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Amines: Topaz or Base Deactivated Liners?

5 Dec 2019

Amines can be difficult to analyze by GC, since they are active and adsorb to surfaces within the chromatographic system, including the inlet liner and the column.  This leads to loss of compound response, and peak tailing.  While deactivations can help to mitigate these effects, the quality of deactivations varies.  Primary amines are especially difficult, due to their susceptibility to interact with active acidic sites like silanols, commonly found in liners and columns.

Restek offers a base deactivation for both liners and columns, designed to passivate glass surfaces with basic sites, lessening the chances of adverse reactions with amines and other basic compounds.  As you may be aware, Restek launched Topaz inlet liners in 2017 as our newest premium liner deactivation.  One question that has come up a few times is “How do Topaz liners compare to base deactivated liners for analysis of difficult amines?” A study was conducted to compare responses of active bases on the two deactivation chemistries.

The method conditions shown in Table 1 were used for the comparison.  A mix containing primary, secondary, and heterocyclic amine groups, as well as an ethanolamine and an aniline, was evaluated on Restek’s base deactivation and Topaz deactivation (See Figure 1).  Two different concentration levels were tested on each liner.  A total of 7 liners from each deactivation were evaluated.  Liners were single taper with wool tested in splitless mode, as this provides a very rigorous test of the deactivation.

Table 1: Method conditions for testing liners for amine response.

Figure 1: Amines that were tested on the liners.  Chemical structures from Wikipedia.

Figure 2 shows how response factors for various amines at 5-10 ng compare on the two deactivations.  Pyridine and 2,6-dimethylaniline had similar performance on both deactivations.  Examining more difficult amines like diethylenetriamine (DETA) and diethanolamine (DEA), the Topaz deactivated liner had higher compound responses, though the relative liner to liner variation for diethanolamine was slightly higher for Topaz compared to the base deactivated liners.  Diethylenetriamine is very difficult to analyze by GC as it contains two primary amine groups and a secondary amine group.  Diethanolamine contains a secondary amine group, but also has two alcohol groups, which can be reactive.

Figure 2: Amines response comparison on Topaz and Base deactivated liners at 5-10 ng splitless.

Figure 3 shows a comparison of the same amines at 2.5-5 ng.  Once again, pyridine and 2,6-dimethylaniline had similar performance on both deactivations.  DETA, on the other hand, was non-detect on the base deactivated liner at 5 ng.  The Topaz liners showed high liner to liner variability for DETA, but did elicit a response on all liners tested.  Topaz also showed better response for DEA compared to the base deactivated liner, though there was more variation from liner to liner.

Figure 3: Amines response comparison on Topaz and Base deactivated liners at 2.5-5 ng splitless. Diethylenetriamine was non-detect on all base deactivated liners tested at 5 ng.

The above study demonstrated that the Topaz liners generally had better responses for the most difficult amine compounds that were tested here, compared to the base deactivated liners.