Chromatography Forum

Johnny Rod,

My HP-PLOT U column dimension is 30 m; 0.32 mm; 10 um. This column is suitable for analyzing hydrocarbons (natural gas, refinery gas, C1-C7, all C1-C3 isomers except propylene and propane).


AICMM,

This is a chromatogram of 500 ppm ethylene with 12:1 split ratio



Thank you.

ismawanto,

Do you have a good flowmeter handy? I would physically measure the split vent flow and the column flow.

Was this a 1 mL or 5 uL injection?

Best regards,

AICMM

According to the HP Flowcalc app, for a 320um column 30m long using nitrogen at 60C you should be using a flow rate of 0.4 to 0.8ml/min which equates to head pressure of 2.3-4.3psi. This will give the best resolution. What have you been using? Whcih peak is whic hin the sample. What is th sample you've shown.

I don't have somewhere to host a pic I'm afraid, peaks are around 10pA tall on an Agilent 6890 for 5ppm ethylene, peak width is more like 0.1min or less (at an RT of 4min) but we're using helium carrier.

Are you sure PLOT-U can separate acetylene and ethylene? I can't find an example chrom with ethylene on it.

AICMM,

Unfortunately we don't have flowmeter.
I manually injected 1 mL air sample (in my reference papers they injected 1 mL air sample for plant ethylene analysis).


Johnny Rod,

Do you mean the flow rate of nitrogen carrier?
In HP PLOT U chromatogram for hydrocarbon analysis there isn't any information about the flow rate of carrier gas.
I used 1.5 ml/min flow rate. Is it too high?

The chromatogram in my previous post is my 500 ppm ethylene standard with nitrogen carrier gas. Ethylene is the second peak (at an RT of 2.6 min).

I am sure HP-PLOT U can separate acetylene and ethylene. I have tried to inject 1 mL of my gas mixture consist of methane, ethylene, ethane and acetylene. The result showed 4 peaks. I have also checked the chromatogram of this column for hydrocarbon analysis in Agilent website.

Nitrogen can be used as the carrier gas for your analysis, but it has a narrow range for the optimal column efficiency. For a 30m x 0.32mm column, the optimal column flow should be about 0.75 mL/min. The gain in theoretical plates by running at this slower speed is about twice what you will see at 1.50 mL/min. A Van Deemter curve illustrates this property of the different carrier gases very well.

The good news is that a U-type porous polymer PLOT column will have no problem at all in being able to separate ethylene, ethane, and acetylene (propane and propylene may coelute, but methyl acetylene will be well separated from these two compounds).

Bill

Yes I meant the column carrier flow as Bill clarifies. Reduce it to about 0.75ml.min and you'll get better resolution although longer run time of course. If you can see the ethylene in your standard but not your sample, maybe there isn't any. Try spiking a bit in your sample but it sounds like it's not present at all.