Leucine, isoleucine, and a difference no balance can measure
Stereochemical inversion changes the molecule, changes its biology, and changes its mass by exactly nothing.
HPLC
Page 3 of 5 of this archive, newest first.
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…
A spectrum is a photograph of an ion, not of a molecule
A brief and unromantic tour of the interface between a liquid sample and a vacuum.
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…
The change that weighs nothing at all
What would be needed to catch each of these, and what it would cost.
A number without a suitability record is a number from an unknown instrument
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…
A spectrum is a photograph of an ion, not of a molecule
The two ionisation techniques in general use produce different charge distributions, different adducts and different failure modes. Certificates almost never say which was…
Deamidation, oxidation, and the two shifts worth memorising
What would be needed to catch each of these, and what it would cost.
Why Janoshik may run electrospray where another laboratory runs MALDI
The two ionisation techniques in general use produce different charge distributions, different adducts and different failure modes. Certificates almost never say which was…
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.
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.
Resolution, tailing, repeatability: the arithmetic before the result
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…
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 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…
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.
The conservative convention: report the lower of two numbers
Aggregates dissociate in the mobile phase and are recorded as monomer. Only a size-based separation reports them.
Resolving power and mass accuracy are two different specifications
Calibration drift is real, unremarkable, and the reason serious laboratories run internal standards.
D-amino acids are invisible to mass spectrometry, and that is the whole problem
Stereochemical inversion changes the molecule, changes its biology, and changes its mass by exactly nothing.