Benzene and naphatalene in methane

[Christian]

Dear collegues, I am developing a method to resolve a gaseous mixtures containing mainly methane (~ 70 % molar), hydrogen (~ 15 %), argon (~ 7 %) and 4 - 4 % benzene and naphtalene, and traces (< 1 %) of other aromatic hydrocarbons. My GC has a Molsieve 13X and TCD, and a CPSil 5 CB (60 m, 100 KPa) and FID systems. My intention is to quantify relative molar fractions (e.g. C6H6:CH4) to be used in a kinetic research.
The injections are made by means of two six-way valves in series (loop volume 0.1 ml).
My problem is with the capillary/FID system.
Methane peak saturated the detector so I had to raise the split ratio; in the meanwhile, I lost definition of some small peaks (traces). I've got the following questions:

How can I get a methane peak within the range of linearity of the detector, without saturating it, and at the same time enough injection of traces to quantify them? Would the use of the split/splitless injection technique be of any help? I tried it: methane peak was reduced, and the other peaks went up. Am I overloading the column with this sample quantity?

If anybody has suggestions I'll be grateful.
Chris

[chromatographer1]

Christian,

You will have accurate measurement of your mixture if you use the proper tools.

You need a column, 9m 2.1mm ID 3.17mm OD of 316SS composition (glass or Fused Silica lined optional) packed with Chromosorb P 100/120 (or 80/100 mesh) coated with 25% SP-2100.

Using a TCD you will be able to measure accurately the methane balance and traces even at levels of <100 ppm with all mentioned analytes baseline separated.

Use He carrier containing 7% hydrogen if you want precise H2 measurement, otherwise measure H2 by difference using H2 carrier.
Argon, Methane, CO2, and the homolog alkanes and the aromatics will be separated cleanly (ethyl benzene and m+p-xylenes will not be baseline separated). You will be able to measure down to levels below 100ppm and will be able to calibrate your detector. With the FID you will be required to use a smaller sample loop and you will not measure H2 or Ar.

It is recommended that you use Argon carrier to measure hydrogen separately for confirmation of the measurement by difference.

You could do the light gases <3 carbons on the TCD and then measure the higher hydrocarbons by switching the column flow to the FID.

With this mixture if you try to split the sample you will introduce inaccuracies to analyte measurements you will probably want to be measured with great precision and accuracy.

You may email me directly for questions you may have.

Rodney George
Gas Separations Research
Supelco
Bellefonte PA USA
rgeorge@sial.com
814-359-5737