New method development

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

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I would like to know your approach on new method development.

1) GC column manufacturers column flow recommendations on a 30 meter 0.25 x 0.25 column for hydrogen carrier is 2.5 ml/min. which results in an average velocity of 72.43 cm/sec for my 175C oven ramp method.
2) The van Deemter curve for Hydrogen lowest (most efficient) HTEP for separations is 38 cm/sec which results in a column flow rate of 1.06 ml/min, which is well below the column recommended 2.5 ml/min!

So it works out that there is no way to use the best HTEP flow rate and still comply with the column manufacturer flow rate recommendations. Not even worrying about shark fin peaks from too fast a flow rate. What approach do you take to make the best method given that all of the peaks are resolved in either of these scenarios? *I have been in the past primarily been relying on the Van Deemter chart information for the column flow for He2 or H2 - and trying to set the average velocity close to the best minimum HTEP in case some unexpected production process impurity could show up some day. I’ve not been giving much consideration at all to the column recommended flow rates. How do I reconcile this discrepancy between these two parameters.

Thanks, for your time,
After posting my question I had some 180 views - and 0 replies! Is this lack of replies due to my question being so dumb as to not warrant a response - or is it that not many forum users develop GC methods? Is there another way that I should approach GC method development other than considering carrier gas flows and column specifications?

Thanks.
Try the manufacturer's method, ideal flow, and one in-between. Pick the one you like best. If you're running MS toss the compounds into a chrom modeler and play around with it, with ECDs you just need to shoot and see where all the non-target garbage comes off.
Technically, although it's essentially just missing the van Deemter "A" value, the better equation to be considering for average linear velocity on a WCOT GC column is the Golay equation. Snow does an excellent job of describing and discussing the differences here: https://www.chromatographyonline.com/vi ... _campaign=

In the interest of getting things done, very few people actually run GC methods at the Golay optimum linear velocity. I really like in Rood's excellent book how he discusses linear velocity to include best practical linear velocity. That's undoubtedly along the lines of what the column manufacturer is suggesting. We've compared 'best' versus 'practical best' linear velocities in our methods, and the improvements do not justify the added analysis time (we're running thousands of samples a week). As ev70 suggests, just start with the column manufacturer's recommendation, and move to the Golay/van Deemter optimum as needed.
Part of the answer is in the shape of the curve (either) - If you speed things up a little by running above the rate at the absolute low point, you give up little in terms of separation efficiency (but you're shortening run time, so Yay!). Where you don't want to be is running well below the lowest point in the curve as you give up a lot of separation efficiency while increasing your run times.

So, if you've identified a critical pair of peaks and have system suitability criteria in mind, run it at as high a flow rate as you can and still have some comfort margin with respect to your SST criteria. That's a thumbnail of how many of us would tend to approach it.
Thanks,
DR
Image
Hmmm - Posted, or tried to, this morning. Anyway, I never focused on Van Deemter. We used both helium and hydrogen as carrier gases, and used typical linear velocity/pressure from charts. I developed GC methods routinely, even published a few.

We'd make up analyte in solvent, tailor conditions to get decent retention time and peak shape. If OK, we'd make up placebo product in same solvent and see if the analyte's retention region was clear. Then we'd analyze placebo product spiked with analyte.

We'd adjust conditions and column as necessary.
Jake wrote:
Technically, although it's essentially just missing the van Deemter "A" value, the better equation to be considering for average linear velocity on a WCOT GC column is the Golay equation. Snow does an excellent job of describing and discussing the differences here: https://www.chromatographyonline.com/vi ... _campaign=

In the interest of getting things done, very few people actually run GC methods at the Golay optimum linear velocity. I really like in Rood's excellent book how he discusses linear velocity to include best practical linear velocity. That's undoubtedly along the lines of what the column manufacturer is suggesting. We've compared 'best' versus 'practical best' linear velocities in our methods, and the improvements do not justify the added analysis time (we're running thousands of samples a week). As ev70 suggests, just start with the column manufacturer's recommendation, and move to the Golay/van Deemter optimum as needed.



Thank you very much Jake (also others who took time to post)! After reading about the Van Deemter and Golay equations I discovered that I have been quite misled, misinformed, or simply ignorant, on the column carrier gas parameters. Your link eventually led me to relevant articles and the fact that Van Deemter was based upon a packed column, and that Golay was based upon a capillary column - BUT in a constant temperature isothermal run situation. Almost all of my chromatography methods are temperature programmed runs - therefore my obsessing over Van Deemter and Golay plots betrays my ignorance.
*This new information directs me back in my method conversion to hydrogen carrier and FID by starting with H2 on 0.25 ID column of a column flow of 2.5 ml/min (per column manufacturer), and slowing down the linear velocity to say an arbitrary 2.0 ml/min. I will check both flow rates to see if upon close zoom examination on my baseline for a typical sample if I encounter any co-elution problems. Barring any evident co-elution, or shark fin peak, I should default to the column manufacturer's recommendations as that would yield the most sample throughput.
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