#hplc-massspec 2025-04-20

right so i was wrong about tailing factor. it is usually the injection or the pH, not the column

retention time is a hint

has anyone ever had a result where the mass was right and the purity was wrong

run a blank

slightly off topic but baseline is drifting badly, is that the column or the mobile phase

shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly

update from 17 months ago: switched to asking for the chromatogram rather than the number and it changed everything

a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you, n of 1 obviously

how do you spot a deletion in a sequence from mass alone

genuine question integration choices move the number by a few tenths easily. that is most of your inter-lab variance

i asked Medutest for the raw trace and they sent it. that is the right answer to that question

two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly, from memory

how do you tell co-elution from a clean single peak

vial that sat in a warm flat for three weeks, then got tested. purity dropped from 99.1 on the vendor sheet to 96.4. is that the heat or was the sheet optimistic

what did the new impurities look like. names or just numbers

two new peaks, one just before the main one and one just after

that pattern is very familiar. the earlier one is usually deamidation, the later one is often oxidation

deamidation converts an asparagine to an aspartate and adds about 1 Da. one dalton on a 4000 Da peptide is invisible on a cheap mass spec but it changes the charge state of the molecule enough to move it on the column

1 Da does something to retention time?

it is not the mass doing it, it is the extra negative charge. an amide becomes an acid. the molecule is more polar so it comes off earlier

which is why the deamidated peak usually sits at RRT just under 1.00 and people misread it as an unreacted intermediate

the usual suspects and where they turn up

so heat plus time explains what i have without anyone having lied to me

it explains it. it does not prove it. the vendor sheet could also have been generous and the heat did nothing

you cannot separate those two after the fact. this is why the split-sample habit exists, test half on arrival and keep half

which nobody does because it doubles the cost

correct, and so we argue about causes forever

what about the truncation thing, whats a deletion sequence

during synthesis a coupling step fails on a fraction of the chains. those chains carry on growing but they are missing a residue.
you end up with a peptide that is very nearly the product, a bit lighter, and often quite close on the column.
these are the impurities that are hardest to remove because they are chemically so similar to the thing you want

and they would still show up as high purity?

they show up as a related substance if the method resolves them and inside the main peak if it does not

so the shallow gradient thing from the other conversation matters here specifically

that is exactly where it matters

batch lookup: GL Biochem lot GLB-24-11882 — 2 community results on file (purity 98.9% / 99.2%), 0 quantity results, last updated 41 days ago

two purity results and no quantity is the fleet-wide pattern honestly

our own dataset has the same bias the vendors do. cheaper test, more of it

one thing i still do not get. if my vial degraded, is it less potent or differently potent

for a deamidated or oxidised fraction the honest answer is we do not know the activity of that fraction from a chromatogram. it may be less active, inactive, or nearly the same. purity loss is not a potency figure

and anyone who converts a purity drop into a percentage potency loss is making it up

ok. thank you both, thats clearer than anything else ive read