Chromatography Forum

We are having some trouble with rippling baseline with our GCFID. We believe it may be down to contaminated gas supply. Do you guys run your gas to empty? we were recommended to not to do this and to change our gas when it gets as low as 400psi. Have any you guys actually heard of this before? We using normal grade hydrogen, nitrogen and instrument air.

You shouldn't run them until they're empty. There can be contaminants in the bottom of the cylinder. I try not to go below 500PSI, but 400 should be fine. I've run out before and haven't had a problem, but I don't do it on purpose. If you've run out and have problems, make sure you change any purifiers you have on the empty cylinders because they are probably bad.

Yes, I first heard of this over 30 years ago (and every year since).

Any contaminants in the gas cylinder tend to remain unvolatilized at the higher pressures and do not leave the cylinder. But if the pressure drops too low the contaminants can turn to vapor and play havoc with your baselines.

Since cylinders are not always cleansed between possibly several fillings, it is a good idea to change out the cylinder when the pressure drops to below 200 psi.

This also gives you a margin to blow out the lines when replacing cylinders.

I have had to clean regulators and gas lines when this procedure was not followed, much to my dislike.

best wishes,

Rod

... Do you guys run your gas to empty? we were recommended to not to do this and to change our gas when it gets as low as 400psi. Have any you guys actually heard of this before? We using normal grade hydrogen, nitrogen and instrument air.

Never, never ! Always leave ca. 5% of cylinder volume (calculation is easy to express it in pressure units). Ask your gas supplier, not users.

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Here is an excerpt from Restek literature:
" As a general rule, change a cylinder when the pressure regulator indicates that there is
200-300psi remaining in the cylinder. As the cylinder pressure drops, the concentration of
impurities such as moisture and hydrocarbons increase. Therefore, column damage or
premature purifier consumption will occur if you attempt to "save money" by using all the gas
left in a cylinder. In addition, if the cylinder pressure drops below the supply pressure required
by the GC, retention times and detector sensitivities can slowly change and affect the validity
of your data."

Thats great thanks for the quick response. We have changed them to stop at 200 psi so hopefully thats good enough. thanks again

Another excerpt, this time from Scott Specialty Gases:

"All cylinders should be returned with at least 50 psig (3 bar) positive pressure to avoid contamination of the cylinder."

That's more the lookout of the cylinder filler to be honest, most of ours have a non-return device which also retains 3-4bar in the cylinder for that reason.

We have ours in pairs on manifolds and only change them when empty (while the other one is in use). Maybe you need gas line scrubbers? We don't use them on all our GCs though.

My observation of shifting baseline was in switching between UHP air and breathing air for the FID's. I got a nice sine wave baseline when they switched over. It did not really make a quantitative difference and it was way cheaper so they stayed with the breathing air.

Best regards,

AICMM

Really??? Breathing air for GC-FID? My heart skipped several beats when I read that post.

I've heard horror stories of ruined GC's when lower purity gases are used. Do you have to clean or change out your detector?

Really??? Breathing air for GC-FID? My heart skipped several beats when I read that post.

Take care of your heart before you read further.

I've heard horror stories of ruined GC's when lower purity gases are used. Do you have to clean or change out your detector?

Urban legends. I use ambient air from chromatography room only compressed and fed through zero air generator.
Do you really think that air is the culprit of dirty FID ? What part of FID ?

I the past, field service engineers for different GC vendors have told me that using breathing grade cylinders was what was contributing to our dirty FID problems. This is since the breathing air would not have the higher purity of the UHP grade air, i.e. possibly more contaminants.

Also, the zero air generator would be scrubbing the ambient air to make it UHP quality.

...

Also, the zero air generator would be scrubbing the ambient air to make it UHP quality.

In fact zero air gas generator sounds great, but it only oxidize trace hydrocarbons to CO2 and water.
Hence only if air before zero air gas generator is UHP then it stays UHP.
Otherwise only hydrocarbons content is reduced. Nothing else.

I the past, field service engineers for different GC vendors have told me that using breathing grade cylinders was what was contributing to our dirty FID problems.

What babies those field engineers are! How about the GC we received (from a warehouse from production facility in a different state) that had been set up to use house air with no purifier for FID? We found maybe 25 ml of lubricating oil from that house compressor in the FID, and some had spilled over and contaminated the oven insulation.

Let me add that oven heating produced a "distinct" odor. We ended up cleaning everything we could, tore open the oven compartment top metal, pulling out what oily insulation we could, and stuffing in fiberglass tape as new insulation. Then we used it for assays under 200C, for non-regulated stuff.

... We found maybe 25 ml of lubricating oil from that house compressor in the FID, and some had spilled over and contaminated the oven insulation...

Moving parts demand lubrication ! Unfortunately they were not aware FID has no moving parts.
And a dose of oil as protection against corrosion never hurts.
Never met such "inventors".

BTW - what was that instrument that had FID volume 25 mL or larger ?

BTW - what was that instrument that had FID volume 25 mL or larger ?

Hewlett-Packard 5890 Series 2. The oil was both on the flow control for the air to the FID and spilled over to the top of the GC - a real mess.

Even after cleaning/replacing everything in the air flow path and FID, and removing what oil-soaked insulation we could, bad smell if oven got over 200C. GC was nicknamed "Smoky", so we never went above 190C on it.