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.

 

6 Responses to “GC Inlet Liner Selection, Part I: Splitless Liner Selection”

  1. Dear Linx,

    Thank you for sharing your results regarding different GC split/spliless inlet configurations.

    I was wondering, why the inlet temperature is not increased close to maximum for the S/SL inlet? The boiling point for C40 is much higher than 300 C. A too low inlet temperature can cause discrimination for the high boiling point compounds. Why not use 375-400 C and graphite o-rings in the inlet?

    Best regards
    Lars Kürstein, Copenhagen

  2. Hi Lars,

    Thank you for reading and commenting! The inlet temperature was chosen to be more representative of the common range at which most analysts would be operating a GC inlet, not necessarily for those specifically looking at hydrocarbons. While there could potentially be some improvement for some of the heavier hydrocarbons at higher inlet temperatures, many analysts are looking at active compounds and cannot afford to go higher than 300 (if they can even go that high) without major deleterious effects. Graphite o-rings are also not as user friendly/easy to seal as standard viton o-rings, so I try to avoid them if I can.

    This work demonstrates that when using a proper inlet liner, you can minimize discrimination, even on high boiling compounds such as C40, and you don’t have to use extremely high inlet temperatures to accomplish this.

    Best wishes,
    Linx

  3. Cyril Maffezzoni says:

    Hi Lynx,

    Nice work! Always nice to see this kind of article. this is actually lot of users don’t have time to study so thank you.

    As this is part I of the serie, are you going to mention the 2mm ID liner for splitless too?

    I like to use 2mm ID liner with whool at bottom (or even without whool, depending on the analyte / solvent volatility) like the single tappered W/whool to increase velocity and transfert time x4. Of course the volume of the liner is /4 so care must be taken to avoid flashback. Thus doing pulsed splitless injection is fine. We both know but wanted to know if you ae going to blog it too for the community?

    2mm liner works better for very volatil splitless application.

    Thank you for your blog!

    Cyril

  4. Hi Cyril,

    Thank you very much for your insight. You are absolutely correct that a smaller ID can help with volatiles, since the faster transfer time leads to sharper peaks, in turn boosting sensitivity. Thanks for pointing out the issue of backflash, as well. I may add a future blog that discusses liner volume/ID, as you suggest.

    Thanks again!

    Linx

  5. Yuki says:

    Hi Lynx,

    Which liner(single taper with wool or double taper cyclo) do you think is more tolerant towards heavy matrix like food samples?

  6. Hi Yuki,

    A single taper liner with wool should work. The wool is very effective at protecting the column from involatile matrix material. Over time, you may see degradation of some active pesticides, signaling that it’s time to change the liner. If a simple liner change doesn’t bring back performance, then trimming the column and replacing the seal should help.

    If at all possible, using a split method will increase the time between maintenance intervals. Detectors such as MS/MS will often have the sensitivity to allow for a split injection and still meet method detection limit requirements.

    Thanks,
    Linx

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