GC Inlet Liner Selection, Part III: Inertness

The inlet liner is the first surface analytes will interact with after introduction into a GC.  It is critical that liners are deactivated, as a number of adverse interactions can occur between analytes and the glass surface.  Deactivations typically involve some type of silanization of the surface to cover active sites inherent in glass, such as silanols.  Most liners are made of borosilicate glass, which in addition to active groups like silanols, can also contain metallic impurities (Figure 1).

Many liners are also packed with glass wool; which serves to aid in vaporization, helps to mix the sample, and protects the column from non-volatile material.  Wool can be either borosilicate or quartz, with quartz being preferable, as it contains less impurities.  The high surface area of wool presents challenges in deactivation, where providing comprehensive coverage is essential.

Figure 1: Examples of active sites found on glass surfaces, requiring the need for deactivation. The steric variations of silanols found on the surface require a comprehensive deactivation procedure.

Given the potential activity shown in Figure 1, there are two main types of adverse interactions that can occur.  One is chemical reactivity and the other is adsorption, explained in further detail below.  It is important to note that not all liner deactivations are created equal.  As seen in Figure 1, several steric variations of silanols are possible, making thorough deactivation difficult.  Depending upon the specific deactivation reagents, as well as the process with which they are applied, some deactivations may have better performance for active analytes than others.  Restek offers Topaz liners, as our premium deactivated liner.  Topaz performs well for a variety of active analytes, including various pesticides, acids, and bases.

Chemical Reactivity

Chemical reactivity occurs when an analyte reacts within the liner to form new products.  The high temperatures in the inlet combined with active sites can lead to chemical reactions for active analytes.  This can affect the accuracy of the GC analysis, since analytes not originally found in the sample will be produced upon injection.  One example of this is the “breakdown” of endrin and DDT, two chlorinated pesticides.  Upon introduction into a GC, endrin can react to form endrin aldehyde and endrin ketone and DDT can dechlorinate or dehydrochlorinate to form DDD or DDE, respectively. Figure 2 demonstrates how liner deactivation can affect compound performance, showing different levels of endrin and DDT reactivity.

Figure 2: Comparison of endrin and DDT breakdown on three different liner deactivations. Breakdown percentage is calculated as the relative percentage of reaction products vs total amount of parent analytes introduced into the system. Only endrin and 4,4’-DDT were injected into the system at 50 ppb and 100 ppb, respectively. Liners were single taper with wool, analyzed in splitless injection mode.

Adsorption

Analytes can adsorb to the liner surface through interactions such as hydrogen bonding and Van der Waals forces.  Adsorption can be reversible or irreversible.  With reversible adsorption, analytes may temporarily interact with the liner surface and then slowly load onto the column, potentially leading to peak tailing.  Irreversible adsorption, on the other hand, results in total loss of the analyte, with the analyte “sticking” in the liner.  Figures 3 and 4 show some examples of adsorption for acidic and basic compounds.

Figure 3: Example of 2,4-dinitrophenol response on two different liner deactivations, one showing low recovery due to adsorption within the inlet liner. Liners were single taper with wool, analyzed in splitless mode.

Figure 4: Example of benzidine response on two different liner deactivations, one showing lower recovery due to adsorption within the inlet liner. Liners were single taper with wool, analyzed in splitless mode.

A Word on Injection Mode

While these interactions can occur when using either split or splitless injections, liner inertness is especially critical for splitless injections.  Splitless injections have lower total inlet flows, leading to longer liner residence times for analytes and therefore more time for adverse interactions to occur. Longer residence times subject compounds to higher temperatures in the inlet. In addition, splitless injections are generally used for trace analyses, where activity can have a much larger impact due to the larger ratio of active sites to analyte molecules.  Because of this, using split injections, if possible, will greatly reduce the impact of inlet/liner activity on your analysis.

Conclusions

It is important to be aware of the adverse interactions that can occur within liners in order to optimize analyte recoveries and preserve the integrity of your analyses.  If you are working with active analytes, such as pesticides, acids, and bases, it is essential to choose a liner with a good deactivation as a starting point.  Keep in mind that with use, liner performance may degrade as a result of build-up of non-volatile matrix material.  This will result in the same types of adverse interactions discussed above.  Monitor analyte performance for any signs of chemical reactivity or adsorption, which signals that it’s time to change out your inlet liner.

Also be aware that if you’re using a splitless injection, switching to a split injection can significantly reduce the impact of inlet/liner activity on your analysis.  The catch is that you must still be able to meet method detection limit requirements in order to make this switch.