#hplc-massspec 2025-05-10

thats co-elution

my peak has a shoulder on it, what does that usually mean

quick one is uv at 214 or 280 better for peptides

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

a warm transit can put a visible degradant peak on a chromatogram. that is what 11 days at ambient does

Verification log updated: CPC — evidence added, status unchanged.

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

within inter-lab range

nobody runs enough blanks, including me

if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result

relative retention time against a standard is comparable between labs. absolute minutes are not
the first peak is the solvent. it is always the solvent

for the archive 214nm sees the amide backbone so it sees everything. 280 only sees aromatics

plus 16 is oxidation

do the arithmetic

ms/ms sequencing is the only thing i would call identity. everything else is circumstantial, thats just me

oxidation is about plus 15.99 Da and is usually methionine or tryptophan

whats a normal gradient for a peptide this size

ask for the trace

co-elution is the failure mode that a beautiful single peak is most likely to be hiding

truncations show as a mass deficit matching one or more residues. you need the sequence to say which
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2

coming back to this two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily

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

genuine question semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2, still working it out

a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you

whats the void volume peak, is that always solvent

why do two labs integrate the same peak differently

shallow gradient

if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic, ask me again in a month