Optimizing Splitless Injections: Inlet Temperature

One of the key parameters that requires optimization for splitless injections is inlet temperature.  With liquid injections, the analyst is relying on the volatilization of the sample upon introduction into the inlet.  The analytes can then be efficiently transferred to the column in a vapor state, where they are refocused, prior to beginning the chromatographic separation.  Since analytes have different boiling points, it’s important to monitor the least volatile compounds.  These higher boiling compounds will require more thermal energy to efficiently vaporize and transfer to the column.  Increasing inlet temperature will generally lead to increased responses for compounds with high boiling points, although at a certain point the gains may become insignificant and possibly deleterious.

As with most things in chromatography, though, there is always a trade-off.  As we increase inlet temperature, we provide thermal energy which can also drive chemical reactions.  Many analytes are thermally labile, that is, they tend to react or degrade at high temperatures.  This creates a problem, as we compromise the integrity of the analysis by forming new products in the inlet that are non-representative of the sample being injected.

So my advice when setting up a new method is to try several different inlet temperatures and observe both the behavior of your compounds with the highest boiling points, as well as behavior of thermally labile compounds.  You may have to compromise, as the temperature at which the high boiling compounds have the best responses may lead to lower responses for sensitive compounds or vice versa.  Keep in mind that some analytes may have both high boiling points and be thermally labile, such as the pesticide deltamethrin.

A good initial inlet temperature is 250 °C, which works well over a wide range of compound boiling points.  If you have a lot of higher molecular weight analytes though, you may want to experiment with higher temperatures.  For instance, start at 250 °C, then try 275 °C, then 300 °C.  Observe the effects on your last eluting compounds, and if there are any active compounds that lose response as you increase temperature.  From here you can choose the best overall temperature for your analysis or further experiment within a range, such as trying temperatures between 275 °C and 300 °C.

The examples below illustrate the effect of changing inlet temperature on a high boiling point PAH, Benzo[ghi]perylene, as well as a thermally labile chlorinated pesticide, endrin.  Benzo[ghi]perylene, which has a boiling point of 500 ⁰C, shows increasing response as inlet temperature increases; however, notice that the gains are not linear and become less significant as inlet temperature continues to increase.  For instance, you don’t gain much sensitivity by increasing the inlet temperature from 275 ⁰C to 300 ⁰C.  On the other hand, notice that endrin continues to experience increased degradation as inlet temperature increases, compromising the quality of the analysis.

Benzo[ghi]perylene, a PAH with a high boiling point, shows increasing response with increasing inlet temperature. Notice that the gains are not linear, however, and increases in peak response become less significant as temperature increases.

Endrin, a thermally labile chlorinated pesticide, shows increased degradation as inlet temperature increases. This compromises the analysis, as new products are being formed in the inlet.

In the introduction to this series, I said that I wouldn’t discuss liners, since I have already blogged on the topic; however, it’s impossible to completely ignore liners in a discussion about inlet temperature.  Using a liner that has wool or some obstruction like a cyclo, increases the surface area and allows for retainment of more heat within the inlet.  Therefore, when you use a liner with wool, you can achieve better vaporization of heavier compounds at lower temperatures compared to a liner without wool.

Along these same lines, the inlet temperature set point is not equal across the entire inlet.  Often this temperature is measured near the center of the inlet; the top and bottom of the inlet are cooler, sometimes significantly so.  Because of this, it’s possible for a sample to vaporize upon introduction into the inlet, but then condense as it reaches the bottom and not effectively transfer to the column.  Selecting a liner with an obstruction, like wool, at the bottom will help prevent this, as it not only prevents the sample from contacting the bottom of the inlet, but holds heat to help with volatilization.

While this series is focusing on splitless injections, note that the general guidance in this blog will also apply to optimizing inlet temperatures for split injections.

For the next installment of this series, I’m going to talk about splitless hold time, another important splitless injection parameter.  Stay tuned!

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