GC-FID make-up gas, and various Newb questions

[Vox]

So I'm discovering I don't know as much as I thought about GC and I apologize for asking so many simple questions, I've searched, trust me.

I've got He as a carrier for my HP5890 GC-FID. I've also bought the H2 for the FID. What other gases do I need? Is H2 the make-up gas? That's what I've been told but i hear people say that they use N2 as make-up gas. I'm confused. On the left side of my 5890 there are only connections for Air, Hydrogen and Carrier.

Also, I know I need Air, but how pure does the air have to be? I'm going to be analyzing ethylene down to 1 ppm. I hope that doesn't mean i need ultra-pure air. Is it possible to run off a compressor with the proper filters. Or do i need to go cylinder gas. I won't be running a bunch of samples, possibly 4000 total samples this year, so i'm sure that cylinder is probably the way for me to go.

As long as I'm at it, around what pressures am I going to want to run all my gases at? I understand I have to test to find out what works best, and it depends on my column. I have a 12m .25 I.D. capillary column.

Any help is greatly appreciated.

[chromatographer1]

HP 5890 Series II have an aux gas connection which is used for make-up.

One can use Helium or Nitrogen for make-up.

Zero Air should be used for FID work or the signal of your FID will be elevated. Noise can also be a problem. Determine by experience if lower quality air will cause you analytical problems. No trap will trap methane but they are useful and recommended for general purpose use.

I have never used bottled air for an FID. I don't know if your gas provider will always give you quality air. A Zero Air generator may be outside your budget, but I have always used them with great success.

Perhaps a lease might be possible.

As mistakes happen it is a good idea to have filters on your carrier gas and your hydrogen supply as well.

As far as pressures are concerned, it is a matter of adequate flow, and sometimes a faster flow than optimal works well. A short 0.25mm ID capillary should require less than 20 psi at most, and probably only 5-10 lbs of pressure routinely.

best wishes,

Rod

[Vox]

I have a 5890A. Any idea where this aux input would be? Also since I have a built in regulator for the carrier gas, would it be possible to run carrier and make-up gas off of the same bottle of HE?

Thanks.

[Earl]

Vox,

Yes, you can run the aux and carrier from the same He cylinder. Just put a T in the line and run 1 line to the aux (which should be on the back of the block which has the igniter switch on it) then the other line to the injection port pneumatics where the carrier line goes (or into the regulator which is how mine is currently set up).

I use zero grade air in a cylinder and have had no problems with this. Though I have a good provider.

The gas flows are restricted by the block with the igniter switch where the on/off switches are. A psi recomendation will not likely do you much good as these blocks can be adjusted for different flows, so you have to set this up based on flow out of the FID.

If you have a flow meter (you should get one if not) set your flows as such to start out then adjust for optimum sensitivity: He 30ml/min, AIR 400ml/min, aux 30ml/min. This will get the flame lit and get you started. Check the flow out of the FID with column installed and column head pressure set. Measure gases one at a time turning them on and off at the block. (you can buy a special adapter from restek to measure flows from the FID, but some people just make one)

I've heard that you can find the operators manual for this GC on the agilent website. This will be very handy for you to get.

[Vox]

Thank you for all your help. The way it is hooked up now it has the HE coming in and being split AFTER the initial internal regulator, then going to the aux gas of both FIDs and both injector total flow knobs. Will this work for using HE as make-up gas? I'm assuming it will since this is how it was connected when i bought it? How will I know how much to turn up the aux-gas knob?

Also I have two small tubes that aren't connected to anything. Attached are some pics. The two small tubes attach to the top right of the FID control blocks. Thanks so much once again.

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[GasMan]

Vox,

You can get most information about gas pressures from the operating manual. I did not have any luck getting the manual from the Agilent web site, but you can download a manual from the following site.
http://www.analytic-consult.de/download ... hp5890.pdf -.

From the pictures that you have posted, it looks as though the make-up gas is tee'd into the carrier gas supply, so if you are using helium as carrier gas you will also have helium as the makeup gas.

Flame detectors work at their optimum with about 30 ml/min of hydrogen, 350/ml air and about 30ml/min of carrier gas. if you are using a capillary column with a much lower flow rate, then you need the makeup gas to bring the 'total' flow of carrier gas up to 30ml/min. On the HP/Agilent systems, this make up gas is tee'd internally in the FID block to the hydrogen line, so you will not see a separate line going to the detector for the make up gas.

The two tubes that you are talking about are, as far as I can remember, vent lines for the air supply. When you press the ignite button, the air flow has to be reduced so that the flame may ignite. Pressing the button diverts some of the air out through these lines.

Hope this helps you

Gasman