#hplc-massspec 2026-05-03

whats a plus one dalton shift usually

the first peak is the solvent. it is always the solvent

update from 10 months ago: switched to asking for the chromatogram rather than the number and it changed everything

retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result

update on the earlier thing 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

plus 0.98 is deamidation

whats the void volume peak, is that always solvent

baseline is drifting badly, is that the column or the mobile phase

thats the solvent peak

i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer

relative retention time against a standard is comparable between labs. absolute minutes are not

coming back after 5 months, what changed in how you read these

while im here baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column, take that with a pinch of salt

if the lab reports area percent without stating the wavelength, the number is less useful than it looks

integration choices move the number by a few tenths easily. that is most of your inter-lab variance
if the lab reports area percent without stating the wavelength, the number is less useful than it looks

why does integration change the number so much

whats a normal gradient for a peptide this size

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

while im here deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant

a warm transit can put a visible degradant peak on a chromatogram. that is what 4 days at ambient does, thats one data point

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

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

Reminder set. I will post here in 3 days.

a certificate with a number and no chromatogram is asking you to trust the integration you cannot see

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

is uv at 214 or 280 better for peptides

ms/ms or it isnt identity

ms/ms sequencing is the only thing i would call identity. everything else is circumstantial

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

need more sample

nobody runs enough blanks, including me

shallow gradient, i think

can you tell a dimer from the chromatogram alone