Gas Chromatography

[kryptonxenon]

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Assuming that the stationary phase is polar, what compounds elute first when gas chromatography is performed?

Do we look at polarity of the compounds or the boiling points to determine the order of elution?

 

[theonlytycrane]

I'm mainly familiar with using a gc-ms (gas chromatography / mass spec) where you inject a tiny bit of liquid into the machine and the liquid heats up into vapor. The order of elution will depend on boiling points so the more volatile compounds will elute first.

This gets coupled with a MS reading that you match against a known database to identify the compounds.

This is a super crude explanation, I know, but it's for MCAT purposes 🙂

 

[aldol16]

I'm mainly familiar with using a gc-ms (gas chromatography / mass spec) where you inject a tiny bit of liquid into the machine and the liquid heats up into vapor. The order of elution will depend on boiling points so the more volatile compounds will elute first.

Perfect. There's essentially no difference between an purely analytical GC and a GC-MS. The MS part occurs after the GC anyway so you can think of them as two distinct machines that just happen to be linked together. That is, each compound that comes out on the GC with a characteristic retention time is then injected automatically into the mass spectrometer to get its mass reading. That's why each peak in the GC trace corresponds to some mass spectrum. If you want to uncouple them, you can although that's more common with LC-MS where you can just do a direct injection into the MS part.

 

[kryptonxenon]

Thank you for clearing it up guys!

 

[Mr. otcoD]

So can you say it's similar to TLC in terms of a stationary and mobile phases? If you have a stationary phase that is polar, the 1st compound to elute first is nonpolar with a small mass?

 

[kryptonxenon]

So can you say it's similar to TLC in terms of a stationary and mobile phases? If you have a stationary phase that is polar, the 1st compound to elute first is nonpolar with a small mass?

@Mr. otcoD, I think comparing the boiling points is your best bet.

 

[Mr. otcoD]

@Mr. otcoD, I think comparing the boiling points is your best bet.

So is it the lower the boiling point, the faster it would elute out?

 

[aldol16]

So can you say it's similar to TLC in terms of a stationary and mobile phases? If you have a stationary phase that is polar, the 1st compound to elute first is nonpolar with a small mass?

So is it the lower the boiling point, the faster it would elute out?

The second post above is correct. The principles are similar, as polar compounds tend to have higher BPs - think of hydrogen-bonding compounds versus non-H-bonding.