In gas chromatography what is column flow, purge flow, split ratio?

Here’s what they have in common:

✅ They all involve carrier gas flow: Each parameter dictates how the carrier gas moves through different parts of the GC system.

✅ They impact sample delivery: Properly managing these flows ensures that the sample is introduced in the correct volume and concentration to prevent overloading the column.

✅ They contribute to system performance: The balance between these flows affects peak shape, sensitivity, and reproducibility in GC analysis.

✅ They are controlled by the injector: The split injector regulates these flows to optimize sample dilution, remove contaminants, and ensure efficient separation.

Together, these parameters help maintain the precision and accuracy of GC analysis.

In gas chromatography (GC), the mobile phase is a carrier gas—typically helium—that transports the analytes through the column. At the inlet of the GC, we use specialized fluidics because the essential job of the injector is to dilute the sample before it enters the column.

Now, if you want the deep version of that, get the on-demand GC Bootcamp or attend a hands-on bootcamp. 

How a Split Injector Works

But essentially, a split injector works by injecting one microliter and typically splitting it 100 to 1, which means that 1% enters the column while 99% goes to waste. This is purely to keep the column happy. What do I mean by that? Well, the column is very small, contains a very small amount of stationary phase, and requires a very small injection volume. We can’t inject 0.1 microliters, but we can inject 1 microliter and dilute it 100-fold—and that’s the whole idea behind a split injector.

In a split injector, we have one flow coming in, which we call total flow, and three flows going out.

1. Septum Purge Flow
   The first flow is called the septum purge. The septum purge is really a low flow. Let’s say we have 100 mL/min coming in—about 3 mL/min goes out through the septum purge. This is just a split stream coming off our helium flow, passing over the septum and straight out to waste. The purpose of this is to remove any impurities from the septum.
2. The septum purge should be set at a small number. If you can set it, leave it alone—3 mL/min is fine. Most instruments don’t allow you to change it anymore, but if you can, don’t touch it. It should be set to roughly 3 mL/min, anything but zero.
3. Column Flow
   The column flow is the amount of carrier gas that actually moves through the column.
4. Split Flow
   The split flow is what goes out through the split vent. Typically, if we have 100 mL/min coming in, 3 mL/min goes out through the septum purge, 1 mL/min goes through the column, and that leaves 94 mL/min going out through the split valve.

Split Ratio  

The split flow (94 mL/min in this case) divided by the column flow (1 mL/min) gives us the split ratio. So in this example, the split ratio is 94:1, meaning we discard 94 parts for every 1 part that enters the column.

Typically, we use more rounded numbers. I always start my methods with a 50:1 or 100:1 split ratio—these are my two go-to choices. They work for 99% of the GCs out there. A 50:1 split ratio is a good starting point.

Adjusting Split Ratio for Peak Size  

• If you want bigger peaks, lower the split ratio:

• 25:1 split → Peaks are twice as big (compared to 50:1)
• 10:1 split → Peaks are five times as big (compared to 50:1)
• If you want smaller peaks, increase the split ratio.

There aren’t many cases where you’d want to make peaks smaller, but let’s say you give me a sample of pure gasoline and ask me to analyze it. The normal hydrocarbons in gasoline are at such high concentrations that they would overload my column. In that case, I might use a 500:1 split ratio to dilute the sample before it enters the column.

Summary  

• Column flow: The carrier gas moving through the column.
• Purge flow: A small stream (around 3 mL/min) that removes contaminants from the septum.
• Split flow: The portion of the sample that is vented out instead of entering the column.
• Split ratio: The split flow divided by the column flow (e.g., 100:1 means 1% of the sample enters the column).

Instead of thinking about individual flows, just think about split ratio. On most modern GCs, you simply input the split ratio, and the instrument sets the flows for you.

Good question!