Causes of Split Peaks in HPLC and UHPLC

Few things are as unsettling as a peak that used to be clean suddenly showing up as two humps with a dip in the middle, or growing an unexpected shoulder. Split peaks distort your data and make integration unreliable. In both HPLC and UHPLC the causes fall into a few well-understood categories — and once you know them, the problem is usually quick to track down. This guide explains the main causes and how to fix them.

To recognise what a distorted peak looks like against a normal one, our guide on how to read an HPLC chromatogram is a useful starting point.

What a split peak is

A split peak is a single compound’s peak that appears doubled — two maxima sharing the same base, or one peak with a distinct shoulder — when it should be a single symmetrical peak. The key diagnostic question, as with tailing, is whether the splitting affects all peaks or just some of them.

The decisive first question: all peaks or one peak?

• All peaks splitting → the cause is almost always physical (a problem at the column head, a void, a blocked frit, or a bad connection in the flow path).
• Only one (or a few) compounds splitting → the cause is usually chemical (sample solvent mismatch or a pH effect specific to that analyte).

The reasoning behind the rule is simple: when every peak in the chromatogram is split or doubled, the sample was not introduced onto the column symmetrically in the first place — the disturbance happens before any separation takes place, so it affects everything equally [1][2].

Cause 1: A blocked or contaminated inlet frit

This is the single most common cause. Debris from the sample, the mobile phase, or a failed pump seal or injector rotor collects on the inlet frit. As it partially blocks, the sample stream arriving at the column inlet is distorted and every peak is distorted with it [2]. Blockage is usually accompanied by a rise in back-pressure, which is a useful confirming clue [3].

The fix: Replace the guard column or in-line filter first — the cheapest and most likely fix. Reversing the column and backflushing to waste for a few minutes resolves the problem in roughly a third of cases; check the column care-and-use instructions first, since not every column may be reversed [2]. Prevent recurrence by filtering the mobile phase (0.2 or 0.45 µm) and the samples before use. A pre-column filter is the standard protection: a 2.0 µm frit for columns packed with 5 µm or larger particles, 0.5 µm for smaller particles [4].

Cause 2: A void or channel at the column head

Over time the packed bed at the inlet can settle or form a channel. Part of the sample then travels faster than the rest, so the band splits before it is properly separated. This typically affects all peaks equally and is often accompanied by loss of efficiency and small retention-time shifts [1].

A frequent underlying cause is chemical rather than mechanical: operating a silica-based column above its permitted pH range dissolves the silica, the bed loses support and a void forms [5].

The fix: If your column allows it, reverse-flush to try to clear the inlet. If the void is structural, the column must be replaced. Staying inside the specified pH range — or choosing a column rated for high pH — prevents the problem in the first place.

Cause 3: A bad connection in the flow path

An improperly made connection — tubing not pushed fully home, an incorrect ferrule depth, or a small void inside a fitting — creates unswept dead volume that splits peaks. Even a small dead-volume pocket can split narrow peaks, particularly early-eluting ones [1].

The fix: Re-make suspect connections carefully. With fingertight fittings, stop the flow, loosen the fitting, push the tubing firmly to the bottom of the port, then retighten. Keep tubing short and connections minimal — this matters even more in UHPLC, where narrower tubing and smaller particles make the system less forgiving of dead volume.

Cause 4: Sample solvent stronger than the mobile phase

This is the most common chemical cause and typically affects the early-eluting peaks. When a sample is dissolved in a solvent much stronger than the starting mobile phase (for example, a high percentage of organic in reversed-phase), the analyte band is not properly focused at the column head. The front of the band migrates faster than the tail, producing a split or distorted peak. Poor sample solubility and incompletely filled sample loops produce similar symptoms [4].

The fix: Whenever possible, dissolve and inject samples in the mobile phase — or in a solvent weaker than the initial mobile phase. Reducing the injection volume is the quickest diagnostic test: inject a smaller volume and see whether the distortion disappears [3]. See our mobile phase basics guide for background on solvent strength. In HILIC this effect is even more pronounced, because water is the strong solvent — see how HILIC retention works.

Cause 5: pH near the analyte’s pK_a

For an ionisable compound, trouble arises when the mobile phase pH sits within about ±1 unit of the analyte’s pK_a. The compound then exists as a mixture of ionised and neutral forms, which travel at slightly different speeds — producing a split peak for that specific analyte. A mobile phase with the wrong pH, or one missing a tail-suppressing additive, commonly shows up as a peak-shape problem together with shifted retention times [3].

The fix: Move the mobile phase pH well away from the pK_a (typically at least 2 units) using an appropriate buffer, so the compound exists in a single form. This affects only ionisable analytes, which is why it usually splits one peak rather than all.

Cause 6: It is not one peak at all

Before rebuilding the system, rule out the simplest explanation: what looks like a split peak may be two compounds that are almost, but not quite, resolved. A published case study of a cough-syrup assay traced exactly this pattern back to a second component appearing at low concentration [6].

The check: Change the detection wavelength, or look at the ratio of the two humps across different sample concentrations. A genuine split tracks the analyte; a hidden second compound does not.

Diagnostic checklist

• All peaks? → guard/frit, column void, connections (physical).
• Pressure also up? → blockage rather than void.
• One peak, early eluting? → sample solvent too strong; inject less and retest.
• One ionisable peak? → pH too close to the pK_a.
• Worse at high flow, with efficiency loss? → suspect a column void.
• Ratio changes with concentration? → probably a second compound, not a split.

Key takeaways

Split peaks are almost always traceable to either a physical disturbance at the column head or flow path (affecting all peaks) or a chemical mismatch such as strong sample solvent or a pH–pK_a clash (affecting specific peaks). Start with the guard column and the “all peaks or one peak?” question, prevent problems by filtering samples and mobile phase, and inject in a compatible solvent — and most splitting disappears.

References

1. LC Troubleshooting Essentials: A Guide to Common Problems and Solutions for Peak Tailing, Ghost Peaks and Pressure Spikes. LCGC International, 2026 — injection solvent mismatch, dead volume and the “all peaks or only some?” rule. chromatographyonline.com
2. Troubleshooting Basics, Part IV: Peak Shape Problems. LCGC International — a partially blocked inlet frit distorts the sample stream before separation and affects all peaks; backflushing resolves it in about a third of cases. chromatographyonline.com
3. Dolan JW. LC Troubleshooting: The Basics. LCGC International — too much or too strong an injection solvent distorts peaks; a blocked frit usually comes with a pressure rise; wrong pH or a missing additive shows up as peak-shape plus retention changes. chromatographyonline.com
4. HPLC Troubleshooting Guide — split and broad peaks from incompletely filled sample loops, injection-solvent incompatibility and poor solubility; pre-column filter specifications. Sigma-Aldrich/Merck. sigmaaldrich.com
5. Peak Splitting in HPLC: Causes and Solutions — void formation, backflushing procedure and preventive practices. Phenomenex. phenomenex.com
6. Dolan JW. Split Peaks — A Case Study. LCGC International — worked example in which apparent splitting was traced to a second component. chromatographyonline.com