In the previous installment of this blog series, I discussed liner selection for splitless analyses (GC Inlet Liner Selection, Part I: Splitless Liner Selection). Today I’d like to discuss liners for split analyses. During a split injection, the split vent is open and the majority of the flow is vented. The split ratio, set by the user, determines the total flow exiting the split vent vs the total flow going to the column. Split injections are essentially dilutions occurring within the inlet and require optimization to assure proper sample transfer since the split can affect sensitivity.
Split injections use high inlet flows versus splitless injections. These high flows lead to narrow, sharp peaks and reduced time in the inlet, which is advantageous for thermally labile compounds. At the same time, because of these high flows, choosing a proper inlet liner is essential, since analytes spend such a small amount of time in the inlet, yet must still be thoroughly vaporized and transferred to the column, uniformly.
To test liner performance for split injections of liquids, I set up a similar experiment to my approach in Part I, this time utilizing a split injection method. Once again, I wanted to compare liners based on recoveries across a wide molecular weight range, as well as reproducibility from injection to injection. Hydrocarbons ranging from C8 to C40 were evaluated for response, as well as injection to injection reproducibility.
The following liner configurations were compared using the split 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 a wide molecular weight range when performing split injections (20:1 split):
Figure 1: Comparison of peak area response across a wide molecular weight range for various liner configurations used in split mode.
As with splitless injections, the use of wool helps to significantly improve response, by providing extra surface area and homogenization of the sample, promoting thorough vaporization and transfer to the analytical column. Wool also helps to catch the sample, preventing it from going straight to the bottom of the inlet, potentially condensing out or getting swept out of the split vent. This is especially critical with fast autosampler injections, where the sample is expelled from the syringe so quickly it may not have time to fully evaporate before reaching the bottom of the inlet. The straight liner with no wool was added to show the drastic effects of performing split injections without some kind of obstruction to catch the sample, mix, and enhance vaporization. While the cyclo liner was more effective than a straight liner without wool, it did not meet the same performance as the liners that had wool.
Figure 2a shows injection to injection reproducibility for the liners compared under split conditions. The straight liner with wool, precision liner with wool, and the low pressure drop liner showed the best reproducibility. The single taper liner with wool had acceptable performance, albeit slightly worse reproducibility. As expected, the straight liner without wool showed variability due to lack of an obstruction to catch and homogenize the sample.
Figure 2a: Liner injection to injection reproducibility comparison across wide molecular weight range.
Figure 2b shows a zoomed in view of Figure 2a to more clearly see results for the liners with acceptable reproducibility. Notice the straight liner with wool, precision liner, and low pressure drop liner all performed very similarly.
Figure 2b: Zoomed in view to show performance of liners with best reproducibility.
Based on performance and cost, the straight liner with wool and the Precision liner appear to be the best choices for split analyses. The straight liner with wool has a cost advantage over the Precision, however, the Precision ensures that the wool cannot move in the liner, causing variability. Wool movement should generally not be an issue, unless rapid pressure changes are made. The low pressure drop liner also performed similarly, but is the most expensive of the wool containing liners shown. A single taper liner with wool will also work, though some slight performance sacrifices are made with regards to reproducibility in split mode. The cyclo liner unfortunately did not show reproducible or complete vaporization and transfer of compounds compared to the liners with wool.
For those concerned about the use of wool, activity is much less of an issue when using split mode (compared to splitless), as analytes have very little residence time in the liner to interact with active sites. When using a good inert deactivation, such as Topaz, wool will seldom be an issue in split mode.
You can see from the data on the straight liner (with no wool), that not having some kind of surface to promote mixing and volatilization of analytes results in low responses and very high injection to injection %RSD’s.