#hplc-massspec 2026-05-02

baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column

plus 0.98 is deamidation

214nm sees the amide backbone so it sees everything. 280 only sees aromatics, thats just me

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

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

integration difference

truncations show as a mass deficit matching one or more residues. you need the sequence to say which

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

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

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

you need more sample for identity than for purity. plan for that when you post it, anyway

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

thats co-elution

coming back to this does retention time on its own tell you what the compound is

run a blank

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

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

Reminder for thistle_tests: dose day is today. Set 9 days ago.

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

update from 3 months ago: switched to asking for the chromatogram rather than the number and it changed everything

within inter-lab range

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

*PeptideMeter not the other one

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

ask for the trace

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

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

coming back to this a warm transit can put a visible degradant peak on a chromatogram. that is what 4 days at ambient does

shallow gradient

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

plus 16 is oxidation

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

how much sample does a lab actually need for identity as well as purity

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

do the arithmetic

whats the void volume peak, is that always solvent

show me the spectrum

thats the solvent peak

the honest position is that hobby-level interpretation of a chromatogram is worth something but not much

is monoisotopic or average mass the one on most certificates

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

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

thats co-elution

do the arithmetic

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

slightly off topic but whats a plus one dalton shift usually