Fast GC

[wrighty]

Evening all,

Could someone provide a quick crash course on fast GC please? i am using an Agilent 6890 with a ZB-5 column (10mX0.1umX0.1mm) and am analysing FAME an FAEE (up to C24:1).

i have been having a go at method development and whilst i am achieving some success i am finding that larger c-chains (>C20:0) are not coming off as well as the smaller c chains (the larger peaks are more broader than the earlier, smaller peaks).

i would have thought that if i was getting a good profile on my old method (on a DB-23 column- ~25minute run time) that i should be able to get a method using a fast GC column?

any pointers would be greatly appreciated.

cheers

steve

[Tom]

What inlet pressure and ramp rate are you using?

[wrighty]

off the top of my head i cant think of the inlet pressure but i know i start with a temp of 180 and ramp to 205 degrees at 20 degreees/min.

the velocity is 46 cm/sec (h2 carrier gas) if that helps.

[Bruce Hamilton]

The reason for using a polar column for FAMEs is to resolve the various levels of unsaturation within each carbon number, and a ZB5 column will seriously screw that up, especially if you are including FAEE.

My recollection was that I needed to use a much longer non-polar column to achieve a lesser separation, and I would probably also needed to go to a higher final temperature. In some cases, unsaturated peaks were superimposed, and orders reversed, eg on aPolar column C18:0, C18:1, C18:2, C18:3 would become C18:2, C18:1, C18:3 very closely bunched, followed by C18:0.

I'd actually use a more polar column for FAMEs, it would be shorter, have less of a temperature gradient, and a lower final temperature - which is nicer for the longer chains and PUFA, than a ZB5.

If you do want to use that column, increase the flows through it, take it to a higher final temperature. Your ramp rate could be a bit steep, taking about 1 minute, I'd be looking at about 10 c/min over 6 minutes.

Just do whatever works for you, but I'd need a good reason to not use a FAME column such as DB225, BPX70, etc.

Please keep having fun,

Bruce Hamilton

[Tony]

Dear Steve,
We routinely use fast GC for FAME analysis, using a SGE BPX70 10m x 0.1mm ID x 0.2um film column under H2. Run mode is constant flow under a 150-220°C ramp, with a 1/200 split on injection. In out hands, we found a high split ratio and rapid injection technique to be important for good peak shape for the early eluters. Elution of FAMEs up to C24 is not a problem; using contant flow rather than constant pressure keeps the later eluting peaks narrow.

I agree with Bruce that using a ZB-5 column won't work well because it isn't polar enopugh to separate all FAMEs. However, if you are only interested in 24:1, this will elute between 23:0 and 24:0 on the ZB-5 and should be resolved OK. I would look carefully at your sample concentration, split ratio, and injection conditions, as it is very, very easy to overload these narrow-bore columns and severely distort peak shape. (I think SGE don't even make a BPX70 in 0.1µm film thickness for the overloading problems with very thin films)

cheers
Tony

[Tom]

I routinely use db-17 10x10x0.1 with helium. The van D optimum should still be a good starting point. I'll run the ramp as fast as the oven will go.
I have to put less on column but the peaks are tall and narrow.
...but I'm not running FAMEs so perhaps Bruce is right about the lower ramp or column phase inadequacy.
Let us know if it works.
Tom

[wrighty]

i know its been a while since i posted this so i thought an update was due. Firstly, the info was wrong in my original post (apologies for this). the column in question that i was originally using a ZB-Waxplus column, the ZB5 column was something from another one of my projects.

It took a while to figure out why people were talking about the ZB5 until i realised my mistake!

Anyhow, i got the BPX70 column ordered (10MX0.1mmX0.2um). This has worked a treat and everybody seems happy with the results.

Thanks for all the replies.

Steve