Revisiting Column Insertion Distance

In our first investigation of column insertion distance, we found that not installing the column far enough into the injection port had the greatest impact on response and reproducibility when using hexane as a solvent and alkanes ranging from C8 to C40. In this work, we will expand this study to evaluate the use of methanol and methylene chloride as solvents with volatile and semivolatile compounds.

In 2024, we published work that determined that proper insertion distance into the injection port is critical for maintaining reproducible results and good peak response, whether operating in split or splitless mode.¹ Split injection was used to analyze alkanes in hexane from C8 through C40, and not surprisingly, the best response was found to be an insertion distance of 5 mm, which closely matches the manufacturer’s recommendation. An insertion distance of less than 5 mm resulted in a 50% drop in the overall average response for the alkanes tested and an 18-fold lower response for C40, while installing the column too far into the injection port decreased the response 8% per 5 mm. In the prior study, we examined splitless conditions and found less variation in overall response, but an increase in variability from injection to injection. Interestingly, the lowest percent relative standard deviation (%RSD) was achieved with the 10 mm insertion distance under specific conditions.¹ Readers had a variety of questions about this study, which can be summarized as: How would volatiles perform under these conditions? Would using solvents other than hexane change the results? In this work, we will examine volatiles and semivolatiles using two other solvents.

Experimental Design

The column used for this work was a cyanopropylphenyl-dimethyl polysiloxane, “624-type phase,” 30 m x 0.25 mm, 1.4-µm (Restek) installed into a flame ionization detector with a flow of 1.7 mL/min, 17 psi constant head pressure. A GC oven program of 35° C (hold 5 min), 20° C per minute to 220° C (hold 5 min) was used for both split and splitless injections. The split conditions of 10:1 were used with a precision-style liner where the wool is held in position by baffles. The splitless hold time was set for 1.0 min using a single-taper liner with wool at the bottom.

The Washington State volatile petroleum hydrocarbon standard was used, containing 15 components, where the following compounds in that mix were monitored during the testing: methyl tert-butyl ether, hexane, o-xylene, and 1-methylnaphthalene in methylene chloride and methanol. Concentrations were adjusted to maintain 5 ng on column for both split and splitless. Split and splitless conditions were tested using an Agilent 7890 gas chromatograph with a standard split/splitless injection port with the following insertion distances: 0.5 mm, 5.0 mm, 10.0 mm, and 20.0 mm. Distance was measured from the top of the ferrule of the column nut (Figure 1). Each of these tests was performed three times, and the average of these values was presented.

Results and Discussion

The manufacturer recommends a column insertion distance of 4.0–6.0 mm past the end of the ferrule on the column nut (Figure 1).² Our goal was to test different insertion distances from short (0.5 mm) to regular (5.0 mm) to long (10.0 mm, 15.0 mm, and 20.0 mm) using additional solvents (methylene chloride, methanol) and volatiles as target compounds. The use of volatiles places the solvent close to the eluting compounds, which can distort peak shape and impact retention in splitless mode.³ In split mode, the sample is thoroughly vaporized by the wool prior to reaching the bottom of the inlet, therefore it would be easy to assume that insertion distance does not make any difference if there is good resolution between the solvent and the target analyte.

Reproducibility for triplicate split injections with insertion distances of 0.5 mm, 5.0 mm, 10.0 mm, 15.0 mm, and 20.0 mm using methanol and methylene chloride is shown in Figure 2. The differences in reproducibility were less dramatic than our previous studies, which examined alkanes up to C40. Methanol had overall higher variability for replicate injections since the methyl tert-butyl ether (MTBE) eluted close to the solvent, and the peak width was twice as broad as that of hexane. Surprisingly, the best result when using methylene chloride in split mode was the 0.5 mm insertion distance. Overall, the differences were not dramatic and highlighted the importance of using the proper solvent and optimized inlet conditions. Our choice of a 17-psi head was chosen to minimize the solvent expansion in the inlet and prevent the vapor cloud from exceeding the volume of the liner, and worked for both solvents.

Splitless injections allow most of the sample (and solvent) to reach the column by closing the split line for a period of time; in this case, we set the hold time to 1 min. The increase in solvent can cause peak tailing or splitting if the solvent elutes too closely to the target analytes,⁴ and in our case, hexane and MTBE peak shapes were poor, making integration difficult. The overall trend, as shown in Figure 3, indicates the 5.0 mm insertion distance works best for both solvents, with the largest contributors to variability being the hexane and MTBE. Evaluating the latter eluting compounds shows less variation, but there is still a slight advantage to using an insertion distance of 5.0 mm or 10.0 mm.

Conclusions

We found in our 2024 study that using hexane as a solvent in split and splitless conditions for a broad range of alkanes required an insertion distance of at least 5.0 mm. Revisiting this work using two other solvents and volatile compounds, along with a thick film column, reaffirms our previous data. In general, variability increases when the column is not installed at a 5.0 mm insertion distance.

References

1. English, C. The Reality Behind Column Insertion Distance. LCGC, The Column 2024, 20 (9), 23–25. https://www.chromatographyonline.com/view/the-reality-behind-column-insertion-distance (accessed 8-18-2026).
2. Agilent GC Support. GC Column Installation Quick Reference Guide—Inlets; Agilent Technologies, 2024. https://www.agilent.com/cs/library/quickreference/public/GC%20Column%20Installation%20Quick%20Reference.pdf (accessed 7-30-2026).
3. English, C. How Much Sample Can I Put on My GC Column? LCGC, The Column 2021, 17 (9), 2–6. https://www.chromatographyonline.com/view/how-much-sample-can-i-put-on-my-gc-column- (accessed 8-18-2026).
4. Taylor, T. Proper GC Column Installation—The Simplest Way to Improve Your Gas Chromatography. LCGC Int 2016. https://www.chromatographyonline.com/view/lcgc-blog-proper-gc-column-installation-simplest-way-improve-your-gas-chromatography (accessed 8-18-2026).