What is causing my GC peaks to split?

Peak splitting is when a single compound produces two peaks or a peak with a visible notch or shoulder rather than a single clean peak. It looks like the compound has been partially separated into two, even though it is a single substance.

Inlet issues

The most common cause. In splitless injection mode, if the splitless time is set incorrectly or the liner geometry does not suit the injection volume, the sample vapour can expand unevenly in the liner. Part of the sample enters the column immediately and part follows after a delay, producing a split peak.

Check the splitless time, injection volume, and liner type. A liner with a restriction or glass wool packing helps focus the sample before it enters the column.

In split injection mode, splitting is less common but can occur if the liner is heavily contaminated or damaged.

Column installation

If the column protrudes too far into the inlet or does not extend far enough, the sample vapour can take two different paths into the column. This produces a split peak. Check the column position against the manufacturer's recommended insertion depth for your inlet type.

Solvent effect failure

In splitless injection, the solvent effect is used to refocus the analytes at the head of the column. If the initial oven temperature is too high relative to the solvent boiling point, the solvent effect does not work properly and peaks can split. The general rule is to set the initial oven temperature at least 10°C below the boiling point of the injection solvent.

Mixed solvents

If the sample is dissolved in a mixture of solvents with different boiling points, the two solvents can vaporise at different rates in the inlet, carrying analyte onto the column in two separate pulses. Where possible, use a single solvent for sample preparation.

How to diagnose

If all peaks are split, the inlet is the most likely cause. Check liner condition, column position, and splitless time. If only early-eluting peaks are split, the solvent effect is probably failing. Reduce the initial oven temperature. If splitting appears after a column change or maintenance, recheck the column installation depth.