Split and splitless are the two most commonly used GC injection techniques. While they share some similarities, each is suited to a particular type of analysis. But how do you know which one to use?
In this video, we explore how these two techniques work. We’ll start with split injection, demonstrate the technique in action, and explain what split ratios are. We’ll then switch to splitless injection, show how it differs from split mode, and cover what splitless hold time is and why it matters. Finally, we’ll look at the strengths of each mode to help you choose the right one for your analysis.
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Split vs. Splitless Injection
Split and splitless are the two most commonly used GC injection techniques. How are they different? And which one should you use for your analysis?
We’ll begin with split injection, which is the most popular thanks to its versatility over a range of analyses. Inside a split/splitless inlet, you will find the carrier gas supply, the septum, the septum purge, the split vent, the liner, and the column. Now we’ll start the gas flow and simulate a split injection.
Let’s rewind and go through what happened. First, the gas entered the inlet with a total flow of 104 mL/min. A small amount—3 mL/min—passed through the septum purge to reduce possible contamination from the septum, while the rest continued on to the liner. A fraction of the gas—1 mL/min—flowed into the column, but most—100 mL/min—is swept away via the split vent.
This ratio—the split ratio—is determined by the user before starting the analysis. Split ratios typically vary between 5:1 to 500:1, with the higher the ratio, the lower the amount of sample that enters the column compared to what passes out the split vent. During injection, the sample is injected into the liner and vaporizes. Since our example has a high split ratio of 100:1, one part goes onto the column while one hundred parts of the sample exit out the split vent.
Now let’s look at an example of a splitless injection. In a splitless injection, the split vent is closed and left closed before and during the injection. As there is no split flow, the total flow is set at a dramatically reduced flow rate. Here, it’s only 4 mL/min. 3 mL/min passed through the septum purge, while the remaining 1 mL/min entered the column. During injection, the sample remains within the liner for longer before entering the column due to the lower flow rate. Once sufficient time has elapsed after injection, the split vent opens to purge the inlet. This duration—known as the splitless hold time—is calculated to be long enough to allow the maximum vaporization and transfer of analytes to the column.
So which injection technique should you use? It depends on the concentration of your desired analytes within your sample, the sensitivity of your detector, and any method requirements.
Split injection is ideal if your analyte concentration is high enough to afford an on-instrument dilution and still meet required detection limits. The higher flow rates through the inlet lead to sharp, narrow peaks, while simultaneously reducing the time for adverse interactions to occur. However, since most of your sample is vented, your detection limits are much higher. While this can be problematic for less-sensitive detectors, a detector with higher sensitivity such as an ECD or an MS/MS help make split injection possible.
But if your analyte concentration is very low, you may need to perform a splitless injection. Splitless injections excel at trace analyses. Since all the flow is directed to the column, you can transfer the majority of your sample. However, the slower flow rate into the column can result in the degradation of active analytes through adsorption and breakdown. It also leads to increased diffusion, causing band broadening. This is especially noticeable for more volatile analytes, resulting in wider peaks.
While choosing the right injection technique for your analysis is essential, it’s also important to optimize your injection parameters. This allows you to maximize both analyte transfer and injection to injection reproducibility. For more information about optimizing for split and splitless injections, visit our resources below, or visit restek.com