Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Bryby on June 21, 2009, 04:59:04 PM
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Hi. I have been trying to find out how gas phase chromatography works. I have searched online, but cannot find a basic explanation for the process. I understand how TLC works, but i don't see how the two are alike. I understand there is both gas and liquid in GLC, but thats as far as it goes. Can anyone give me an explanation i can grip?
Thank you.
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All chromatography works by the interaction of separate phases. The different "solubility" in each phases allows the separation.
In thin layer chromatography, these two phases are liquid and solid. The liquid is the solvent moving up the plate and the solid is the interaction with the solid (often silica).
Other chromatography using these two phases are high pressure liquid chromatography and paper chromatography.
The normal gas chromatography is gas liquid chromatography. But the liquid is hard to spot. Normally, the liquid is a thin film (often of a silicone material) that coats the inner surface of the capillary column. The gas going through the column interacts with the thin liquid film, forming the partition.
However, there are many other forms of gas chromatography. If you are using molecular seive columns, you are doing gas solid chromatography. And the variations go on and on.
Since the instruments often perform a temperature program (increasing temperature with time), GC is sometimes called (sarcastically, I think) modified boiling point chromatography. The idea being that boiling point has more to do with the separation than chromatography.
Hope this helps.
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Thank you, that does help. However, i still have some questions. In TLC the solid moves up the silica, and based on the distance up it went you can tell what the substance is (if you have comparisons.) In Gas/Liquid chromatography what are you measuring? I just don't understand. Is it how far the liquid moves? I am utterly confused. If someone can give me an example that would help, i think.
Thank you again.
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Thank you, that does help. However, i still have some questions. In TLC the solid moves up the silica, and based on the distance up it went you can tell what the substance is (if you have comparisons.) In Gas/Liquid chromatography what are you measuring? I just don't understand. Is it how far the liquid moves? I am utterly confused. If someone can give me an example that would help, i think.
Thank you again.
In TLC, the distance the "analyte" moves up the plate is dependent on the ability of the liquid phase to partition the analyte from the solid phase in order to move up the plate.
In GLC the "Liquid Phase" does not move, the mobile phase is the gas (usually helium) which travels through the column. Separation is achieved when the analyte ceases to be retained in the Liquid Phase and passes into the moving gas phase.
http://www.chemguide.co.uk/analysis/chromatography/gas.html (http://www.chemguide.co.uk/analysis/chromatography/gas.html)
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Ok, so i read that explanation and here is what i am getting, with simplicity: You inject a substance into the column. The substance boils into a gas phase and at some point perhaps the substance condenses. A carrier gas, helium acts as a solvent and separates the substance into separate molecules. At some point these molecules will enter the detector oven. Positive ions given off from the burning molecule in the flame will be attracted to the cathode, negative ions to the anode. This results in an electrical current which is recorded in peaks. Each peak represents a different molecule.
Now, i don't understand at which point the substance condenses back into liquid state. The Red section is my problem, i think. I think it makes sense if the gas state goes into the detector, and no condensation occurs. Perhaps i have misread the article. This is difficult for me to understand, like doing mental aerobics.
I really appreciate it. Thanks.
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The sustance is supposed to dissolve, in it's gaseous state, in the carrier gas. The GC injector vaporizes the sample, which then dissolves in the carrier gas. As it works its way through the column (which is in the heated oven box), it partitions into the liquid coating on the column surface. It works back and forth between the two phases, gas and liquid. This causes the separation.
As the material exits the column, it goes to the detector. The detector you seem to ge describing is a flame ionization detector (FID). But the list of possible detectors is very large. The detector is usually just outside of the oven. The gas plus sample runs through the detector and the signal is sent to the computer/ integrator, etc.
Is the partitioning between phases bothing you? It is the basis of chromatography. But lab experience shows most lab employees don't understand the process.
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Yea, its definitely the partitioning that bothers me. I don't understand what is going on inside the column. I also don't understand why or how it works between gas and liquid phases. If a gas condenses to liquid, why would it return to gas phase? But now that i think about it, that website posted earlier stated that the heat in the oven isn't constant. Does a change in temperature bring about the phases changes that is troubling me?.. It also makes sense that the more gas to liquid phase changes then the more separation between molecules, right? And does the detector oven require a gas phase molecule?
I know there are website out there that explain the process, but as you can see they have not helped me thus far. I am somewhat of a pain, and i apologize. I do appreciate all the help though, I'm very grateful. Thank you.
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Yea, its definitely the partitioning that bothers me. I don't understand what is going on inside the column. I also don't understand why or how it works between gas and liquid phases. If a gas condenses to liquid, why would it return to gas phase?
