[13] What do Chromatograms tell us? Tailing peaks due to Aggressive Matrix: Destruction of column inlet section: Amine – Water samples

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 If the stationary phase is attacked, the components will elute as a "chair" type peak

In chromatography separations can stay the same for a long time if the sample is pure and all components elute from the GC column.

Practically this is not always the case. More realistically, sample preparation is kept minimal as it adds cost. The price to pay is, that sample inlet systems (liners) and columns get contaminated and need periodic maintenance.

A typical challenging application is amine analysis. It is known that amine analysis is difficult because of the acidity of glass surfaces (including liner and column). Columns have to be base-deactivated in order to eliminate unwanted surface interactions.

Fig. 1 shows a typical challenge that occurs when doing amine/water matrix. After a few injections the amine-peaks do not elute as symmetrical peaks.  This is typical for a destruction process happening at the inlet section of the column.  The impact depends on the way the phase is bonded and the type of surface treatment used.

To ensure lifetime, one can:

• Use guard– or pre- columns: These have to be replaced after every X analysis; Lifetime depends on the technology and quality of the pre-column;
• Inject smallest possible amount: this will always increase lifetime. It’s not always possible as often small levels must be measured;
• Use columns with high capacity:  meaning: Thick-films. Thick films will have good shielding effects and the analytes will elute at higher temperatures. (peaks will look better because of reduced adsorption);

Fig.2 using base surface deactivation like use in the Rtx-Volatile amines, allows amine-water samples to be injected with a more acceptable lifetime

Restek developed a base surface deactivation, which is used for the Rtx-5amine and the Rtx-35 amine. It was also used together with a thick-film, which resulted in the Rx-Volatile amine column. This showed a significant better resistance to water/amine mixtures compared with standard phase technologies; Fig 2 shows a result after 40 amine/water injections. The amine peak shape is still very good. 

Another thing to consider is that when doing amine-water injections, often the syringe plunger gets stuck.  This can be solved by regular rinse with iso-propanol.  Iso-propanol acts as a lubricant which prevents the plunger from being stuck.

One can also use a syringe with a PTFE tipped plunger.