Method development. How to split co-eluting peaks?

The discussion re:delta 10 vs CBC brought up a problem I’m having setting up an SRI 310MM .

It’s got a Hayes sepD packed column, but did not come with a preinstalled “solvent” method. I grabbed samples of all the solvents I had lying around, and ran into a problem: pentane and acetone are co-migrating.

I’ve only tried 140C for 12min, 180C for 10min, and a ramp from 140 to 180, but clearly I’m just flailing around with no conceptual framework on how this works.

Is there an analytical chemist in the room with a spoon to spare?

@srihugh1? @MagisterChemist @Dr_Jebril?!?

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@adinaOE is an incredible analytical chemist I just met with her in Reno last night and was super impressed with her range of knowledge and skills- hit her up

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I’ll admit, I’m more well versed in liquid chromatography, but I think I’ve dabbled enough in GC to get your wheels turning.

There are a few variables you can start with:

  1. injection volume
  2. Carrier gas type
  3. Carrier gas flow rate
  4. Column type - I’m not sure what your column is made of, but Restek makes a reliable fused silica column for RS. Obviously if your analytes are not attracted to the stationary phase, you won’t get separation
  5. Oven temperature/ramp rate

Here is a good link from Restek showing great separation between pentane and acetone:

Check it out and let me know if you are still having trouble. I have a hunch it’s going to come down to your oven parameters. Maybe try something along the lines of what Restek has, I think starting at 140C is a bit hot for the volatile compounds.

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I think you’re starting way too hot too fast. Try ramping from 40-100.

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too hot indeed

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What method do you want to set ? Cannabioids? in flowers/extracts/isolates ? Or in oil tinctures ?

Thank you.

180 was the (random?) temp @srihugh1 used in video on the capabilities of this machine.

It seemed too high. So I tried 140. No joy. I did try starting at 100C (and considered 40C) but that gave me a retention time of about 18min for pentane and at that point I had to get to the airport & head home for mantled turkey :slight_smile:

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Yeah, I’d gotten that far :wink:

Not changing the column (or what it’s packed with).

Nor the carrier gas (not an option).

Could change the gas flow, probably. Although it’s a built in hydrogen generator, and does not have the same interface (set screw) as the 8610C for adjusting gas flow. Operator would then have to undo said change in flow rate to run cannabinoids in the other column.

Lowering the injection volume certainly gives better shaped peaks, but I saw no changes in retention time.

Which leaves me with fiddling with the temperature controls. Which I tried, without much success.

Ha no my bad, now I understand you want the solvent. As writen above, start at the lowest temp you can, old for 1-2minutes, and then ramp up toward 120 at 5 degree C per minutes. And then ramp up more rapidly, like 30C/min toward high temperature (at least 280, or 320 in case of oils) and hold at least 3 minute to clean out the line.

I don t know this type of column, you should try variations around this.

Goal is to be able to look at extract samples and see how much solvent is still in there. Using headspace. Which will be qualitative rather than quantitative given no auto-injector or heated lines.

I can probably work with what I’ve got, in that I should know if an extract saw pentane vs acetone, but it would be nice to calibrate each and not have the operator “guess” which solvent they’re looking at.

Solvents in lab were; methanol, ethanol, isopropanol, pentane, acetone.

The acetone is only around for cleaning and prepping samples for the GC. So technically I would not expect to see it in my finished extract samples

This is the machine I’m using

Just noticed that Hugh used 140C here.

So it’s possible I pulled 180C out of nowhere…

So…as pentane and acetone seem to have very similar affinities for the stationary phase (Hayes sepD?!?), my best bet is to start below the boiling point of one of my analytes?

So 40-50C, isothermal for a min or 4, then ramp to something warmer to get everything else moving and through the column. ?!?

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Yeah that’s what I’d do

Subbed for some gc goodness. One day I’ll find an eBay unit with an fid for under 500 dolla

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Yes, that’s exactly it. The iso hold allows for separation. Can you go even lower than 40C on your initial temp?

We currently supply a 3 foot packed column that is one half Haysep R and the other half is Haysep D for this application. We used to just use the Haysep D. The combo column works better.
I would be happy to send you a free Haysep R/D.
I think acetone and pentane separate on this column at pretty much any temperature program, but the one I currently favor is 180 isothermal for 10 minutes.
Call me a 310-214-5092 and I will get the column shipped to you.
Hugh Goldsmith
SRI

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Apparently I am gonna try changing what the column is packed with…

Big thanks to @srihugh1 !!

(And every one else who chimed in).

Had I not run out of time before my plane flight, I probably would have gotten around to the 40C start temp…but guessing and not making progress isn’t my favorite pastime :shushing_face:

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It will always seem more worthwhile in retrospect. You’ve probably heard the old Edison quote “I haven’t failed, i’ve discovered 10,000 things that don’t work”

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As in I wasn’t hallucinating?

that makes me feel better about starting with 180C… :rofl:

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I’ m sorry in fact Im not aware of how it works with such packed column. The progressive temperature gradient I and other here advised works well for long capilarry column where solvents are weakly retained.

As Mr Goldsmith appears to favor a hot isothermal approach, I guess the separation relies in that case much more on the interactions of the solvents with this kind of column, rather than on their differences in boiling point.

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