Missing Hexane Peak

[jelwell]

Hello All,

I am new to GC (as in, JUST starting out) and already, issues... I'm tasked with analyzing reactor gas, but that is not where my problem lies. I'm struggling with the standard! I'm hoping this will be a duh moment for me, so any input would be greatly appreciated!!!

I am running GC/FID on a gas standard that contains C1-C6. No matter what oven conditions and flow rates I run, I am missing a peak, which I believe to be hexane. I am certain I am getting methane, as the retention time is reproducible from another standard. and there are 4 other peaks, but no 6th peak. All components are at 100ppm in He. I am not getting full separation between methane and ethane, but there are 2 distinct retention times. I have varied enough the wrong way to completely coelude methane and ethane, but I can't seem to pull whichever component is hiding out!

Here are the ranges of variables I tested:
Carrier Gas: He (varied between 1 and 2 mL/min)
Inlet: 300 C
Detector: 300C
Oven Temp: 35C, 40C, 50C, 75C, 100C, 40C for 4 minutes followed by a 15c/min ramp to 250C
Split: 50mL/min, 10:1, 100:1

Please feel free to offer the most obvious solutions. Like I said I am very new to this, and think I should not be struggling so hard with a standard. Also, If you need other info, just let me know and I'll provide it.

Thanks for helping out a newbie!

[rb6banjo]

Are you sure it's not "methane" and propane that you're seeing and the ethane is buried in what you're calling methane? Methane/ethane can be tough without subambient oven temperature. What is the stationary phase and length of your column?

[jelwell]

No, I am not sure what is all clumped in the first 2 peaks. I am running a 30 m 0.320mm diameter HP-5 column.

[rb6banjo]

I can barely get methane/ethane at 40 °C with my 105 m Rtx-1 (similar to DB-5). I think your ethane is buried and what you're barely separating is the propane from the other 2.

[jelwell]

Great... Is there anyway I can force the system to identify both, or did I fall victim to listening to a sale rep when configuring my system...

[rb6banjo]

There are other columns that will make this separation easy. Some of those won't be good for heavier materials (long elution times). But, if you expect your sample stream to be pretty consistent you can compensate for that. Here are a couple of examples from Restek:

http://www.restek.com/chromatogram/view ... %7C74-98-6

http://www.restek.com/chromatogram/view ... %7C74-98-6

Other column manufacturers make similar phases.

[rb6banjo]

Here's my separation on the Rtx-1 (Restek). It's a 105 m x 0.53 mm x 3.0 µm column.

[jelwell]

Thank you for this input!! I need to determine how important the C1/C2 separation is for our application... I think our focus is going to be mainly on larger hydrocarbons, so maybe I will be able to just accept it for what it is.

Thanks again, you helped alleviate some stress on this Friday before a holiday. Much appreciated!

[jelwell]

I just saw your plot. I think you are right. With this column length, I think I'm lumping C1&C2. The separations of the other peaks appear to be in the same relative distances as what you have.

Thanks again for your help.

[rb6banjo]

You are welcome. I'm running at about 58 cm/s (linear velocity of the carrier gas) which is pretty fast for helium but I need to empty the loop on the rotary injection valve quickly so that I can get good injections. These conditions work for me.

[lynoguchi]

The DB-5 is not the most indicated column to do the hydrocarbon separation, in this case, the best choice is an HP-1/DB-1 column or an Alumina column. The HP-1 separates the compound by his Boiling Point, the Alumina by his polarity (separates olefins from paraffins). In ambient temperature you can use the Alumina, if you have cryo the HP-1 is the best choice.
The DB-5 has 5% of Phenyl in his composition, that interferes in hydrocarbons separation.