John V. Hinshaw

Why Do Peaks Tail?

The familiar Gaussian peak shape really is only an approximation. It arises from a simplified peak elution theory that assumes, among other things, that interactions between solutes and the chromatographic system are symmetrical and independent of solute concentration. In a real instrument, solutes encounter extra mobile phase volumes in inlets, connections, and detectors, and the solutes' chemical interactions with the system are composed of multiple, often nonideal, concentration dependent encounters with the stationary phase, the support, and other materials.

Figure 1

A number of problems can cause volumetric peak tailing in GC. Installing a fused-silica column with the tip of the column too low in a split–splitless inlet can create tailing peaks because the full carrier gas split flow cannot reach the column entrance and sweep residual sample away. A similar problem exists in some detectors if makeup gas is not used: solute molecules can become trapped in unswept detector areas.

Volumetric peak tailing is a potential problem in splitless injection as well. Sample vapors expand rapidly and fill the inlet liner during the initial splitless injection period after sample has been deposited in the inlet liner. The split vent flow is disabled, so sample vapors flow through the liner and enter the column at the column flow rate: as much as 2 min can be required to transfer 80% or more of the vaporized sample into the column. Allowing this low flow condition to persist for much longer results in a severely tailing solvent peak that might obscure early eluted peaks. By establishing a high split flow after sample transfer into the column, the back of the solvent peak is cut off effectively.

Lack of sufficient septum-purge flow also can cause peak tailing when some solute or solvent escapes from the inlet liner up into the septum area during injection and is trapped there. The volatile material will escape slowly, and if not swept away by septum purge flow, it will be free to diffuse back into the carrier gas split flow and on into the column.

Tailing due to adsorptive interactions: As solutes move through a chromatographic system, they can undergo interactions other than normal adsorption–desorption onto or solution–dissolution into the stationary phase. One effect familiar to gas chromatographers is caused by active sites on surfaces exposed to the solutes — such as a poorly deactivated inlet liner or exposed areas of the column wall or support material. Such sites can engage in additional solute retardation mechanisms that hold solutes back from the main peak and cause tailing peak shapes. These interactions tend to manifest at lower solute concentrations, and as such, usually are present to some degree. Higher solute concentrations can provide enough additional material to saturate the active sites so that only a small fraction of the total solute present is affected.

Poorly cut capillary column ends can expose active surfaces to the sample, as will the careless insertion of a column into a ferrule without making a clean cut afterwards. Accumulation of nonvolatile sample residues in the inlet or the column entrance also can lead to peak tailing, which will be cumulative as the residues build up from one run to the next. Sometimes overheating a column, or exposure to large amounts of the wrong solvent, will remove or strip the stationary phase and result in peak tailing from the exposed, more active surfaces.

It is difficult to avoid some degree of peak tailing for polar or labile peaks that strongly interact with surface imperfections. Carboxylic acids as well as strongly basic compounds will tail strongly, although specialized stationary phases are available for many such problem samples.