
One measurement, twenty companies, forty compounds
The Journal has read several hundred certificates from twenty companies. The number is almost always there; the method behind it almost never is.
Why VendorInvestigate 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.
Leucine, isoleucine, and a difference no balance can measure
Stereochemical inversion changes the molecule, changes its biology, and changes its mass by exactly nothing.