I'm trying to calibrate a Buck GC 910 for CO2 analysis. I varied the concentration of CO2 and N2 diluent using mass flow controllers and passed them directly through the GC. I realized that for different total flow rates but same concentration I get different area percentages. For example, a flow rate of 1ml of CO2 in 9ml and 3 ml CO2 and 27ml of N2 gave different area percentages. Why does this happen? Is the flow rate meant to be constant for various calibrations even if the sample volume of the TCD(valve switch) is constant?
Yes, TCD is very sensitive to flow and the relationship is not linear. You need to set your flow rate to optimum sensitivity and then vary the concentrations in that same flow rate
See the 'Van Deemter' plots.
http://ion.chem.usu.edu/~sbialkow/Classes/361/GC/GC.html
TCD is concentration sensitive detector. When concentrations are same you should get same areas. When you change the sample flow rate, are you changing the reference cell flow ?. The sample flow rate and reference flow rate should be approximately same.
Confusing element in one answer:
(1) My math says 3 in 30 is 10%, and 1 plus 9 is 10, so 10%.
Advice:
Never believe a concentration based on flow rate control and mixing without an external check of flow rates
TCD measures lots of effects besides composition of mixtures.
TCD detectors are flow rate sensitive, since they are usually wheatstone bridges set up to measure very small differences in thermal conductivity. They are also non linear for the same wheatstone reason.
TCDdetectors respond to thermal conductivity differences, so there are many extraneous items which give response and some which give non linearity.
TCD detectors are pressure sensitive, so their response changes with anything which affects the pressure in the tcd cell via the PV=nRT (rearranged to n/V= P/RT) and thermal conductivity equations.
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