Understanding constant pressure vs. constant flow in gas chromatography.
Gas chromatography is incredibly powerful, but like any technique, the instrument will occasionally behave in ways that make you stop and ask, “Wait… is that supposed to happen?” One common head-scratcher comes from our Axion Labs video, “GC Troubleshooting: The Most Common Problems You Will Encounter.”
A viewer asked:
“What could be wrong if my inlet total flow falls when the oven temperature is ramping up?”
It’s a smart observation—and if you’ve ever watched your total flow tick downward mid-run, this article explains exactly why it happens and how to prevent it.
What Is Total Flow in a GC Inlet?
When we talk about total flow, we’re really discussing three flows combined:
- Column flow (the carrier gas moving through the column)
- Split flow (the flow vented out during split injections)
- Septum purge (a small flow preventing septum particles from entering the system)
Add them together, and you get the total flow your GC displays.
Most users don’t monitor tiny changes in these values, so if you spotted a shift during a run—great eye.
The Real Issue: Constant Pressure vs. Constant Flow
This is where things get interesting.
Modern GCs can operate in two modes:
- Constant Pressure
- Constant Flow
Before the 1990s, all GCs were constant pressure—because that’s all the technology allowed. Later, Electronic Pressure Control (EPC) made constant flow possible, and honestly, it changed everything.
Why This Matters During a Temperature Ramp
If your GC is running constant pressure (the default setting on many instruments even today), you’re pushing a fixed pressure—say 20 psi—onto the column.
That pressure may give you your ideal flow at room temperature.
But once the oven starts heating:
- Carrier gases like helium, hydrogen, and nitrogen become more viscous as temperature rises.
- More viscous gas flows more slowly through the same column at the same pressure.
Dr. Polite explains it perfectly:
“Water flows easily through a straw. Honey does not. Hot helium? That’s the honey.”
So What Happens?
As your oven temperature increases:
- Column flow decreases
- Total flow may drop (especially if the column flow is a large fraction of the total)
In many setups, the change is tiny—for example:
- 1 mL/min column flow → 0.5 mL/min during the ramp
- Total flow might go from 104 → 103.5 mL/min
Most people never notice it.
But if you’re using:
- A megabore column, and
- Higher column flows (e.g., 10 mL/min)
- A low split ratio (e.g., 2:1)
Then the shift is obvious:
- 10 mL/min → 5 mL/min is a big drop
- Total flow might shift from 30 → 25 mL/min
That’s when the symptom becomes truly visible.
The Fix: Switch to Constant Flow
Dr. Polite puts it bluntly:
“In my mind, there is one right answer—and that’s constant flow.”
Why Constant Flow Is Better
- It keeps your column flow steady throughout the temperature program
- You get more consistent retention times
- Better reproducibility
- Better chromatography
- No mysterious drops in total flow
How to Change It
In your GC’s software:
- Go to the Column or Flow settings.
- You’ll see options for:
- Constant Pressure
- Constant Flow
- Select Constant Flow.
- Save the method.
That’s it.
From that moment on, your GC will automatically adjust pressure as temperature changes—maintaining the flow you selected.
Bottom Line
If your total inlet flow drops during a temperature ramp, your GC is most likely running in constant pressure mode. As the oven heats and the carrier gas becomes more viscous, the column flow decreases—and that change shows up as a drop in the total flow.
Switching to constant flow eliminates the issue and gives you smoother, more reliable chromatography.This was a fantastic question—and one that hits on a subtle but critical concept in GC method setup. Keep the questions coming, and keep learning!
