Chromatography Forum

I have submitted gas samples for GC analysis for O2, N2, CO2, CO, C1-C4 hydrocarbons and H2S to two different labs. The results of analysis between the labs (using direct sample injection from canister to instrument from duplicate samples), both using a GC FID/TCD, do not agree. One lab reports H2 gas 2-3 orders of magnitude greater than the other.

The analytical procedure for the one lab, reporting [H2] of 1700 ppm is as follows:

Sequence of analysis: CO2, O2, N2, C1-C4 were run then the sulphurs and finally the H2.

- The samples were not diluted when the CO2, O2, N2, C1-C4 were run. The canisters were pressurized just before the sulphurs were analyzed.

- GC Model: 7890A GC FID/TCD

- Oven temperature: Run starts at 40 and goes to 200 at varying ramps. H2 comes out between 140 and 200

- Single inlet

- Argon Carrier Gas

- 1 mL Sample loop volume

- Backflushing time: 0.5min with carrier gas

- Column specs: Supelco 60/80 Carboxen 1000 15’X1/8” SS (2.1mm ID)

- 2X dilution with ultra high purity nitrogen

The other lab reports [H2] of 810,000 ppm. I am waiting to get details of analytical procedure for this lab.

Can you suggest possible gas interferences, or analytical errors that might cause this 2-3 order of magnitude discrepancy in Hydrogen gas?

Thank you!

1 - mis-identification of valve upset peak as a hydrogen peak

2 - calibration mix error

3 - human error

You realize that asking this question without GC hardware and parameters and chromatograms displayed is a big waste of time ? IMHO.

Where's the BEEF ?

best wishes,

Rod

810,000 ppmv is 81%, what are the concentrations for N2, O2? The total should be close to 100%.

Do you have any knowledge of the sample composition that would allow you to rule out one of the results? H2 may elute very near a valve actuation depending on the GC configuration (as chrom1 points out).

I am very surprised to see that you think that hydrogen elutes between 140°C and 200°C with a starting temperature of 40°C. Hydrogen is hardly retained by any column that I know of, and I would expect it to elute VERY early in the chromatogram. This looks like misinterpretation as mentioned by chromatographer1.

Gasman

While this column does retain hydrogen to a small degree, more than almost any other packing I know, with a 6m 2mm ID column at 120°C with helium carrier at 30cc/min (45 psi) the retention time of hydrogen is about 1.62 min.

best wishes,

Rod

Gasman

I think you were misled by the numbers w/o a label. I think they meant 140 to 200 seconds.

Rod

Post the cromatograph of both TCD detector. I assume that there is a data missinterpreted.