Carryover: When the Previous Sample Appears in Yours
An autosampler injects one sample after another through the same needle, loop and tubing. When traces of the previous sample remain and appear in the next chromatogram, that is carryover. It is common, it is well understood, and on a peptide method it can manufacture an impurity that was never in the vial.
Why peptides are prone to it
Peptides adsorb to surfaces. Basic residues interact with residual silanols and with steel; hydrophobic sequences stick to polymer tubing and seals. A hydrophobic peptide that barely elutes from the column under the gradient is, by the same chemistry, reluctant to rinse out of the flow path.
Concentration makes it worse in a specific and predictable way. Analysing a concentrated standard immediately before a dilute sample is the classic arrangement for producing a carryover peak, because the residue left behind is large relative to what follows.
What it looks like on the trace
A small peak at exactly the retention time of a compound from a previous run. That precision is the tell: a genuine impurity has no reason to elute at precisely the retention time of the last sample’s main peak, and a carryover peak does.
Two consequences follow. In a purity determination, the phantom peak is counted as an impurity and the reported purity is lower than the material deserves. In an identity or trace analysis, it can suggest the presence of something that is not there at all.
How a laboratory rules it out
- Blank injections. A solvent blank run immediately after a sample should show nothing at the sample’s retention times. Blanks bracketing a run are the standard control, and their absence from a method is a gap.
- Needle wash. Most autosamplers rinse the needle between injections. The wash solvent has to be strong enough for the analyte — a water-rich wash will not remove a hydrophobic peptide, and a wash that works for one compound may not work for the next.
- Run order. Sequencing from low concentration to high, and separating unrelated compounds with blanks, limits the opportunity for carryover to matter.
Carryover checks sit alongside the other pre-run controls in system suitability, which establishes that the instrument was capable of a meaningful answer that day.
Why it matters when comparing certificates
A small unexplained impurity peak on one laboratory’s chromatogram and not another’s is sometimes a real difference between samples and sometimes a difference between run sequences. Neither laboratory is wrong; they analysed the same material in different company.
It is one more entry on the list of reasons two reports disagree, alongside wavelength, column, gradient and integration convention. Why certificates of analysis disagree covers the set, and peak integration covers the one that moves the number most often.
What to look for
On a published chromatogram, an impurity peak with no corresponding entry in the impurity table, or a peak the report does not attempt to assign. Neither proves carryover, but both are worth a question, and the answer — whether blanks were run — is a fair thing to ask any laboratory.
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