Chromatography Forum

Hello!
I am facing huge differences in area response by valve injection and manual injection of gaseous standards into GC. For the same amount, concentration and split ratio, the manual injection gave responses around 200% higher. I tried change the load time and injection time between the range of 0.5 and 1 min and didn't note any differences.
Any one has any idea that could help in this case?
I recently started in the field of gases analysis.

With a gas sampling valve it is common to let the sample drop to ambient pressure before injection in order to get a consistent pressure inside the loop. Since you a pushing with a syringe, that may be causing part of the problem

For really consistent results, you should stick with the sampling valve. Put a rotameter or beaker of water on the outlet of the valve and sample. When the bubbling has almost stopped, make your injection.

It would be useful to know what types of compounds you are sampling by the way....

Best regards,

Thank you very much for the tips, AICMM.
The gas valve sampling entry is connected to a flow controller. We used this flow controller to adjust the levels to make the calibration curve for H2, CO, CO2, CH4. Since our flow controller only allows to mix two gases at a time, we did the calibration of each gas separately and than, we used a Tedlar gas bag to make a mixture of all of the gases in the mid-level concentration. This mixture was previously checked with another method and machine and it was okay. But when we injected this mixture manually in the machine through the appropriate entry, we got inconsistent results, despite we had sustained the sample volume and split ratio.
Best regards

Lilian,

Please clarify something for me. Did you take a sample of your Tedlar bag mix and inject it via the gas sampling valve as well or did you inject it with, for example, a syringe into the injection port?

Best regards,

AICMM

Depending how you have the valve connected to the GC inlet you will be getting different split rations between introducing the sample through the carrier gas inlet with the valve, or into the body of the inlet with a syringe. In particular, you will lose valve sample out of the septum purge.