Dissolved Gas Analysis-Gas Chromatography

Dissolved Gas Analysis-Gas Chromatography

Gas Chromatography is always an interesting topic to write on. There is so much technology involved from sampling, to extracting and testing and analyzing it can fill volumes. 

I have gone over some sampling procedures in earlier posts so here I will only mention that the sample has to be obtained carefully and it’s a good idea to never let it be exposed directly to the atmosphere.

Extraction is a job in itself. Once a sample has reached the lab the testing technician has to extract the gases from the oil in order to be able to put them into the chromatograph for testing.  An accurate measure of the oil used for extraction is needed first. The tech will usually use 35 to 40 milligrams of oil. This is normally in a syringe which is easy to measure or sometimes in a steel bulb which is a little harder. Oil poured out from a bottle can be tested but exposure to the atmosphere can skew the results.

In the lab a measured amount of oil is injected into a glass vessel under vacuum. The oil is then stirred by means of a magnetic stirring device under the vessel. Observation shows the oil will foam like a bottle of ginger ale when first injected. It will continue to bubble while being stirred. The stirring action exposes all of the oil surface to the vacuum and helps force the gases out of the liquid faster.  It is very much akin to opening a bottle of ginger ale. The contents,  under pressure holds gas, carbon dioxide, in the liquid. When the pressure is removed by opening the bottle the gas bubbles form inside the liquid and float to the top. Since the gas is compressed into the liquid you don’t see a volume change.  Once equilibrium is reached the gases no longer bubble out and the ginger ale turns “flat”. Put it into a vacuum and it will again begin to bubble.  

Once the oil is “flat” and won’t give up any more gas the collection portion of the apparatus is sealed off with a valve. The extracted gas is then compressed using liquid mercury for a piston.  At a given pressure the volume is measured. This enables the technician to calculate how much gas was in the given amount of oil. This forms the base for the parts per million of total gas found in the sample.  Most of the gas is then extracted from the chamber using a syringe then immediately injected into the gas chromatograph.

Sounds simple, but when working with vacuum it is very difficult to keep leaks from forming. The glassware used has to be specially designed for the purpose, and all hazardous materials need to be contained and handled carefully.

The glass vessel is then removed from the apparatus to dispose of the oil. The magnet is retrieved and put back into the vessel. The sealing edges of the vessel are then coated with vacuum grease. The apparatus is then tested to make sure it’s ready for the next sample.

Any comments or suggestions are always welcome. If you have a question my email address is: transformerbob@gmail.com

Quote of the month:

"My mom said she learned how to swim. Someone took her out in the lake and threw her off the boat. That's how she learned how to swim. I said, 'Mom, they weren't trying to teach you how to swim.' " --Paula Poundstone