[5] What do Chromatograms tell us? Peak Shape: Tailing solvent peak

Chromatograms are like fingerprints.  If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.

Fig.1 Solvent peak shape appears with a strong tail.

A peak shape for the solvent peak is observed as shown in figure 1.  After installing a new column, the solvent is tailing very strong while normally the peak looks like the trunk of a tree.  The tail of the peak contains residual solvent molecules that are injected after the main injection is done.  Very often the tail is caused by a dead volume.

Dead volume will occur:

Fig.2 To prevent dead volumes, cutting fused silica columns needs to be done corrcetly. A 90 degree angle will guarantee the best connections. A simple ceramic wafer will often do.

When there is a coupling and the column ends do not meet exactly. Also column ends may not be cut off at a 90 degree angle. Make sure you use scoring wafer and a good cut, like shown in fig.2;

When column is not installed in correct position:  in the liner (injection port) or when the column is not positioned close to the detection point in the detector; Check with manual from instrument manufacturer.

During splitless injection if split line is not opened after injection time. In splitless injection the injection port must be flushed immediate after the injection time. Sometimes split lines can be blocked due to contamination, see: Contamination of Injection System Split-Vent-Lines: A Maintenance item not to Underestimate

If connection with uniliner is not sealing well:  when using this liner for direct injection technique, a good seal is required. If seal is not optimal, some solvent will go below the seal and will initiate the tail.

Other possible causes for increased solvent tailing:

The injection system may be cold.  This will impact sample transfer. Check inlet temperature;

Too large solvent volume for liner used; solvent will backflash the liner and end up in carrier gas lines.  This will also initiate memory effects/carry over. Check the maximum volume of solvent gas vapor your liner can accommodate. Note that this volume is strongly depending on the molecular weight of the solvent.   Inject less or use a larger diameter liner

Column may show some activity for the solvent. This can happen with polar solvents (methanol). Depending on column brand this can be more or less. New generation columns like Rxi-series, tend to be more inert and will show improved peak shape. Also upon installation some graphite may have entered the column. Make sure that after the graphite ferrule is put on, a 1-2cm piece of column is cut-off;

If the tail develops in time, the column may also be activated by sample matrix or carrier gas contamination. May consider options for guard columns and carrier gas filtration.