[showandy]

Hi, I am doing a senior project involvling gas chromatography and have been working on it for like 7 months (couple times a week).  The purpose and goal of my experimentation is to create a more affordable version of a working gas chromatograph so that my High School Teacher can use a classroom set to teach with.  I also need to have this done in like a week so that I can present for my final prject grade.  The reason I am here is to see if ANYONE has any ideas of how to 1) make a gas chromatograph in the first place and 2) knows how to modify and tweak aspects to make the performance even better.  I am currently using using a mixture of Methylene Chloride and Chloroform as the gases which I am trying to separate. If anyone knows a different gas/chemical that can is relatively easily accessible and available that works better than one or both of those for gas chromatography, I would really appreciate it.  In my homemade gs chromatograph, I have a centimeter (ish) diameter glass tube that is about two feet long (I also have a one footer as well).  Packed inside the tube is some packing material to separate the gases based n formular weight (size of the molecules). Does anyone know how tightly the material should be packed inside of the tube or if there is any outstanding technique that is critical to success?  The information that I have says to pack it tight, but not too tight (really helpful, I know).  Also the carrier gas that is propelling the test gases through the tube is methane (natural gas from classroom as jets, therefore the gas flow is constant).
If anyone has tried to make a gas chromatograph and succeeded, please tell me if there was a very important detail/technique/whatever that was the determining factor in you success.  Thanks a million...

[Dude]

Looks like you got the easy project.  The other student in your class had to design and build a car cheaper, more reliable and more fuel efficient than a Toyota Camry in one week.  

Gas chromatographs are highly engineered pieces of equipment that have evolved over 50+ years.  Every part has a history of engineering to it.  There was a review article a few years back in "Trends in Analytical Chemistry" called The History of Gas Chromatography describing the advances to the columns, detectors etc.

Here is what I would do.  Buy some Swagelok fittings to connect to your nitrogen line (I would use nitrogen as the eluent - not methane), a gas regulator and a pressure gauge in the 1-100 psi range.  Buy some semi flexible steel or aluminum tubing of the same dimensions as the Swagelok fittings.  Find an old oven that you can drill into the side of.  Pack some diatomateous earth loosely (pressure drop should not be greater than 30 %) into the tube and then coil it neatly.  Run the sides of the tube in and out of the oven and seal the vacant space with an insulator to prevent heat loss.  Where the nitrogen comes from, connect a "T" tube with one side going to the tube in the oven, the other to the nitrogen source and the third to a septum (a piece of rubber) that will seal the gas in but allow your test mixture to be introduced via syringe into the flowing nitrogen.  Split the tube coming out of the oven into a T also and run the tubing into two separate solutions.

Here's the tricky part.  Try to find two organic chemicals of moderate to low volatility that will give a visible chemical reaction in the solution (ie color change, precipitation, heat with a thermometer in the solution etc)  The differential in time between the appearance of the visible reaction will represent the retardation factor related to either separation or volatility.  In reality, most GC's use polydimethylsiloxane (a viscous goo) lined on the walls of a highly coiled glass capillary column and the separation is due to boling point  / volatility variations more so than adsorption.

THe variables would be
1.  Pressure of the flow gas (observable by regulator - faster should cause material to come out faster)
2.  Temperature of the oven (higher = faster).
3.  Create another column of longer / shorter length or different packing material

It should qualitatively give a student an understanding of the variables affecting hold time in the column.

[showandy]

Thamks a lot for the tips and info.  I will see what I can do.  One of the reasons that I am using methane instead of something else is because that is just what I have available.  But I will check to see if nitrogen is better as a eluent.  Thanks again.

[Unknown discoverer]

hi ....it is gooooooood discusion...

but where is the detector ..how you will know law concentrations...better to get old GC from scrap and simulate..it


thanks