Is a 99.4% result from Janoshik comparable to 97.1% from Janoshik?

Details up front: 99.4% · Janoshik · 97.1% · Janoshik.

I have used one of these for a while and I am considering switching, which requires a reason.

What I care about is reproducibility, because a result I cannot repeat is not useful to me.

What does each option buy me, and what does it cost me?

More usefully, reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.

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.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

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

It helps to be literal here: start from what the detector sees, because that tells you what the number means.

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

The relevant detail is that 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.

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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."

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

On the detail: gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

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

edited 2 Nov 2024 by stopper_core — fixed an arithmetic slip in the third paragraph

Put another way, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

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.

Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.

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