[Bryby]

But wont an increase in tempterature increase the vapor pressure, and make the molecule more volatile?

[Phlogiston]

But wont an increase in tempterature increase the vapor pressure, and make the molecule more volatile?

True, but some things have such a low vapor pressure that for practical purposes, they're non-volatile.  For polymers, for example, raising the temperature will cause them to decompose before they evaporate.

[marquis]

Now you are getting to the part of GC that gets interesting.

There are many accessories that can be added to a GC that extend its capabilities.
Much of what the instrument can analyze depends on the specific GC you have, the detector set up, and the accessories.  For example, a pyrolyzer accessory can break down the molecule into fragment compounds.  By analyzing the fragments, you can identify the parent molecule.  Changing the injector settings, changing the column, etc can make a drastic difference.

 There are also chemical ways to extend the abilities of the instrument.  Many people will derivatize a family of molecules to identify them.  A common one is FAME (fatty acid methyl ester) analysis.  The methyl ester is usually more volatile than the base fatty acid.

Having said that, each extra step adds to the complexity and the difficulty in interpretation.  It's usually been biological molecules that give me fits.  I couldn't get anything when soybean oil (and other triglycerides) was injected into the GC, for example.

On the other hand, I could identify many fatty acids without having to derivatize. 

[Bryby]

Trust me when i say I've been interested this whole time. GC sounds like it can answer alot of my questions regarding "what's in stuff." Correct me if i am wrong, which i probably am, but it seems like you need to know the molecules that go in the GC in order to learn the concentration. Also, i am interested in learning what special accessories are needed to find environmental toxins, more specifically PCB's.

Does the ion detector that is used with GC work like a smoke detector - only the GC records the voltage? I was looking to learn how it works online, and i came across this site: http://en.wikipedia.org/wiki/Gaseous_ionization_detectors. I thought maybe they used the same principle. Unfortunately, i am more confused now because evidently some ion detectors use helium in some way. Do all flame ionization detectors use another gas besides hydrogen, and in what way?

[marquis]

The most common GC detectors I've run across are thermal conductivity (TCD), flame ionization (FID), electron capture (ECD), nitrogen phosphorus sulfur (NPS), and mass spec.  Each has it's strengths and weaknesses.

 A TCD just uses the carrier gas (usually helium) for detection.  But it is relatively insensitive.  The concentration of unknown must be relatively high.

An ECD typically uses radioactive sources.  It is often ignored because of regulatory issues.  But it works well.

Now for the FID.  Think of it as a conductivity meter in a flame.  You run your carrier gas into a burning flame (usually of hydrogen and air).  The flame has two electrodes in it which measures conductivity.  As a compound elutes it changes the conductivity.  This is measured and sent to an intergrator.  It's an oversimplified explanation, but it gives the general idea.

You can identify unknowns by retention time match, although that is not very specific.  If you are using mass spec, you can get a better match through the spectrum.  While you can estimate the concentration, most procedures require you to make a calibration curve.  While a single point curve is possible, it usually is not well regarded by regulatory agencies.  Multi point calibration curves are the normal rule. 

Usually, you have an idea of what is present when you make and injection into the GC.  This is especially true of detectors like FID.  It is less true with an MS detector, which can also produce a spectrum match.

[JGK]

For more on detectors see

http://en.wikipedia.org/wiki/Gas_chromatography#Detectors

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm

http://www.chemistry.adelaide.edu.au/external/soc-rel/content/gc-det.htm

[Bryby]

Now for the FID.  Think of it as a conductivity meter in a flame.  You run your carrier gas into a burning flame (usually of hydrogen and air).  The flame has two electrodes in it which measures conductivity.  As a compound elutes it changes the conductivity.  This is measured and sent to an intergrator.  It's an oversimplified explanation, but it gives the general idea.

You can identify unknowns by retention time match, although that is not very specific.  If you are using mass spec, you can get a better match through the spectrum.  While you can estimate the concentration, most procedures require you to make a calibration curve.  While a single point curve is possible, it usually is not well regarded by regulatory agencies.  Multi point calibration curves are the normal rule. 

So is there a table that is used to match a time or conductivity up with a molecule? I dont understand how that works. Thank you so much for the replys though, they've all been helpful.

[Phlogiston]

Now for the FID.  Think of it as a conductivity meter in a flame.  You run your carrier gas into a burning flame (usually of hydrogen and air).  The flame has two electrodes in it which measures conductivity.  As a compound elutes it changes the conductivity.  This is measured and sent to an intergrator.  It's an oversimplified explanation, but it gives the general idea.

You can identify unknowns by retention time match, although that is not very specific.  If you are using mass spec, you can get a better match through the spectrum.  While you can estimate the concentration, most procedures require you to make a calibration curve.  While a single point curve is possible, it usually is not well regarded by regulatory agencies.  Multi point calibration curves are the normal rule. 

So is there a table that is used to match a time or conductivity up with a molecule? I dont understand how that works. Thank you so much for the replys though, they've all been helpful.

The problem is that retention times / conductivities / etc are not very reproducible from instrument to instrument- part of the reason is that you can't have two columns that are exactly alike- variable coating thickness (stationary phase) and other issues make it hard. 

To get around this, people often add a known amount of what your suspected unknown is, to see if they come out as one peak or two.  Or, add two or more reference compounds and see how your unknown comes out relative to them.  Mass spec makes this easier.

To get a quantitative measure of how much unknown you have in your sample, the calibration curve marquis alluded to is standard.  Make up a range of known concentrations of the compound, see how much signal each produces, and interpolate to find your unknown concentration.

[simpleton]

we will have series of standards (e.g. if you analysing let say propionic acid recovery with GC-FID). We will prepare known concentration of standards and then we will also analyse the samples. Based on the peak area, we will be able to calculate the amount of PPA present in the samples to.

For better reproducibility, we usually have an internal standard so we can correlate. Hope it helps.