[22] What do Chromatograms tell us? Response of Target Analyte is much Lower as Expected

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.

The chromatogram obtained in fig. 1 shows a low response for the target analytes. Although the peak shape is not that bad, response is too low. This phenomena happens when columns are not installed the right way in the detection port.  Although a lot of attention is required to avoid problems with injection port, the detection side is often neglected. The detection side also can cause significant challenges.

Fig 1: when the column is not put in correct position in FID, compounds can show lower response.. this was a 6890 system.

When using a FID, the detection port liner can become very active because of the H2/air mixtures can enter the detection port liner. This will happen any time the detector is “hot” and the system is opened for column replacement.

Good practice should teach us that we only replace columns when system is cooled down, but do we really do that?

The moment the column is disconnected from detector, air/H2 will enter the detection port liner. At High temperature a lot of reactivity can develop. 

Other reasons for a lower response of target analyte, can be the quality of the injection.  If the sample is not transferred to the column in a quantitative way we will see discrimination. For this, liner choice is very important.

Restek developed several liners to minimize discrimination, but this depends on injection technique used.
Fig.2 Liners recommended for Splitted injection

For split injection, the best liner is the “Restek Premium-precision liner”.  This will give RSD’s < 1%. The trick is, to use deactivated quartz wool which make evaporation very reproducible.  If sample matrix or components react wit matrix, a liner  is needed without quartz-wool. Then we use the Cyclo-splitter, see fig.2.  It’s a more expensive liner, but it will produce good data.

For splitless injections, we need the Restek Premium-single taper with wool at the bottom as the primary choice. This will transfer analytes perfect onto the capillary.  If the matrix or components interact with the quartz wool, use the double-gooseneck or cyclo-splitter for best response. See fig. 3.
Fig. 3 Liners recommended for splitless injection.