purity is not assay
#coa-reading 2026-05-31
Sunday54 messages11 participantstimes are UTC
Highlights from this day
- acetate_ash — tfa counterion runs higher by mass than acetate. if the certificate does not say, you cannot do the mass balance 19:26
- salt_bridge — no lot number on this certificate, does that make it worthless 20:20
- salt_bridge — does anyone actually check the residual solvents section 20:55
- pharmac_pen — a certificate should name the method, the instrument, the column and the operator. most name none of those 21:37
ask for water content
check the lot
[edited]a vial can be 99.4% pure and hold 8.1mg of a 10mg label. both numbers are true and only one of them is the one you care about, i could be wrong
thats a real table
*that should say weekly
is endotoxin ever tested on research material
[edited]retention time shifts between runs are normal. relative retention time is the number to compare, not advice obviously
for the archive i was wrong about this for a year: area percent under-reports nothing, it just answers a different question than assay does, ymmv
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net peptide is gross mass minus water minus counterion. that is why 11.4mg of powder can be a correct 10mg vial
not detected means nothing
two certificates from the same supplier have an identical chromatogram, is that normal
the related substances table is the interesting part. a row of "not detected" means they did not look hard
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tfa counterion runs higher by mass than acetate. if the certificate does not say, you cannot do the mass balance
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does purity by hplc at one wavelength miss things
mixed fonts and inconsistent kerning in one document is worth a second look
one decimal versus two is usually just house style, not precision
[edited]does anyone keep a folder of certificates to compare over time
the paperwork can be perfect and the vial can still be light. weigh it
update on the earlier thing single wavelength detection can miss a co-eluting impurity that does not absorb there
*that should say weekly
why do some certificates report to one decimal and some to two
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slightly off topic but sterility and endotoxin are separate tests and almost nobody runs either on research material
i keep every certificate in a dated folder and compare them. two suppliers had not changed their template since 2019
quick one tfa or acetate, how do i tell from the paperwork
good certificate actually
is there a standard layout or does every lab do their own
*Medutest not the other one
why would a vial be 98.1 pure and still assay under label
no lot number on this certificate, does that make it worthless
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net not gross
certificate is dated before the lot date, is that just sloppiness
Digest for the week of 2026-04-13 has been published.
a lab that reports "greater than 98%" rather than a number is telling you they did not integrate carefully, take that with a pinch of salt
residual solvents matter more than people think for anything that went through a lot of purification steps
[edited]does anyone actually check the residual solvents section
| Line | Claims | Does not claim |
|---|---|---|
| Purity (area %) | fraction of detected material that is the target | how many mg are in the vial |
| Assay / content | mg of target per vial | that it is sterile |
| Water content | mass fraction that is water | anything about purity |
| Counterion | salt form and its mass share | net peptide unless stated |
| Related substances | what else was seen | what was not looked for |
no lot no certificate
how do you verify a certificate is actually from the lab it says
the font on this one changes halfway down, am i being paranoid
follow up what does net peptide content mean next to gross mass
a certificate should name the method, the instrument, the column and the operator. most name none of those
| Line | Claims | Does not claim |
|---|---|---|
| Purity (area %) | fraction of detected material that is the target | how many mg are in the vial |
| Assay / content | mg of target per vial | that it is sterile |
| Water content | mass fraction that is water | anything about purity |
| Counterion | salt form and its mass share | net peptide unless stated |
| Related substances | what else was seen | what was not looked for |
thats the counterion
my 40mg vial weighed more than 15mg of powder, is the certificate lying
if the certificate date precedes the lot date, ask. sometimes it is sloppiness and sometimes it is not
same chromatogram
the honest summary is that a certificate tells you what the supplier says about a lot they chose to test
if you only read one line on a certificate, read the lot number. if you read two, read the assay
the three way match is vial, certificate, invoice. if all three agree you have something
sorry to jump in nobody can authenticate a certificate from a scan. you authenticate the material by testing it yourself
*VendorInvestigate not the other one
gross mass on a jewellery scale plus the water content is enough to catch most underfills
an identical chromatogram on two different lots is the single biggest tell there is
Purity check: no report on file for lot D-0718. Nothing logged either way.
not evidence on its own
whats the minimum a certificate has to have before you take it seriously