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Offline Cantacoxinha

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GC parameters question
« on: February 07, 2019, 04:07:31 AM »
I'm a noob in gas chromatography and I need an advice. I recently started working on an old GC-FID which was running for years with packed columns. I was told to switch to a brand new megabore column with internal diameter = 0.53 mm. The stationary phase is unchanged.

However, when I switched them, the peaks of the compounds I'm measuring were almost absent for working stock solutions, and had an ugly shape. I think that the gas flow rates might be the problem. The rates for the old packed column were 17 psi for nitrogen (carrier), 34 psi for hydrogen and 13 psi for air.

How should I adjust the flow rates when switching from packed column to megabore? Should they be increased or decreased?

I think that the installation was done properly, although I heard two different opinions on how deep the column must be inserted into the FID, one stating that the end of the column should be placed just above the FID coil, while the other saying it should be inserted approx next to the hydrogen inlet, which is before the FID coil.

Any advices?

Offline Cantacoxinha

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Re: GC parameters question
« Reply #1 on: February 13, 2019, 05:30:11 AM »
How should I adjust the flow rates when switching from packed column to megabore? Should they be increased or decreased?

I think that the installation was done properly, although I heard two different opinions on how deep the column must be inserted into the FID and injector, one stating that the end of the column should be placed just above the FID coil, while the other saying it should be inserted approx next to the hydrogen inlet, which is before the FID coil.

Any advices?

Anyone? Trying to fix this, but had no progress in a last couple of days...

Offline hypervalent_iodine

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Re: GC parameters question
« Reply #2 on: February 13, 2019, 10:26:42 AM »
How should I adjust the flow rates when switching from packed column to megabore? Should they be increased or decreased?

I think that the installation was done properly, although I heard two different opinions on how deep the column must be inserted into the FID and injector, one stating that the end of the column should be placed just above the FID coil, while the other saying it should be inserted approx next to the hydrogen inlet, which is before the FID coil.

Any advices?

Never used megabore columns, sorry. You could try asking over at chromforum.org? They are quite helpful with this sort of stuff!
Anyone? Trying to fix this, but had no progress in a last couple of days...

Offline P

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Re: GC parameters question
« Reply #3 on: February 14, 2019, 10:24:46 AM »
How should I adjust the flow rates when switching from packed column to megabore? Should they be increased or decreased?

I think that the installation was done properly, although I heard two different opinions on how deep the column must be inserted into the FID, one stating that the end of the column should be placed just above the FID coil, while the other saying it should be inserted approx next to the hydrogen inlet, which is before the FID coil.

Any advices?

I can't give a direct answer, but have you tried running standards or calibration samples with both scenarios to see how they compare with what your expected results should be?

I had to strip and reassemble an old GC once when doing work experience. It took a lot of fiddling about to get it working and calibrated.


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Offline marquis

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Re: GC parameters question
« Reply #4 on: February 14, 2019, 07:12:00 PM »
I'm not surewhat size your regular packed columns are, but there is a large difference in flow and theoretical plates when you switch from a packed column to a megabore.  Megabore columns normally have many more theoretical plates, but take much lower flow.  Best answer, call the manufacturer.  They normally have the adapters you need and can tell you what difference in flow rates to use.

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