Purge Time and Purge Flow

[rfazily]

I have a GC run where my Ethyl Alcohol peak shows up at 0.8 minutes, IPA at 1.0 minutes and Butanol (Internal Standard) at 2 minutes. The first two peaks are sometimes clumped together affecting my results.
It is a splitless injection, run in an Agilent System with a 30m X 0.53mm X 3.0um column. My purge flow rate is 15ml/min and purge time is 1.0min. The solutions are prepared in water.
There seems to be tailing in my ethyl alcohol peak and slight fronting in the Butanol. What should I toggle with to get better separation of the first two peaks?

[James_Ball]

I have a GC run where my Ethyl Alcohol peak shows up at 0.8 minutes, IPA at 1.0 minutes and Butanol (Internal Standard) at 2 minutes. The first two peaks are sometimes clumped together affecting my results.
It is a splitless injection, run in an Agilent System with a 30m X 0.53mm X 3.0um column. My purge flow rate is 15ml/min and purge time is 1.0min. The solutions are prepared in water.
There seems to be tailing in my ethyl alcohol peak and slight fronting in the Butanol. What should I toggle with to get better separation of the first two peaks?

What is your column flow?

You only need to hold splitless until the liner is swept with carrier about 3-4 times. A typical open splitless liner is no more then 1ml volume, so if you have a 4ml/min column flow then purge valve opens at 1min, but if you are flowing 7-8ml/min open it at 0.5min. You can sometimes get away with 2-3 volume sweeps which would be even shorter purge valve times. Easy way to do it is take liner volumn divided by flow rate in ml/min times 3.

For water though it should probably be a split injection, assuming you have enough sensitivity.

[rfazily]

Flow rate is 30ml/min.

[rfazily]

Flow rate is 30ml/min.

[dblux_]

Flow rate is 30ml/min.

Column flow 30 mL/min ?
Much to much.

[Steve Reimer]

30ml/min? could that be 30 cm/min?

[larkl]

0.53 is a megabore column. It could be 30 ml/min flow. What's the column? I'd be more concerned that that chemistry could be the source of the tailing/fronting.

[rfazily]

Yes the flow is 30ml/min. I thought it was too much too. But our main company in Michigan used the same parameters and they got good separation. The makeup flow is 2.3ml/min.
They used an Agilent system just like mine. We are doing an intermediate precision.

[James_Ball]

Yes the flow is 30ml/min. I thought it was too much too. But our main company in Michigan used the same parameters and they got good separation. The makeup flow is 2.3ml/min.
They used an Agilent system just like mine. We are doing an intermediate precision.

Putting 30m x 0.53mm ID x 3um into the flow calculator yields:

30ml/min = 151cm/sec optimum high flow rate is 4.2ml/min (assuming helium carrier)
30ml/min = 185 cm/sec optimum high flow rate is 5.3 ml/min (assuming hydrogen carrier)

conversely

30 cm/sec = 4.6ml/min with helium which in at the optimum high flow rate

30 cm/sec = 3.84ml/min with hydrogen which is in the optimum flow range.

Make sure the Michigan lab is not using 30 cm/sec instead of 30 ml/min. It sounds as maybe they sent you the wrong units for carrier flow.

[dblux_]

Make sure the Michigan lab is not using 30 cm/sec instead of 30 ml/min. It sounds as maybe they sent you the wrong units for carrier flow.

He gets ethanol peak at 0.8 min. If carrier velocity is 30 cm/sec then unretained peak is in 100 sec.
Carrier gas velocity is presumably higher to get ethanol peak at 0,8 min.
It would be the best solution if original poster simply copy all relevant GC parameters from Michigan lab and then if in trouble consult here.