Gas chromatography: Heat stable & volatile compounds

[SaintJude]

An admin QofQuimica saidthis regarding gas chromatography:

The important thing to understand about GC for the MCAT is that you cannot ever use it to separate compounds that are A) nonvolatile (no biomolecules!), or B) heat-labile (nothing that decomposes at high temperatures).

1.) I understand that GC is suitable only for relatively heat-stable, volatile molecules since the mixture is dissolved into a heated carrier gas. Has anyone come across for a mixture that is actually suitable for GC?

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[chiddler]

Water and acetone, methanol.

Or any two liquids that generally are miscible and have different boiling points.

Are you just looking for specific examples?

 

[SaintJude]

You listed three as if they were in one mixture. Is that what you meant?

 

[chiddler]

You listed three as if they were in one mixture. Is that what you meant?

No, sorry i meant water and acetone, water and methanol. They are both miscible but have very different boiling points.

Another thing to help understanding, for me anyway, was the effect of fractional distillation. This is placing extra surface area on the path that the vapor traverses such as placing beads or some other solid. This accentuates the boiling point differences because the more volatile fluid will ignore the beads while the less volatile will bind to the beads. So you can see how the liquids interact with each other for distillation to be possible.

 

[MedPR]

An admin QofQuimica saidthis regarding gas chromatography:

1.) I understand that GC is suitable only for relatively heat-stable, volatile molecules since the mixture is dissolved into a heated carrier gas. Has anyone come across for a mixture that is actually suitable for GC?

Where did you read this? It looks like a good place to get a quick outline on lab techniques and I definitely need that!

 

[mcloaf]

An admin QofQuimica saidthis regarding gas chromatography:

1.) I understand that GC is suitable only for relatively heat-stable, volatile molecules since the mixture is dissolved into a heated carrier gas. Has anyone come across for a mixture that is actually suitable for GC?

Basically everything you ever work with in Ochem lab works great for GC/MS (assuming you don't put in a bunch of acid or something, in which case you'll have a very unhappy ochem lab instructor).

 

[SaintJude]

MedPr. Unfortunately, I can't find it myself again. But thanks for your responses. I think what I didn't understand earlier was that GC was based upon differences in boiling points. So not only do the two compounds have to be volatile, they also have to have boiling point that's dissimilar.

 

[mcloaf]

MedPr. Unfortunately, I can't find it myself again. But thanks for your responses. I think what I didn't understand earlier was that GC was based upon differences in boiling points. So not only do the two compounds have to be volatile, they also have to have boiling point that's dissimilar.

Well, not really. Boiling point is related, but the separating power of GC comes from the interaction of the vaporized compound with the stationary phase of the capillary through which it is passing. GC is basically plain old silica gel column chromatography except that rather than a liquid solvent, the mobile phase is an inert gas. So really you're separating based on polarity, which in turn means that the compounds probably do have differing boiling points--but the boiling point is not what's allowing you to separate them.

 

[QofQuimica]

Well, not really. Boiling point is related, but the separating power of GC comes from the interaction of the vaporized compound with the stationary phase of the capillary through which it is passing. GC is basically plain old silica gel column chromatography except that rather than a liquid solvent, the mobile phase is an inert gas. So really you're separating based on polarity, which in turn means that the compounds probably do have differing boiling points--but the boiling point is not what's allowing you to separate them.

Correct. All of the sample is vaporized simultaneously, and the compounds separate because they have different interactions with the mobile versus stationary phases of the column. As you said, the mobile phase is an inert gas; the stationary phase is often a highly viscous liquid that coats the inner surface of the column (called an open tubular column). Also, the separation is possible in part because GC columns are extremely long; if you've never seen one, ask your instructor if they'll open up the oven part of the instrument and let you take a look.

SaintJude, you often *can* separate volatile compounds by differences in boiling point; that's how distillation works.

 

[SaintJude]

Wow. QofQuimica actually posted in this thread.

Hmm. So components of the mixture will reach the detector at varying times due to differences in polarity?
That seems so odd to me when the a crucial part of GC is heating the column and thus causing the mixture to evaporate. But I watched a video by RSC that said that the components leave in "order of volatility". I guess I just don't understand how these two traits (volality & polarity) work together to partition the mixture.

How are differences in polarity affect how the gas "acts" ? Helium or Nitrogen is an inert gas so they don't react with the gas mixture...

When would you use GC as opposed to HPLC?

Edit: Link to video on GC I'm referencing. http://www.youtube.com/watch?v=08YWhLTjlfo