Chromatography Forum

Recently, we have being faced with the challenge of having contamination (i.e. air interference) with our hydrocarbon gas samples. We use Agilent 7890 model GC with Packed columns attached to a Thermal conductivity detector. The columns separate the Nitrogen, Carbon dioxide and also Oxygen(if present in the analyte) compositions. Since we know air is abundantly of Nitrogen (about 78%) and Oxygen (about 21%), we use this ratio to knock out the presence of air in the sample.
Recently, this ratio has failed when we have such contamination in low pressure and low volume hydrocarbon gas samples.
What can we do please?
Thank you.

Are you running a filter on your carrier gas and is the filter close to the instrument? Are these samples in headspace vials? Tedlar bags? How are you getting samples on the instrument?

Yes, our carrier gases have filters and they are close to the GC. We manually inject the gas from the sampling bottles via liners into the GC. So no vials needed here.

BlessingNasara,

Don't inject with a syringe. Going from standard to injector allows introduction of air. Use a gas sampling valve instead. Pressure fill the loop, allow it to come to atmospheric pressure and switch the valve to on the column. Much more repeatable, less contamination, better accuracy.

Best regards,

AICMM