Twelve minutes or forty: what gradient slope buys
We set out the parameters a method disclosure would contain, all of which are known to whoever ran the sample and none of which is commercially sensitive.
Mass spectrometry
Page 5 of 6 of this archive, newest first.
What competing on one number does to a market
A market that competes on one measurement will optimise that measurement. There are legitimate ways to raise a purity figure that involve no change to the product at all.
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.
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.
The figure of merit this trade chose, and the four it did not
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.
Quadrupole, time-of-flight, orbital trap: what each class can and cannot see
Parts per million sound impressive until they are converted into daltons at the molecular weight of the thing being measured.
What a mass spectrometer does to a peptide before it measures anything
The ionisation method determines the charge states you see, the adducts you must account for, and the modifications you might destroy in the process.
Where the solvent front ends is a decision, not a fact
We work through a single chromatogram twice, under two integration conventions, and show where the difference comes from.
Manual reintegration is legitimate, and it should be declared
We work through a single chromatogram twice, under two integration conventions, and show where the difference comes from.
Matrix, laser, needle: the plumbing that determines what you see
A brief and unromantic tour of the interface between a liquid sample and a vacuum.
Soft ionisation: the twenty-year-old trick that made peptide identity routine
A brief and unromantic tour of the interface between a liquid sample and a vacuum.
Why two wavelengths on one run would settle several arguments
Non-chromophoric components — counter-ions, salts, some excipients — contribute mass to the vial and nothing at all to the chromatogram.
D-amino acids are invisible to mass spectrometry, and that is the whole problem
What would be needed to catch each of these, and what it would cost.
Sixteen daltons: methionine, and the easiest degradation to find
Deamidation adds 0.98 daltons. On a low-resolution instrument at incretin molecular weights, that is inside the noise.
Accuracy, precision and the tolerance nobody states
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
Mass spectrometry answers a different question, properly
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.
Resolving power and mass accuracy are two different specifications
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
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…
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.
Why PeptideMeter may run electrospray where another laboratory runs MALDI
A brief and unromantic tour of the interface between a liquid sample and a vacuum.