Optimizing Splitless Injections: Initial Oven Temperature and Solvent Polarity

Beyond optimizing the inlet parameters of temperature and splitless valve time, initial oven temperature also plays an important role for splitless injections. When a liquid sample is injected into a GC, the first goal is to vaporize the sample within the inlet and transfer it to the column. As you know, this sample volatilization and transfer takes longer during splitless injections due to the slow inlet flow. Because of this, we want to condense (focus) the sample at the head of the column, to prevent it from moving through the column before analytes have completely transferred onto the column. If we do not focus properly, broadening and poorly shaped peaks will result, especially for early eluting compounds. This can also lead to poor resolution of volatiles.

In order to focus the sample in a tight band at the head of the column, a low initial oven temperature is required. There are two different approaches for setting this temperature, based upon your solvent and analyte boiling points. If your sample contains relatively volatile analytes that have a boiling point close to that of the solvent, utilize what’s known as “solvent focusing”. This involves setting the initial oven temperature below the boiling point of the solvent, to condense the solvent, which will trap volatile analytes. Ideally, set the temperature 20 °C below the boiling point of the solvent to take full advantage of this effect. For some solvents this may not be possible or practical without cryogenic cooling; in this case, get as low as practical, even if setting near or just slightly below the boiling point of the solvent. If this results in poor peak shapes of early eluters, you may have to consider cryogenic cooling or a different injection technique.

When analyzing compounds that have boiling points above the solvent, more than 150 °C, you can start with higher initial oven temperatures. Solvent focusing is no longer required in this case and instead the goal is to “cold trap” or focus the analytes at the head of the column. To do this, use an initial oven temperature that is lower than the boiling point of your most volatile analyte. This technique is referred to as “analyte focusing”.

Figure 1 shows examples of how initial oven temperature can affect peak shape. Higher oven temperatures lead to poorly focused initial peaks. While temperatures that are slightly too high may affect the first few peaks, as temperatures continue to increase, more peaks will be affected, as shown in the figure.

Figure 1: Example of the effect of initial oven temperature on peak shape. Using an initial oven temperature that is too high can lead to broadened and deformed peaks.

Solvent Polarity

Besides boiling point, solvent polarity can also affect your splitless analysis.  From your first chemistry classes, you’ve probably heard the expression “like dissolves like”.  This rule also applies to dissolving your solvent within your GC stationary phase.  For instance, if you have a non-polar stationary phase and you inject a polar solvent, your solvent may “bead up” instead of dissolving into the stationary phase.  Analytes that are cold trapped in the solvent can then become split between these “beads”, resulting in split and deformed peaks.  Figure 2 provides a visual demonstration of this phenomenon, as it occurs at the head of a capillary column.  To avoid this, always match column polarity with solvent polarity.  For instance, very polar solvents like water are best on polar phases such as wax columns.

Figure 2: When solvent and column phase have similar polarity, the solvent will dissolve evenly within the phase, forming a uniform film of solvent. If the solvent polarity is vastly different from the column phase polarity, beading of the solvent will occur, affecting focusing of the analytes.

Relating back to the initial topic of oven temperature, if your analytes have sufficiently higher boiling points than your solvent, and you can start with a higher oven temperature, the solvent polarity/column polarity will not have as great of an effect, since you are not utilizing solvent focusing.  Likewise, when it comes to split injections, this also doesn’t matter as much, since you are both injecting less solvent and not relying on solvent focusing.

Conclusion

This will conclude my blog series on optimizing splitless injections. To summarize, some important parameters to consider for splitless injections include liner type, inlet temperature, splitless valve time, initial oven temperature, and solvent polarity vs column polarity.

I hope these blogs have been informative and useful for your method development. Please feel free to comment and share your experiences.

2 Responses to “Optimizing Splitless Injections: Initial Oven Temperature and Solvent Polarity”

  1. Dear Linx,

    Thank you for the installment. Very nice! A table containing suggested initial oven temp and injection port temperatures for a variety of the most common polar and non-polar solvents would probably be “nice to have”…?

    Best regards –
    Lars Kürstein, Copenhagen

  2. Hi Lars,

    Thanks for the idea! That would make a nice addition to this post.

    -Linx

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