Set the threshold at a tenth of a per cent and the small impurities vanish
We work through a single chromatogram twice, under two integration conventions, and show where the difference comes from.
HPLC
Reversed-phase high-performance liquid chromatography — the method behind almost every purity figure in the trade.
One method is one opinion
Aggregates dissociate in the mobile phase and are recorded as monomer. Only a size-based separation reports them.
The instrument was calibrated on Tuesday. Your sample ran on Friday.
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
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.
Peak capacity, and the honest measure of a separation
Gradient slope is the single most consequential method parameter for a reported purity figure, and it is the one most often omitted.
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.
When a better instrument changes the answer rather than the confidence
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
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…
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.
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.
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.
A percentage is not a specification
This piece takes the measurement apart into the decisions it is made of, because each decision moves the answer.
Five things a purity figure cannot tell you
A chromatogram is silent on everything that does not absorb: counter-ions, water, inorganic salts and non-chromophoric excipients all contribute mass to the vial and nothing…
Calibrated, qualified, accredited: three words that are not synonyms
System suitability is the set of checks demonstrating that the instrument and method were performing adequately when your sample was injected. It is recorded as a matter of…
Tandem mass spectrometry, and the difference between weighing and reading
An intact mass measurement establishes elemental composition, at best. The number of distinct sequences consistent with a given composition is astronomically large.
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.
Counter-ion, water, salt: three masses with no chromatogram
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…
Nought point nine eight of a dalton: the modification that hides in plain sight
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.