Chromatography Forum

Hi,
I'm helping another group get a GC with TCD up and running. They were initially analyzing N2, CO2 and O2.
Barring any column issues can the TCD also detect Argon, methane, Helium and Hydrogen (assuing the carrier gas does not interfere)?

Additional background. We initially did this analysis out of house by GC-MS. We tried to bring it in house cheaply (hence the GC only). I know the carrier gas will have to interefere with either the N2 or He. Any other options for a carrier gas?

Thanks!

Jesse,

You can measure these gases, but you may have to use different carrier gases. The most common carrier gas used in the US is helium, mainly because it is inert and gives good sensitivity for most samples except hydrogen and helium. To measure hydrogen and helium reliably you should use nitrogen or Argon as carrier gas.

What are the concentration levels you will be looking at, as this is important to know for a TCD.

Gasman

You must look at the thermal conductivities of the gases involved and the column you wish to separate the gases you wish to analyze.

To detector hydrogen or helium or other noble gases at low levels you must use a gas that has a different conductivity. You could use a hydrocarbon like methane, or preferrably, nitrogen (or argon?).

To detect low levels of hydrocarbons, carbon monoxide, carbon dioxide, oxygen, nitrogen, you should helium, hydrogen (or argon?)

Many analysts use two different carrier gases and two TCDs to check for everything possible.

Not all column packings are equal in quality, even though they all may say Molecular Sieve 13X or 5A, not every lot works the same.

Not many other gases are inexpensive enough for routine work or are available at all.

Hydrogen, helium, nitrogen, and argon are your best choices.

best wishes,

Rod

Here are our compounds of interest and common levels (by GC-MS)

N2- 10%
O2- 1%
Argon- 0.15%
CO2- 87%
H- 1.2%
He- <1%, usually not there
Methane- 0.12%

The oddity is that there should never be any methane. Is it possible that the GC-MS analysis lab is using methane as a carrier and reporting trace levels?

Also- Because of the compounds we are finding, it looks like there are no good carrier gases that will allow for detection of all compounds in one sample run.

a single run? no, all the good carrier gases are part of your analysis !

The Ar and O2 peaks require low temperatures to separate cleanly for the TCD to measure them.

Plus you wish to measure hydrogen and helium at low levels while still seeing nitrogen.

I do have one suggestion. After comparing thermal conductivities of all the gases, could you use CO2 as a carrier gas? Measure each small amount of the other gases, and then calculate CO2 by difference? Methane might be the only problem gas to measure if my memory is accurate.

Please check and let me know. Thanks.

Rod

I do have one suggestion. After comparing thermal conductivities of all the gases, could you use CO2 as a carrier gas? Measure each small amount of the other gases, and then calculate CO2 by difference?

Probably not as CO2 is the "critical gas in this analysis. We are pushing for CO2, N2 and Other as the acceptable analytes.

If you can accurately measure the small components accurately (1% error) then your measurement of CO2 by difference will be more accurate than if you measured CO2 directly with the same accuracy.

If 15% impurities are measured with a 1% error (0.15%) and you take the difference to be [CO2 = 84.85%-85.15%], that is more accurate than measuring CO2 at 85% with a 1% error (0.85%) [84.15 - 85.85%].

It is assumed that no other gases are present that are not being measured. (ie water)

This is done in the process monitoring industry routinely. It requires all impurities to be counted though.

If CO2 and Nitrogen are to be measured directly as primary measurements, then helium and hydrogen are your best gases.

best wishes,

Rod

Conductivity Values for fixed gases
Weist Handbook of Chemistry and Physics
value Carrier gases difference of response
Helium Nitrogen Argon Hydrogen
Hydrogen 471.1 95 405 426 0
Helium 376.1 0 310 331 -95
Nitrogen 65.7 -310.4 0 20 -405.4
Argon 45.5 -330.6 -20 0 -425.6
Oxygen 68.2 -307.9 3 23 -402.9
Methane 89.3 -286.8 24 44 -381.8
Carbon Monoxide 63.9 -312.2 -2 18 -407.2
Carbon Dioxide 43.8 -332.3 -22 -2 -427.3
Ethylene 55.0 -321.1 -11 10 -416.1
Ethane 58.3 -317.8 -7 13 -412.8
Propane 48.4 -327.7 -17 3 -422.7
Butane 43.4 -332.7 -22 -2 -427.7

Sorry about the previous post all the formatting disappeared. Put the values into columns

First column Gas
Second column Conductivity value
Third column Using He carrier cond diff
Fourth column Using N2 carrier cond diff
Fifth column Using Ar carrier cond diff
Sixth column Using H2 carrier cond diff