Troubleshooting a flat chromatogram

A flat chromatogram means the display shows no peak you are willing to integrate. The decision is which of six causes to test first, because replacing the column is the slowest and often the wrong one. Separation modes that can legitimately put your analyte in the flow-through are summarised in chromatography methods in life-science labs.

A flat line is a signal about the signal

Chromatography can be proceeding while the detector reports nothing. The trace is detector response against time or volume. If the detector is dark, mistuned, or scaled out of the conversation, the bed can be full of analyte and the screen stays quiet. Work from the detector inward, then the injector, then the plumbing, then the chemistry.

Confirm the lamp or the source is ignited and within the life the manual treats as usable. An ultraviolet lamp that has failed, or a fluorescence lamp that has not warmed, gives a flat or noisy floor. Read the wavelength. Proteins poor in aromatic residues are quiet at 280 nanometres and obvious near 214. A peptide method left on 280 after a protein method will look empty. A fluorescence detector with the wrong excitation or emission pair is dark even when the ultraviolet channel on the same flow cell is busy. If a mass spectrometer is the detector, a flat total-ion trace can mean the spray stopped. Look at the spray current or the calibrant before you blame the column.

Look at the scale. Integrators remember the last big peak. A 10 milli-absorbance peptide on a 2 absorbance axis is a straight line to the eye. Autoscale the raw channel. Look at the whole run, including the first minute, where a solvent disturbance proves that liquid reached the cell.

Did anything get injected?

An autosampler can report a finished method and never have entered the vial: an empty position, a needle that stopped above the liquid, a vial cap the needle did not pierce, a volume of zero left over from a previous line. The cheapest test is a standard of known response in a vial you can see, injected twice. Two flats mean the injector or the detector. A standard peak and a flat sample means the sample vial, the sample solvent, or the chemistry. Watch the syringe if the instrument allows a diagnostic stroke. A leak at the needle seat can send the injection to waste while pressure barely changes.

A leak upstream of the column drops pressure and usually drips. A leak downstream, at the cell outlet, can leave pressure almost normal and still starve a fraction collector, while the detector itself still sees the peak. If the screen is flat and the bench is wet, stop and find the spray. Do not keep injecting. Solvent on a hot lamp housing is a different incident from a dull trace.

Chemistry that is honestly quiet at the detector

Affinity chromatography and ion exchange are supposed to give a quiet elution when the target never bound. The sample is then in the flow-through. Collect it and assay it. A histidine-tagged protein in the presence of EDTA, or a protein loaded onto ion exchange in high salt, will not appear later in the gradient. The trace is "flat" in the elution window because the separation worked as the physics required. Assay early tubes before you change the wavelength. The companion page on affinity and ion-exchange protein purification is the binding logic. Here the rule is only: an empty elution window is not an empty column until the flow-through has been tested.

Some analytes have nothing for the chosen detector to see. A sugar on an ultraviolet detector, a protein without tryptophan on a 280 nanometre channel, a molecule that does not ionise in the mass spectrometer. UniProt will show whether a protein sequence is poor in aromatic residues. That predicts a weak 280 nanometre peak. It does not prove the tube is empty. Switch detector principle or assay the fractions. Identifications, when you eventually have ions, belong to the reporting practice HUPO discusses, not to the absence of a line.

Leaks, air, and a pump that is not moving liquid

A pump that has lost prime delivers little volume. Pressure sits low and the retention clock is fiction. You may see no solvent disturbance at all. Purge as the manual describes, then confirm flow by collecting into a graduated vessel for one minute. If the collected volume matches the set point and the standard is still flat, return to the detector. If the collected volume is near zero, stay with the pump. Air from an empty solvent bottle is the usual reason. A check valve held open by a precipitated buffer is the reason that looks like air and does not purge away until the salt is dissolved.

What to select instead of a new column

Specify a detector that can see the analyte, with the wavelength or ionisation mode written down. Specify a standard and the response you expect at a stated injection volume. Specify a leak check and a flow check as the first system-suitability lines, before any sample whose absence would be scientifically interesting. Specify that flow-through will be collected on affinity and ion-exchange methods. A column purchase does not answer a dark lamp. When you do replace hardware, match the cell path and the wavelength range so yesterday's method is still a method.

In a laboratory where power drops, a detector that rebooted can come back with the lamp off and the previous scale loaded. Make lamp status and autoscale part of the restart, and inject a standard before the sample set. A flat afternoon after a power cut is an instrument state until the standard says otherwise.

Safety and the claim

A flat trace is not evidence of absence for a hazard. Treat the unseen sample as still in the vial, the flow-through, or a leak pan. Solvent leaks are chemical exposures. Biological loads stay inside your institutional rules. The WHO laboratory biosafety manual frames the biological side. It does not find a peak. This page is not a diagnostic interpretation of a blank clinical chromatogram.

Enquiry

Name the detector, the wavelength or mode, the standard that should have responded, the injection volume, whether pressure was normal, and whether the flow-through was assayed. Ask for hardware that reports lamp state and that fits the column chemistry you already run. Use the scientific instruments catalogue and the quote request. A protein method question can be discussed from the custom protein expression and purification reference as an enquiry prompt. ---