Chromatography Forum

Dear all,

Can anyone give me the pros and cons of choosing either a TCD or an FID + methanizer, for detecting CO2 in clean dry samples at levels just over ambient? (390 - 500ppm perhaps)



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We were trying for a long time to get an old GC-TCD working, were having LOTS of different problems and have finally given up and have found some funding to fit a new TCD into an unused GC with functional FID.

I am aware that FIDs have better sensitivity and linearity, but I don't know what the efficiency of a methanizer would be, and I can't get any good answers from my superiors as to why we are fitting a TCD rather than a methanizer, but there seems to be some resistance.

The GC-FID is a PE autosystem XL GC (is that a name??), and we would probably be using packed columns with porapak-Q.

thank you!
Nancy

Nancy,

The FID/Methaniser is usually used for the low concentrations that a TCD will not easily detect. This would be in the ppb to say 10 ppm of CO2. The concentrations that you need to measure are no problem for the TCD. Most methanisers are designed for a particular maximum concentration, and above that you will not get full conversion, so you should check with PE on what their limits are.

Gasman

GasMan is right, though you probably won't be able to go lower than 100-200 ppm on a TCD for CO2.

I would suggest an HID but I have a clear bias in that perspective in that I make an HID. I do not believe they are nearly as fussy as people make them out to be and they have wonderful sensitivity. Furthermore, if you have the separations with a TCE then you would have sufficient separations with an HID. Of course, you would have to have access to good purity helium to go this route. I believe the Valco HID and I know my HID will retrofit to systems that currently support an FID electrometer so the hardware investment would be minimal.

The other advantage of the TCD and HID is that you would also be able to monitor other constituents besides CO2 in the same run.

However, if you want more selectivity, then conversion to methane would provide a very good solution. If you do go with a methaniser, you want to be careful not to introduce a high amount of carbon into the system as this poisons the catalyst eventually. You can also adjust the loop volume used on the injection to compensate somewhat for the working range of the methaniser.


Best regards.