"fast" GC

[GT]

Dear all,
I've got 3 GC methods to evaluate solvents purity by GC FID, using DB-WAX column, 30 mt, 0.53 mm ID, 2um film, with run time about 30 min, carrier nitrogen.
Is it possible to reduce run time to about 10 min with 0.25 ID or smaller column?
I use an Agilent 6890 with electronic pressure control but without any accessory for fast GC.
Could you also suggest me some websites speaking about this?

Thanks a lot for your help
Regards
GT

[Peter Apps]

Changing the carrier gas to helium will approx halve the analysis time, and changing to hydrogen will halve it again without changing anything else except the oven programme. Check out the forum archives for good discussions on optimum gas flow and temperature programme rates for columns of different diameter.

Peter

[virtu]

Hi,
1. column i.d. (decrease, 0.32mm)
2. column length (decrease, 10m)
3. thick of film (decrease, 0.5mkm)
4. velocity of carrier gas (increase)
5. He, H2

Fast GC?

[Csaba]

Hi,
It depends on on the resolution you have, and the one you are satisfied with. And the temp rate limitation if using a temp gradient.

We have methods that are <10 minutes using 0.32 µm columns, 25 m HP1 and HP5 columns using He with more than enough resolution.

Just try!

[aldehyde]

Like they said helium will help, hydrogen will help even more. Because the separation efficiency maximizes for hydrogen at a lower velocity you you can increase your temperature ramp to decrease the over all run time without ruining your chromatography.

Decrease column ID, change to helium or hydrogen, post details of your flow rate and oven temperature program. What kind of detector?

[beate]

Hi,

I decreased a FAME analysis with 37 peaks from 64 to 13 minutes.
Column: 0.1 ID, 5m length
carrier gas He (as it was before)
You have to increase the flow as the Van Deemter curve is different at this ID.
Increase your sampling rate (decrease sampling time) as the peaks width decreases, so that you have enough data points for each peak (20).
In the short oven program you will have high temperature rates, the oven must handle this reliable, should not be such a problem if the GC is not very old.
To have an optimal separation over the whole chromatogram I have 3 different oven rates, (3 steps).
Good luck
Beate

[AICMM]

GT,

As Peter notes, changing carrier from nitrogen to helium or hydrogen will shorten your run time significantly. However, be a bit more wary of phase and i.d. When you start doing that, you start to change the loading capacity of the column and, depending on what you are shooting, you can degrade the separations by not paying attention to this fact. So, for example, if you are shooting neat materials (or percent levels) you probably do need 0.53 and thick film. Depends on your application.

Simplest thing to do is change carrier and then decide where to go from there.

Best regards.

[RCBarata]

Well if the goal is just solvent purity I'd really shorten the column and maybe worl in isothermic mode, besides changing to hydrogen, if possible.

Any comments on this sugestion?

[Peter Apps]

Whether the column can be shortened depends on the resolution required for the most diffcicult to separate pair of peaks. Running isothermal can reduce cycle time, but unless the peaks have a restricted range of elution times (which implies limited resolution) the increase in run time will more than cancel the time saved by not having to wait for the oven to cool. Also, if you run isothermal the later peaks are wider, which makes them less detectable.

Peter

[GT]

Dear All,
thank you very much for your replies.
We made some tests in the meantime:
moved from gradient to isothermal methods, reduced the resolution for critical peaks to values around 2 (is this value enough? Before it was greater than 4), obtained without change column. In this way we obtained a run time from 25 to about 10 min.
We made also tests changing carrier from nitrogen to helium without any improvment of separation /resolution between peaks, so we came back to nitrogen.
Thanks again
GT

[Peter Apps]

Hi GT

Thanks for the feedback. In going to isothermal, presumably your column temperature is now at the high end of the programme you used to use ?

At their individual optimum flow rates each of the carrier gasses gives the same (for all practical purposes) resolution - the benefit with helium and hydrogen is that you get this resolution in a shorter time.

Peter

[aldehyde]

To add to what another person was saying about fast oven ramps I have seen problems with large diameter columns in GCs with small ovens (such as the agilent 6850). With the 6850 25 C/min is about the max you want to do because the oven will heat up but the actual column will not absorb the heat quickly enough.

I've only seen this be a problem in one case but its worth thinking about when you're optimizing .

[TonyG]

Why would a smaller oven size affect the column heating rate? I would assume that the column thermal capacity and air circulation rates are the same... This is of interest to me as I use a 6850/5973N.