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Calculation Question with Thermal Desorption Tubes

Posted: Sat Feb 03, 2018 8:48 pm
by rw228
Hi Everyone,

I test thermal desorption tubes on my GC FID and I have a calculation question based on some samples that I'm testing. Sorry if this is the wrong section to post it but, I thought this might be for the best.

I currently dilute a calibration gas standard into three levels and load it onto thermal desorption tubes to run on my GC for my calibration curve. I typically load the samples onto the tubes at 5psi. I need to test something that had a sample loaded onto it at 20psi. How would I take account for that in my results if everything is run the same?

Is it best to assume my standard gas and sample gas behave like ideal gases to calculate for the compensation of the pressure change? In addition would the equation be like this: (P1V1)/(n1) = (P2V2)/(n2)

Thanks for any help that you guys can provide.

Re: Calculation Question with Thermal Desorption Tubes

Posted: Wed Mar 20, 2019 12:56 pm
by bearcla
Do you know the flow rate at 5 psi? Just use
(flow rate at 5 psi)(concentration)=(concentration)(New total flow rate at 20 psi). If not pretty sure you can figure out the flow using PV=nrT and then just timing how long you load your tubes for.
mL or L/min

Re: Calculation Question with Thermal Desorption Tubes

Posted: Wed Mar 20, 2019 3:11 pm
by rb6banjo
I agree with bearcla. It's all about the flow rate through the sample tube and the time the gas sample flows.

5 psig through 1.0 meter of 320 µm tubing (like a GC column) gives an outlet flow (at atmospheric pressure) of about 33 mL/min.

Image

That's about the right flow to reliably trap most things on Tenax. Then, you just need to know how long you sample to get the amount trapped (or volume of gas passed through the sample tube).

That same 5.0 psig through a 530 µm column gives about 248 mL/min through the sample tube. That's too fast. Under these conditions, you might just be blowing your analytes through the sorbent. Most of the app notes I've ever seen for gas sampling indicate that they go at 20-50 mL/min through the sample tube.