GC Inlet Liner Selection, Part I: Splitless Liner Selection

Splitless injections are used when detection of trace amounts of analytes is necessary and the goal is to recover close to 100% of all analytes that are injected into the instrument.  During a splitless injection, the split vent is closed for a predetermined amount of time, directing all inlet flow onto the column (with the exception of the septum purge).  Because of the slow flow rates, splitless injections can be tricky.  These slow flow rates can contribute to band broadening (wider peaks), as well as longer residence times in the liner, leading to increased interactions with any active sites.

I conducted an experiment to compare various liner configurations for use with splitless analyses of liquid injections.  I wanted to compare liners based on recovery across a wide molecular weight range, as well as reproducibility from injection to injection.  To do this, I injected a series of hydrocarbons ranging from C10 up to C40.  As these compounds are all in the test mix at equal mass, ideally, peak area responses for all compounds should be the same.  A common phenomenon, known as molecular weight discrimination, occurs in the GC inlet when there is incomplete vaporization and therefore incomplete transfer of heavier compounds, with the heaviest compounds showing much less recovery compared to lighter compounds.

The following liner configurations were compared using the splitless conditions listed in Table 1 below.

Table 1: Instrument conditions for liner comparisons.

Figure 1 shows how some common liner configurations compare for peak area response across the molecular weight range when performing splitless injections:

Figure 1: Comparison of peak area response across a wide molecular weight range for various liner configurations used in splitless mode.

As previously mentioned, an ideal liner will minimize molecular weight discrimination, leading to equal responses across all compounds.  You can see that the single taper liner with wool and the double taper cyclo liner achieve this.  Both the wool and the cyclo corkscrew provide extra surface area, which enhances vaporization by increasing the heat capacity of the liners.  The low pressure drop liner had similar performance for C20 and higher; however, there is some loss of more volatile compounds, perhaps from the wool being located higher up in the liner, leading to losses out of the septum purge vent. The presence of a taper helps to direct the sample to the column, as well as minimize interactions with the gold seal, which can otherwise be detrimental to performance with the slow carrier gas flows used in splitless injections.

When it comes to reproducibility, the liners with wool and the cyclo corkscrew also performed best (See Figure 2).  These features create a turbulent zone, allowing for reproducible mixing with the carrier gas upon injection.  They also serve to “catch” the sample, preventing analytes from hitting the bottom of the inlet where they can condense and get lost.

Figure 2: Liner reproducibility comparison across wide molecular weight range.

Conclusions

Overall, I would recommend the use of a single taper liner with wool or a cyclo liner for use with splitless injections of liquids.  The single taper liner with wool is the more cost effective solution; however, for those that do not want to use wool, the double taper cyclo is a viable second choice if you’re looking for the best splitless performance.  As you can see from the data above, if your analytes are on the more volatile side of the spectrum, the use of wool or a cyclo may not always be necessary for recovery and reproducibility.  Depending on matrix, though, these features can help to catch involatile material, as well as septa particles, preventing column contamination.