Chromatography Forum

Hey everyone, I'm trying to set up an Agilent 6890 GC with a TCD detector to separate and detect mixtures of H2, CO, N2, and O2. Unfortunately the concentrations of H2 and CO are very low, so I need to push the limit of detection of the instrument as far as I can.

I've been using helium as a carrier gas, and I realize that the sensitivity for H2 would increase by switching carrier gases, I'm afraid that I'll lose too much sensitivity for CO.

Here's my setup:
Agilent 5A mole sieve column (25m, 530 micron)
Helium carrier gas
Inlet Temp: 250 C
Oven Temp: 70 C (no ramp)
Detector Temp: 300 C
Inlet pressure: 32.0 psi
Reference flow: 32.0 mL/min
Makeup flow: 5.0 mL/min
Splitless injection

These settings give good separation for all the peaks, but as I stated above, I'd like to push the limit of detection a bit further (especially for H2) if at all possible.

Any advice on what I can do? Thanks for the help!

Relative Thermal Conductivity to Helium Carrier Gas:
H2 = 1.18, CO = 0.16, N2 = 0.17, O2 = 0.17

Relative Thermal Conductivity to Nitrogen Carrier Gas:
H2 = 6.97, CO = 0.955, N2 = 1, O2 = 1

Relative Thermal Conductivity to Argon Carrier Gas:
H2 = 10.2 CO = 1.39, N2 = 1.46, O2 = 1.47

If you can only use one TCD, then an Argon carrier gas provides the optimal LOD for analysis of H2 and CO based on relative thermal conductivities. Hydrogen gets better but CO gets worse. Keep in mind this will make the LODs for your other analytes worse.

The best way to perform this analysis is to use a GC with two TCDs, two sets of columns, and two carrier gases. So you detect H2 using N2 carrier on one channel and CO using He carrier on the other channel simultaneously.

With standard disclaimer here... An HID would do this analysis without any problem, especially at low levels. Think about this alternative if you are thinking of another detector. For a lot less than another TCD and column set.

Having no money to spend but lots of time (common with some people and students) then you should consider running H2 as carrier on the early part of the week to catch the CO from your experiments and running Argon on the latter part of your week to catch the H2. Considering the insensitivity of the TCD this may still not do it but it is about the best you can do in the circumstances.

Best regards,

AICMM