Does the Outer Diameter of the Liner Matter?

Split injection can be considered a dilution of the sample inside of the injection port. Jack Cochran coined the phrase “shoot and dilute” to describe split injections (1). This technique has several advantages over splitless injection, including narrower peak bandwidths, a higher GC oven starting temperature, a decrease in solvent effects, and a decrease in solvent interference, among others (2). The obvious downside is the decrease in sensitivity, which is directly and precisely proportional to the split ratio. From our work in The Column, we know that the inner diameter of the liner is critical to efficiently transferring the sample onto the column (3), and that for a split ratio of 5:1 and 50:1, using wool in the center provides the best overall sample transfer when evaluating a broad range of compounds from C8 to C40 (4). We also have data comparing a variety of different liners using a 20:1 split; if the liners contain wool, they offer a similar performance when measuring the response and reproducibility for a series of alkanes from C8 to C40 (5). Finally, we have generated a 6-point calibration curve that covered a split range from 400:1 (1 ng on-column) to 10:1 (36.4 ng on-column) with a correlation coefficient of 0.99948 (6). Since the main difference between a split (6.3 mm outer diameter [o.d.]) and splitless (6.5 mm o.d.) liner is 0.2 mm, we would expect more flow restriction when using the splitless liner in split mode with higher flow rates.Now the question remains, does the outer diameter matter when using a split ratio of 100:1, 500:1, and 1000:1? In addition, do other liner configurations impact the results? This experiment will evaluate five different liners using a standard containing polyaromatic hydrocarbons (PAHs), with the concentration of the standard adjusted for 2 ng on-column concentration for all injections to allow for easy comparisons.

Experimental Design

The column used for this work was a 5% diphenyl 95% polydimethylsiloxane, “5‑type phase,” 30 m × 0.25 mm, 0.25-µm (Restek) installed into a flame ionization detector (FID) with a column flow of 1.00 mL/min. A GC oven program of 40 °C (hold 3.5 min), 28 °C/min to 260 °C (no hold), 3 °C/min to 290 °C (no hold), and 25°C/min to 330 °C (hold 10 min) was used for the three different split injections. Split ratios of 100:1, 500:1, and 1000:1 were used with the following liners: 6.3mm-o.d. straight with wool; 6.5-mm-o.d. straight with wool; 6.3-mm focus with wool (precision); 6.3-mm taper focus (low pressure drop) with wool; and 6.5-mm-o.d. single taper with wool (Figure 1). Compounds chosen were PAHs that covered the volatility range; naphthalene, acenapthylene, acenaphthene, benzo[a]pyrene, and benzo[g,h,i]perylene. Concentrations were adjusted to maintain 2 ng on-column for all split ratios. Conditions were tested with a 7890 gas chromatograph (Agilent) with a standard split/splitless injection port and a 7693A Autoinjector (Agilent) using a 5-µL syringe. Each of these tests were performed three times and the average of these values presented. Following the initial results, additional testing was performed using split ratios of 10:1 and 200:1 to verify system performance.

Results and Discussion

The total average area counts for the first four liner geometries using a split ratio of 100:1 and 500:1 were under 4%; averaging the areas for the PAHs for all three split ratios were under 8%. Aside from the single taper with wool, the liners efficiently transferred the sample onto the column (Figure 2). The first two liners in the experiment were both straight liners with the same amount of wool located in the same position; the only difference was the outer diameter. The location of the wool was measured prior to installation and re-checked after the liners were removed, as the position of the wool in the liners can impact the results (4). One of the most interesting results was the 0.6% relative standard deviations (RSDs) using the 6.3-mm-o.d. straight liner with a split of 100:1. RSDs of under 1% were also observed with a 10:1 split using this liner. For non-active analytes, this liner performed remarkably well. The geometry of the 6.3-mm-o.d. straight liner and the 6.3-mm-o.d. straight focus liner (precision) are similar and therefore it is not surprising that they offered a similar performance. The 6.5-mm-o.d. straight liner is a single taper liner where we drilled out the taper using a diamond drill bit with the liner submersed in water. There appears to be a slight difference between the two straight liners related to outer diameter. The 6.5-mm-o.d. straight liner results were re-run to verify % RSDs, and the results were again above 2%. Another interesting observation is the tapered focus (low pressure drop) liner, which has a bump at the bottom of the liner to reduce back pressure and turbulence. This liner did not perform as well as the straight focus liner (precision) or the 6.3-mm-o.d. straight liner. The 6.5-mm-o.d. single taper liner with wool at the bottom resulted in less analyte on‑column, part of which could be the position of the wool, as the bottom of the inlet is cooler than the middle. The combination of the position of the wool, the taper, and the larger outer diameter impacted repeatability, with an RSD of 23.7% for triplicate injections using the 1000:1 split ratio.

Conclusions

Liner outer diameter—even with a high split ratio (1000:1)—has minimal, but measurable impact on reproducibility. The straight liner and straight focus (precision) with wool have similar geometries and the results are similar. The single taper liner with wool at the bottom offered the weakest performance due to a combination of the 6.5-mm-o.d., taper at the bottom, and the position of the wool (4).

Acknowledgment

Special thanks to Jaap de Zeeuw (Restek) for technical advice and review.

References

(1) Cochran, J. Split Injection GC: The Benefits of “Shoot-and-Dilute’ GC. The Column 2015, 11 (21), 14–20.

(2) Cochran, J. Split Injection GC: Inlet Liner Choice for Shoot and Dilute GC. The Column 2016, 12 (4), 10–15.

(3) English, C. Liner Volume Matters. The Column 2025, 21 (2), 37–40.

(4) English, C. Does the Position of Wool in the Inlet Matter? The Column 2024, 20 (6), 14–17.

(5) Waclaski, L. GC Inlet Liner Selection, Part II: Split Liners. ChromaBLOGraphy. 2019, https://uk.restek.com/chromablography/gc-inlet-liner-selection-part-ii-split-liners?srsltid=AfmBOoqWUz-wDM6xfQyQ_ro6LV8JXa9LXVvrpCNY40Vs2X3YkxItdxZI(accessed 2025-08-05).

(6) Cochran, J. Split Injection GC: Setting the Split Ratio in Shoot-and-Dilute GC. The Column 2016, 12 (8), 10–14.

Chris English has managed a team of chemists in Restek’s innovations laboratory since 2004. Before taking the reins of the laboratory, he spent seven years as an environmental chemist and was critical to the development of Restek’s current line of volatile GC columns. Chris holds a BS in environmental science from Saint Michael’s College, USA.