Chromatography Forum

Hello, I am relatively new to GC world and I would like to receive some advice or suggestions/precautions on collecting gas samples.

I am currently a graduate student in an engineering department and we just recently placed an order for a GC for a simple gas analysis to complement our research. We ordered a system with its barebone setup, nothing fancy, just a detector with a manual injection port and we do not have any plan to modify the GC setup at the moment.

We have a separate setup with a pressure vessel with our sample gases, our goal is to collect gas from this pressure vessel and inject the sample manually into the GC. What would be the best simplest way to collect gas and inject into GC without contaminating it?

I was thinking of either:
1. collecting the gas in a small stainless-steel vessel then use the syringe to collect and inject it into GC or
2. Use a small balloon to collect the sample from the vessel and put the balloon into a vial then use a syringe to collect and inject the sample.

Our samples are either going to natural gas or normal hydrocarbons. Would collecting these samples into stainless steel vessel affect the properties? if I use balloon approach, can I just buy any balloon from the local store or do I need to order some special balloon?

My background is mostly in mechanical works and I do not have any chemical analysis experience also I am the one of few laboratory guy in a simulation research group so, any inputs or suggestions are much appreciated. Thank you!

If you were looking for the best precision, I would say mount a rotary injection valve in the oven and use the gas to fill a loop of fixed volume and inject at atmospheric pressure by rotating the valve after filling the loop. I have a system that is used to analyze beverage-grade CO2 and that's how we get the best precision.

If you are stuck with making syringe injections, I still advocate making injections that are atmospheric pressure. You need to make sure that the volume you inject does not overfill your inlet. I would recommend injecting the smallest amount possible that still allows you to detect with confidence the analytes of interest.

If you can reach inside your vessel with your syringe, you should be ok with what you have planned. You might run into precision problems if the pressure in that vessel is too high. Just try it a few times and see how you do. Is the error in your analysis of replicates of the same sample within some acceptable tolerance? For example, if the error in your measurement is 20%, are you ok with that? If you're trying to tell the difference between ethane in your sample at 100 ppm, at 20% RSD you'll have to be ok with saying that a 120 ppm ethane content is still 100 ppm within the error of the measurement.

A balloon made of a synthetic material (e.g., latex) is a bad idea. Don't do that. If you want to fill a bag-like material like that, buy some sampling bags made of this material called Tedlar. It is synthetic but lots of environmental sampling is done with those. They're good as long as your samples won't be sitting around for long periods of time. They also have a rubber septum on them so you can reach inside with your syringe. They're also fairly low-pressure devices.

Good luck!!

Several of my customers use an inner tube from a wheelbarrow to collect samples (remove the needle stem.) Need to have a slight positive pressure but other than that works quite well.

Best regards,

AICMM