#hplc-massspec 2026-05-13

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

retention time is a hint

why does integration change the number so much

nobody runs enough blanks, including me

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

while im here tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5

need more sample

how would a truncation sequence show up

monoisotopic and average diverge more the bigger the molecule. say which one you are quoting

ok so i have a number at about a third of what i expected, is that a charge state

On this day 6 years ago this channel logged 28 messages.

do the arithmetic

follow up semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2

New independent result logged — WWB, lot F-1330, purity 99% (PeptideMeter).

ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
214nm sees the amide backbone so it sees everything. 280 only sees aromatics

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

thats co-elution

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

plus 16 is oxidation

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

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

is monoisotopic or average mass the one on most certificates

whats the void volume peak, is that always solvent

integration difference

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

Purity check: no report on file for lot B-0114. Nothing logged either way.

update from 5 months ago: switched to asking for the chromatogram rather than the number and it changed everything

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

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

why do two labs integrate the same peak differently

does ms/ms actually prove identity or just make it very likely

plus 0.98 is deamidation

run a blank

does retention time on its own tell you what the compound is

is uv at 214 or 280 better for peptides

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

a shoulder on a peak is information, and usually bad information

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