Remember it's an equilibrium, and a dynamic one. For instance, a sealed vessel with water is constantly evaporating and condensing (at the same rate).
But now that i think about it, that website posted earlier stated that the heat in the oven isn't constant. Does a change in temperature bring about the phases changes that is troubling me?
Don't worry about the changing temperature, it's an add-on. The fundamentals of GC are still there even if the temperature is constant.
.. It also makes sense that the more gas to liquid phase changes then the more separation between molecules, right? And does the detector oven require a gas phase molecule?
I think this is what is referred to as the number of theoretical plates that a column has. The more condensation / evaporation events, the better the separation.
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Please forgive this example.
Remember the like dissolves like rule in chemistry. For example, polar compounds tend to dissolve other polar compounds. And non-polar compounds tend to dissolve other non-polar compounds.
Here comes the weird part. Imagine a long hall filled with voters. Two candidates, one from each party and running for election, must go through the hall. Both candidates want to get home as soon as possible.
We will have both candidates, one republican and one democratic, enter the hall at same time. We'll stack the voters in the hall. All of the voters in the hall will be from one party (say democratic). Which candidate will leave the hall first?
Usually, the republican candidate will leave the hall first. He won't have as much in common with the voters. They won't talk as long. So he will be able to exit the hall first.
On the other hand, the democrat candidate will have a lot in common with all the voters. He will talk longer to them and talk to more of them. He will stay in the hall longer and exit much later.
Replace the candidates with polar and non-polar compounds. The long hall is the GC column stuffed with a polar compound.
You have many thousands of interactions. The compound will travel in the gas, meet one of the voters (chemical molecule), shake hands (be somewhat retained), go back into solution in the gas, travel to the next voter (chemical molecule) and slightly retained, and so on and so on.
The other compound won't shake hands with the voters and will travel through the column very quickly.
Stupid example, but hopefully it will help you visualize the process.
Weir
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No no the voter thing was good, it helped. Thank you. Now let me see if i can put this all together. The substance is injected into the injector oven where it is heated and vaporized. The carrier gas, usually helium, acts like a solvent and seperates the molecules. It carries them into the column. Each molecule type is held together by their polarity. In the column the temperature may remain the same, but because of the different chemical properties each molecule will undergo gas<->liquid phase changes at different rates, causing further seperation. Each time the gas condenses to liquid, it partitions itself on the waxy substance column wall. Because of the equalibrium, eventually that liquid will return to gas phase and exit the column into the detector oven. By the time the molecules get to the detector oven they are already seperated enough.
This process is making more and more sense. Thank you so much!
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Remember as well that for separations based on boiling point, the injector will be heated to a temperature high enough to vaporise the analyte which will then pass into the column. Normally, the column oven will be at a lower temperature (initially, this may change if a temperature gradient is employed) which will encourage condensation/diffusion into the "liquid phase" of the column.
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Ahhh ok thank you. I am getting such good information here. One last question though. What chemicals will they use a GLC test for? I am interested in environmental toxins. I had a professor tell me probably PCB's, but will this work on any chemical? I know it would be real hard to pick up trace toxins that are in the ppb.
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You need to know more specifics on the chemicals to answer that. Then there are questions about the specific detector being used, the specific column, etc.
The most common complaint about GC/MS (gas chromatography/ mass spectrometry) is that it won't analyze enough molecules. In order for GC to work, the molecular species must be volatile.
As for detection limits, ppb is relatively easy with GC/MS. The exact limit depends on the sample prep and detection scheme used. Usually, GC/MS uses a scan of the ions so the spectrum can be obtained. If you know the specific ion you are after, you can park the spectrometer on that ion and dramatically increase the sensitivity.
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Can you please clarify by what you mean when you say it wont analize enough molecules? By the way, Thanks again.
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Can you please clarify by what you mean when you say it wont analize enough molecules? By the way, Thanks again.
He just means that given all the molecules that exist, GC-MS only works for some of them, those that have an appreciable vapor pressure. So, it won't work for large polymers, or salts (not technically molecules), or some other things.
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But wont an increase in tempterature increase the vapor pressure, and make the molecule more volatile?
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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.
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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.
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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?
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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.
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For more on detectors see
http://en.wikipedia.org/wiki/Gas_chromatography#Detectors (http://en.wikipedia.org/wiki/Gas_chromatography#Detectors)
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm (http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm)
http://www.chemistry.adelaide.edu.au/external/soc-rel/content/gc-det.htm (http://www.chemistry.adelaide.edu.au/external/soc-rel/content/gc-det.htm)
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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.
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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.
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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.