[4] What do Chromatograms tell us? Peak Shape: Overload using Adsorbents in PLOT

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 Separation of Hydrocarbons on an alumina column; separation of butene isomers is not possible due to tailing. Tailing in adsorption GC is often related to overload.

A mixture of hydrocarbons is analyzed using an Rt alumina BOND/KCl capillary. The peak shape for the components is observed as shown in figure 1.  The butene component shows a strong tail which causes a separation problem.   Normally tailing is caused by active sites that are present in the column.

In this case the stationary phase is alumina, which is an adsorbent. The separation process here is based on gas-solid chromatography, which is different from gas-liquid chromatography.  In gas solid chromatography, the components interact with the surface of the adsorbent.  Overloading phenomena are also quite different. Fig 2 shows what the impact is. Using polymers like Rtx-1 as stationary phase, an overloaded peak slowly increases in response, and sharply falls back on the base line. In gas solid chromatography the peak shape is opposite. When the component elutes, the signal increases rapidly, followed by a slow decrease, which results in a tail that starts at the top of the peak and continues until the signal is back to base line.

Fig.2 Peak shape of overloaded component in Gas-Liquid (left) and Gas-Solid (right) interactions

This is what we observe in figure 1: the butane peaks are overloaded.

Figure 3 shows the impact of injecting 5 and 50 ng per component on to an Rt Alumina BOND/KCl column. The impact of overload is very clear.  Also note that the peak maximum of the overloaded peak, elutes at a shorter retention time.

This phenomena will occur with all types of PLOT columns (Porous Layer Open Tubular), as well as their packed equivalents. Popular adsorbent materials like Shincarbon, Molsieve 5A and 13X, Silicagel, Rt-Q, QS, S and U BOND, Alumina BOND / MAPD and CFC will behave all similar on overload.

Fig.3 Injection of less sample on to the column results in a more symmetrical peak. Also note the change in retention time when the component is overloaded.

To improve the peak shape:

Inject less sample on to the alumina column. This can be done by reducing the injection volume or using a higher split ratio; Best is to work at high sensitivity setting;

One can also use a column with a higher loadability/capacity. As for PLOT columns, there are limitations on layer thickness, this is not always an option. For highest loadability, best is to use the 0.53mm series.  One can also consider to use 0.53 micropacked MXT column, although plate number is sacrified.