Hello,

I wonder any of you know the relationship between coulumn i.d. and injection volume and what is preferred injection volume for 2.1mm of i.d.

We normally use 5 ul injection size for that diameter column, occasionally 10 ul (flowrate 0.25 to 0.5 ml/min, water/ACN approx 35:65, injection solvent methanol). More causes peak distortion for us. If your sample/standard is made up in mobile phase, maybe can go higher. Try injecting smaller volume: if peak SHAPE improves, you previously injected too much.

If you are injecting in mobile phase, you want the injection volume to be fraction of the peak volume, maybe no more than 20 % of the peak volume. You can inject significantly larger volumes, if you dissolve your sample in a solvent composition that is a weaker eluent than you mobile phase. If you do this correctly, you can inject milliliters.