Does carrier gas/reference gas flow rate really matter?

Discussions about GC and other "gas phase" separation techniques.

4 posts Page 1 of 1
Hi,

There is no peak when I used my TCD detector on HP5890 series II.

The reference flow rate is 18 ml/min and the column flow (no makeup gas) rate is 6 ml/min, there is no peak. Currently I just want to see a peak regardless the quality of the peak. Can I see a peak if the flow rate ratio is not correct? or maybe the peak quality is not good but I can still see a peak? What change should I made for the flow rate if I want to see a peak? Increase the reference flow rate or decrease it?

Thanks so much! I checked everything I can for my TCD detector but there still no peak. Please help!!!
I must admit that I have never used a TCD in my professional career. It is not sensitive enough for what I do. However, my understanding of that detector is that you will get the best sensitivity if the conditions in the flow cell and reference cell are matched as closely as possible. Have you simply tried to inject a ridiculous amount of analyte? I don't know what your columns are but if helium is your carrier gas, try to inject 1 mL of 100% CO2 (at atmospheric pressure of course). Something that elutes early so you don't have to wait all day for your results. I have a small lecture bottle of 40% methane in nitrogen in my lab that I use to set flow rates. Inject a lot of something like that and see what happens.
Not speaking to this particular thread but to the TCD in general, the 5890 TCD needs about 3 times as much reference as carrier and make-up. This is unique to the design of the HP/Agilent TCD. Speaking to TCD's by others, in general the reference flow should match the carrier + make-up (if any) flow.

Having said all that, the TCD is a concentration dependent detector such that sensitivity decreases with increasing flow (increased flow acting as a diluent.) But not in this case because the flows are quite reasonable.

Without having read all the other threads on this particular TCD, did you check the flow out of the end of the column or the end of the detector?

Best regards,

AICMM
Hello!

You do not define what is your carrier gas and what compounds you want to identify. Therefore, I give a general suggestion.

I think that first you should compare the termal conductivity of your carrier gas and those compounds you are interested to detect. When you are using a TCD detector, always have to check this parameter, because if the thermal conductivity of both your carrier gas and compounds are similiar, the signal or peak intensity would be weak, and you could not identify and see easily a peak signal. I tell you this because I tried to identify methane using two types of carrier gases, He and N2, and the signal generated when I used He was bigger than using N2. Finally, I decided to use N2 as carrier gas due to I wanted to identify H2 too.
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