Chromatography Forum

Hi, I’m having trouble with peaks coeluting at the beginning of my GC/FID run. I’m using a Restek Rxi-5Sil MS, 20m column, 0.18mm diameter. Initial temp 50°C, max 320°C with 2 min hold. Rate 1: 20°C/min to 119°C, hold 1 min. Rate 2: 27°C/min to 300°C, hold 2 min. Split ratio 20:1, constant flow 1.2 mL/min, injecting 1µL. Any suggestions to improve efficiency or apps for adjusting parameters? I do want to include that the run must be around 15 min or so. any suggestions will be much appreciated.

There's a lot you're not telling us...
but the first thing I'd try is lowering that initial ramp rate.

Hi,
I increased the split ratio to 25:1 using hydrogen as the carrier gas. The flow rate is now 1.8. Initial temp set to 55°C with a 1-minute hold. Ramp 1: 120°C at 20°C/min with a 2-minute hold. Ramp 2: 300°C at 30°C/min with a 4-minute hold.
I'm still seeing some coeluting peaks. it seems to be a little bit better, but as one thing improves, another area gets worse. The coeluting peaks are happening after 4 and half minutes and I'm not sure what else to adjust
Does any one work with Rxi-5Sil MS 20 meter with 0.18 mmID , 0.18 um df column ?

That column is essentially non-polar. You could try a different phase column, but better if you tell us the identities of the two co-eluting compounds so we can look at their structures.