How does a GC work?

We all talk about GC/MS analysis. But few of us outside of the archane world of a chemist's lab truly know what they are.

Was looking for something else in my archives and came across this, from a chemist in Connecticut whose real name always remained a mystery.  He was an online friend who knew a LOT about Essential Oil chemistry, and delighted in teaching it.

The topic of the message was "pretty smelling mystery oil" so it would appear I had been talking about something that I couldn't identify...  and the subject of analysis came up.  this was his explanation of "how it works":

From: DrPriszm@aol.comDate: Sun, 13 Oct 1996 01:29:59 -0400

Well, not really, but you're close!  Get ready for the $1.39 course on gas

chromatography! (But only if you promise to attend my real class in person!)

A gas chromatograph is really like an oven.  Inside the oven is a hollow tube which is coiled round and round like a garden hose so it will fit in the oven, because it may be 25 meters long.  Inside the tube is something similar to sand, very fine sand. There is some type of gas always flowing through the tube, from one end to the other.  

 

On the front end of the tube, where the gas is coming from, is an injection port.  It is through this port that the sample is injected onto the column, or tube.  The flowing gas carries the sample through the column.  As the sample travels through the column, the

various components of the sample separate, and form bands in the column.

Picture a marching band shoulder to shoulder marching down a street, and then gradually they start separating until they are 10 feet apart of each other.  Now they're marching single file, but they're separated by that 10 feet?  O.K. so far?  Good. 

 

Now at the end of the column, there is a detector. There are many types of detectors, but it really doesn't matter.  As each component marches past the detector on its way out of the column, the detector produces an electrical signal.  If the component is present in a high concentration, the detector produces a big signal.  If the component is only present in a small concentration, the detector produces a small signal. 

The signal then goes to a strip chart recorder.  You know what that is? Picture the device that the old fashioned lie detectors used to use.  You know, the pen would move on the paper and produce a wiggly line?  Same thing. The strip chart recorder draws peaks when the detector "sees" a component. Big peak for high concentrations, small peaks for small concentrations. 

This is the printout of a very simple oil with one major component

.

 

 

Here comes the math, be brave.  Simultaneously when the detector sends its signal to the strip chart recorder, it sends an identical signal to a computer processor.  The computer automatically computes the area under the peak of each component, and compares it to the area of the peak of a standard.  Then, by a simple proportion, it calculates the concentration of the unknown peak to the concentration of a standard compound whose

concentration we do know.  

Let me give you an example.  I take a known amount, say one gram of eucalyptol, the major component of eucalyptus oil, and inject it into my G.C. (gas chromatograph).  I get a peak, and that peak is always the same height as long as I inject one gram of eucalyptol into the G.C.  If I inject 1/2 gram of eucalytol, I get a peak 1/2 the height, and so on.  So I tell the computer that when you see a peak that is exactly the same height as my one gram standard, report that one gram of eucalyptol is in my sample.  Now the peaks are not all on top of each other, because you couldn't tell them apart.  They are separated on the paper just as the components are separated on the column.  The first peak to appear corresponds to the first peak that comes through the column.  

 

Your chromatogram of essential oils may contain hundreds, or even thousands of peaks!  How then, do we identify which peaks correspond to which components?  Well, we have to inject hundreds or thousands of standards of known components into the G.C. We then note the time it takes, from injection, until the compound hits the detector in

minutes.

 

We call this the elution time, or the time it takes for a compound to elute from the column.  Each compound has a unique elution time, and can be identified this way.  Now, let me tell you that it is WAY more complicated than that, and I could go into the various types of columns, injectors,detectors, gases, etc., but that is the gist of it.      

 

 

GC of one of the most complicated oils - Rose Otto. See the multitude of peaks?

I had one supplier who would send a gc upon request...  with the major peaks marked by hand.  Their equipment was not hooked up to a machine... he identified the peaks by hand.   Also, the detail (and accuracy) an analyst can give is determined to a great part by the depth/breadth of the "Library" of analyses they have access to.  

I'm hoping this description makes a little bit of sense!  (Yes, it's simplistic. There may be those who will scoff at the analogy of the marching band.  Feel free to scoff.)