Changing Gases - Urgent Please....

[sadsal123]

Dear all, my current set up for gases we use on our GC is as follows: H2 and Air for FID and N2 as carrier.

Taking up advice from many of you kind people, I have suggested using H2 as carrier and luckily managed to talk company into doing this for the obvious benefits.

My question is, if I change the carrier gas, do I need to change the other gases? Will it be ok just to change the carrier on the current set up? I need the answer urgently as I may not be here too long and want to sort it out before I go.

[Peter Apps]

Hi Salma

Congratulations on talking some sense into your company.

As long as you work with ordinanry capillary columns you only need to swap the carrier gas supply. The little bit of hydrogen that comes out of the column will not affect the FID.

You could use the nitrogen as make-up on the FID.

Regards Peter

[sadsal123]

Thank you Peter , I have been waiting for YOU to log on for a while now

I suggested the 2 FID and 2 Column system but that will cost nearly £4500, so it was a no-no. This will cost us much less. Everything boils down to cost

[Bruce Hamilton]

Depeneding on the instrument and laboratory, remember that H2 is highly flammable, so split vent should be vented to an agreed safe location for flammable gases.

If your carrier gas has a purifier train, it's usually better to buy a new one for H2 carrier, especially if your FID hydrogen system only has the driers and activated carbon. You can keep the nitrogen as both an alternative carrier gas and FID makeup gas.

Bruce Hamilton

[EnvironLab]

Just a little question: What happens if your column happens to break inside of the oven near the detector end?

[Peter Apps]

Having the column break near the detector is no problem. 2 ml/minute hydrogen leaks into the oven from the upstream side of the break and a little bit from the downstream side (nealry all of the FID fuel hydrogen still goes out through the jet because of the resistance of the peice of column still in the detector, and this is the same no matter what carrier gas you use). Hydrogen diffuses so rapidly that at these flow rates it leaks out through the oven walls, and around the door and ventilation flaps etc so rapidly that it never reaches the lower explosive limit.

The only times that hydrogen as carrier gas poses an extra hazard is a break in the column very near the inlet, or the carrier gas on and no column installed, and with a high split ratio (total flow). Many GCs will shut down the carrier gas under these conditions anyway.

Peter

[Bruce Hamilton]

[quote="EnvironLab"]Just a little question: What happens if your column happens to break inside of the oven near the detector end?[/quote

The main risk is when no column is installed. In the 1980s, during a visit here, Walter Jennings once said that the only explosion he had was when he forgot to hook up a 1/4" glass packed column and turned the pressure-controlled hydrogen carrier on . The lower explosion limit is 4% by volume, which is difficult to achieve, given that most GC ovens aren't hermetic

Modern GCs have sprung-loaded oven safety vents, or equivalent. On Agilent GCs, the inner door seal is sprung-loaded with securely-latched outer door that will vent when internal pressure rapidly increases, and then reseal when it drops. The danger from a hydrogen explosion is that a closed, sealed oven would disintegrate.

From my experience, even forgetting to put in capillary column for almost an hour at 100 ml/min H2 doesn't seem to cause a bang on a HP5890, or if it did, I didn't notice .

Bruce Hamilton

[gcguy]

I have heard a story that Agilent (or HP back in those days) tested a gc to find out what happens to it during a hydrogen explosion. The story goes that they had to connect a seperate line straight into the oven as there was not enough flow through the inlet to acieve the magical 4%. After they had acieved an explosive atmosphere they still had to use a sparking device to get it to go off even with the oven on!
The explosion was contained entirely within the oven and the gc was still usable if a little round at the corners.

Not sure if this is all true but it would nice if it could be confirmed.

GCguy

[GasMan]

gcguy

As part of the developement stage of all Agilent GCs, and previous HP GCs, the oven hydrogen explosion test is a standard part of the initial testing of the product. This is probably true of all GC manufacturers.

gasman