Settings to detect octane in DCM

Discussions about GC and other "gas phase" separation techniques.

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I am using an Agilent 5973N GC/MS to detect octane in dichloromethane.
My octane concentration is 1 mM.

The GC method is as follows
-0.1 uL splitless injection
-Solvent delay of 2 mins
-Hold at 40C for 2 mins
-Ramp up to 310C @ 15C/min
-Hold at 310C for 2 mins

With the above method, the octane just barely makes it onto the chromatogram at an elution time of ~3.5-4 mins.

Is there way to make the octane appear later on the chromatogram? For example, would decreasing the ramp rate to 5C/min help?

Yes it would help. The bigger impact would occur if you can start the oven temperature lower than 40 °C (35?, 30 if you can get it). Don't start to elevate the oven temperature until after the octane elutes.

I tried to get to 30 °C on one of my GC's (coming down from 40). My lab temperature is 22 °C (72 °F). I got to 32 °C in about 4 minutes but couldn't get any lower after 10 minutes. 35 would be a reasonable starting temperature for me.

If you have to do multiple analyses, only ramp up the oven temperature hot enough to get the heaviest thing off of your column. If you don't have to cool from 310 to 35 °C, your gc cycle time will be shorter.

If time is of the essence, you might consider a subambient cooling option for your oven. Most GC's either have that capability or it can be added. Then, you use liquid N2 to cool the oven. You can get to very low temperatures with that option.
With a boiling point of ~125 C, Octane should be able to elute just by bringing the oven around 70 C. I'd try a starting temperature of 35 C and holding it for about 5 minutes, then do a 5 C/min ramp up to 80 or 90 C and see when octane elutes. You didn't mention your column flow rate, you could always lower that a bit to achieve a later retention time for octane as well.
0.5 uL split 5:1 might be (might) a magic bullet hear. Certainly sharpens up the solvent front.

Best regards,

The most elegant solution will be to use a thicker film column - but since you do not share what column you have it is not possible to make specific recommendations.

Peter Apps
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