Carryover between injections

Carryover is sample from the previous injection appearing in this one. It is a plumbing and a washing problem before it is a chemistry problem. The decision is whether the peak you are about to integrate belongs to the vial in the tray. How separations produce true peaks is covered in chromatography methods in life-science labs.

Where the previous sample hides

Chromatography assumes the band you injected is the band you detect. An autosampler needle, a seat, a loop, a rotor seal, and the detector cell all touch that band. Hydrophobic peptides, basic compounds, and proteins at high concentration adsorb to those surfaces. The next injection's mobile phase redissolves a little of them. You see a peak at the old retention, often a few tenths of a percent to a few percent of the previous area, sometimes much more after a dirty sample. The exact fraction is an empirical measurement, not a constant to copy.

A weak needle wash is the usual reason. Water after a sticky peptide does not clean a steel needle. A strong solvent, often the organic solvent the method already uses, with the same acid if the hardware allows, is the class of wash that works. Some samplers have an external wash and an internal flush of the needle lumen. Both matter. A programme that dips the outside of the needle and never flushes the inside will still carry over. Follow the manual for solvent compatibility. A strong wash that attacks a seal becomes a leak, and the leak looks like missing area rather than carryover.

Manual injection has the same disease in a syringe. A syringe rinsed once with mobile phase and then used for a blank will write last sample into the blank. Rinse with a strong solvent and then with starting mobile phase so you do not inject a strong plug. ISO 8655-1 speaks to volumetric apparatus. It does not replace a rinse. The volume you deliver can be accurate and still be the wrong liquid.

The blank injection is the test

Run the stickiest sample you care about, then one or two blanks in identical vials. Integrate the blank at the sample's retention. Carryover falls on the second blank if it was a surface film. A peak that does not fall is coming from the column or the solvent, which is a ghost-peak problem, and more washing of the needle will not fix it. Swap the order: blank, sample, blank. The first blank should be quiet. If it is not, you have not started from a clean system, and any carryover percentage you calculate is a fiction.

Quantify carryover as area in the blank divided by area in the sample, and write the number in the method. There is no universal pass mark in this article. A trace impurity assay cannot tolerate a carryover that a preparative fraction collection might ignore, because the preparative decision is a gel of a new load, not a 0.1 percent area. Match the test to the claim. If you report a small peak after a large one, the blank between them is part of the result.

Affinity chromatography and ion exchange carry over through valves and through resin that was not stripped. A blank load after a concentrated elution, collected as fractions and assayed, is the equivalent of the HPLC blank. Activity in that blank is leftover protein. Clean as the insert allows before the next real load. Do not rely on a short equilibration to erase a high-capacity bead.

Branching the fix

If blanks fall once you strengthen the needle wash, lock that wash into the method and keep a periodic blank after the dirtiest sample. If the wash is already as strong as the seals allow and carryover remains, reduce the mass injected, add a blank injection as a scheduled rinse, or change needle and seat materials. A scheduled blank that you do not look at is not a control. Look at it.

If carryover appears only after samples in a particular solvent, that solvent is leaving a film. Dry-down and redissolve in the starting mobile phase when the chemistry allows, or dedicate a loop to that solvent and flush it. If a single sample poisons the column so that every later injection shows the peak, the analyte is sticking on the bed, not only on the needle. Extend the gradient or the strip the insert allows, then blank again. Replacing the autosampler will not cure a column that is still loaded.

Shared vials and reused pipette tips imitate carryover. A tip that touched a high standard and then a blank has done the autosampler's job. Fresh tips and a blank vial that never held sample are part of the test. So is the order of the tray: do not place a blank or a low calibrator immediately after the highest standard unless a wash and a blank injection sit between them on purpose.

What you must not conclude

A blank that is clean does not prove the sample peak is pure. It proves the injector did not donate yesterday's sample at that retention. Co-elution inside the sample is a separate limit. A mass spectrometer makes carryover look more dramatic because it is sensitive, and it can also show that the blank's peak has the same mass as the previous analyte. That match supports carryover. Reporting it as an identification in the blank sample would be a HUPO-relevant mistake if you treat the blank as a specimen. The sequence in UniProt does not belong to an empty vial.

Safety

Needle-wash solvents are often the flammable or acidic ones. The wash bottle is a chemical hazard, not a minor accessory. Waste from blanks is still solvent plus whatever bled off the needle. Biological samples leave biological residue on the needle. Your institutional rules decide how that residue is handled. This troubleshooting path is not a clinical contamination policy.

Enquiry

State the analyte class, measured carryover in a blank after a stated load, current wash solvents, and the seal materials you must live with. Ask for a sampler and a wash programme that can use a strong solvent safely. The scientific instruments catalogue lists instrument classes. Send the measurement with the quote request. If carryover sits in a protein purification workflow, the custom protein expression and purification reference can frame the method discussion. ---