Wednesday, October 10, 2007

GC, columns, and hydrocarbons

The Chemistry department at a major university in our area has a program through which they get rid of "stuff." When labs close or move, they have a ton of stuff they need to get rid of. Some of the stuff is junk, but there is always some very useful and valuable stuff. Fritted funnels, ground glass joints, TLC plates, cannulas etc...

My colleague recently made a trip and picked up 5 GC columns. The condition (and for some the type) of these columns was unknown. I've been installing them in our Varian GC and conditioning and testing them.

I was testing a column from J&W. DB624, 30m long, 0.32 mm bore, 1.8 micron film, apparently designed to analyze pesticides. I injected some hexanes (mixed isomers). The separation was awesome. See below:

I thought, wow, for $0 we really got an awesome column, and we did. The column is in perfect shape. Probably used 5 times. Who knows.
I continued to test it and decided since I got such good hydrocarbon separation, I would inject some ligroin. Ligroin is a poor mans replacement for hexanes. It is essentially defined as a hydrocarbon mix with a certain boiling range. For ligroin the range is 60-90°C. According to the Wikipedia article it consists of C7 to C11. It is less refined and thus less expensive.

I injected some into the GC and here is what I got:
Look familiar? From what I can tell based on this one analysis, the cheap ligroin one buys from Sigma-Aldrich is identical in composition to the more expensive hexanes.

Here's a side by side comparison (hexane is on top and ligroin on the bottom. It's not as dark because it was in red and got grayscaled by the printer):

I guess you learn something everyday.


Captain Catalysis said...

Maybe the ligroin just has more of the high MW greasy stuff that won't fly by GC.

Chemgeek said...

But if this ligroin was the fraction that was distilled at 60-90°C, I'd be surprised if there were any high bp molecules left.

I was expecting the ligroin to be similar to hexanes, but not this similar. For most of my purposes (in teaching labs) I'm replacing hexanes with ligroin.

Ψ*Ψ said...

Huh. That's kinda cool.
I'd KILL for a free frit or two, although I did pick up a few free roundbottoms and a good condenser from a really nice Chemglass guy. :)