[8] What do Chromatograms tell us? Peak Shape: Peak-splitting by poor focusing in on-column Injection

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 peaks that are splitted or even peaks-on-a peak.

The peaks in the chromatogram in figure 1a show “peaks on a peak”. Fig. 1b shows even concrete peak splitting

Fig.2 Use of a retention gap will correct the injection "error" we make when introducing a liquid in to the capillary. Because there is little retention, all components will focus at the head of the coated analytical column

Splitted peaks are usually related to an injection problem, but not always.  In the old days the oven programming was not as sophisticated as today. The peak shape shown in fig 1A was obtained in some GC’s that were developed around the 1980’s. In those days the oven temperature heating was done in “pulses”. The moment a component elutes, the peak could travel faster/slower by a small change of temperature. To fix that, the column outlet had to be isolated using an isolator.  The later oven designs did not show this problem anymore.

The peak shape in 1B is still seen very often and is related to insufficient or non-correct band focusing when doing an injection where a lot of liquid is entering the column. Typically, this is a phenomena that occurs when on-column injection is applied.  The liquid injection onto the column generates a broad and splitted band. To fix this, retention gaps are essential Fig 2 shows how a retention gap works.  The retention gap is a 2-3 m section of capillary that is connected or integrated in front of the analytical column. The retention gap has no phase, is only deactivated and by this, it has very little retention. The sample components will also be spread out over the retention gap, but as soon as heat is applied all components will move fast to the analytical column where they will focus.

 

For info on retention gaps see also:

Using Guard Columns and Retention Gaps in GC (Part 1)

and

Using Guard Columns and Retention Gaps in GC (Part 2)
Fig.3 Retention gaps allow focusing. Here is was used for FAME in Biodiesel application

 

Fig 3 shows the difference when a retention gap is applied in biodiesel triglyceride analysis (bottom) and when not (top);

Important for good operation of retention gaps: one needs a retention gap that is wettable for the solvent of the sample; If droplets are generated in retention gap a new problem will be created. See another blog for this.