PeptideStack
5.2kquestions
20kanswers
220users

How do I reconcile 97.1% from Medutest with 96.8% from the supplier?

Asked 19 Nov 2024Modified 17 months agoViewed 18k times
29

Setup, so nobody has to ask: 97.1% · Medutest · 96.8%.

I noticed this today and I have not touched anything since, in case the state is diagnostic.

I have not discarded anything yet, so a test is still possible if that is the recommendation.

How do I distinguish the benign explanation from the one that matters?

purity
purity

Purity as chromatographic area per cent - the fraction of detected material that is your target peak. It says nothing about how much material is…

788 questionsbatch-testing
batch-testing

Testing at the batch or lot level: sampling plans, how many vials from a lot need testing to say anything about the lot, and the difference…

865 questionscoa
coa

Certificates of analysis: what fields a useful one carries, how to tell a real analytical report from a marketing document, batch and lot…

749 questions
shareeditfollowflag
AH
askedanja_hellstrom13k1619 Nov 2024
6Two of us worked through this independently and arrived here, so it is at least reproducible. – halvard_ness 6 months ago
7Worth adding that the method section is where the answer usually is. – tandem_gradient 7 months ago
add a comment

3 Answers

Accepted answer first, then by votes
34

Accepted answer

The part that matters: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

Compare purity within a single laboratory on the same method, never across laboratories.

edited 21 Feb 2025 by lane_transit — fixed an arithmetic slip in the third paragraph

shareimprove this answerflag
LT
answered · acceptedlane_transit42k3822 Jan 2025
Thank you — the worked example is what makes this usable. – tare_weight 2 months ago
Related: the same reasoning applies to the counter-ion question. – RP_C18 4 months ago
add a comment
Sponsored

PeptideMeter - Independent Peptide Analytics

Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.

Browse results
34

Put another way, the honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

On the detail: integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

shareimprove this answerflag
TU
answeredtenth_of_a_unit40k3831 Dec 2024
22

The relevant detail is that read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

If you are ranking vendors, specify a method and have all samples tested at the same place.

shareimprove this answerflag
DS
answeredDr_Hanne_Solberg40k3811 Jan 2025
The distinction between purity and content cannot be repeated often enough here. – day_seven_trough 5 months ago
add a comment

Your answer

Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.

Not medical advice. Research-use-only compounds are not approved for human use.