gas chromatography and UV spectroscopy

[bananaboat]

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i understand TLC and paper chromatography (virtually the same). but gas chromatography confuses me. can anyone explain how it works? is all i really need to know is that the polar stuff will elute last?

finally, i read that UV spectroscopy was in the mcat syllabus. it was not covered by the TPR, and is not easy to follow in EK orgo book for me. can anyone explain the gist of this lab technique? i know it has something to do with conjugated compounds.

thanks a bunch 😳

 

[87138]

i understand TLC and paper chromatography (virtually the same). but gas chromatography confuses me. can anyone explain how it works? is all i really need to know is that the polar stuff will elute last?

finally, i read that UV spectroscopy was in the mcat syllabus. it was not covered by the TPR, and is not easy to follow in EK orgo book for me. can anyone explain the gist of this lab technique? i know it has something to do with conjugated compounds.

thanks a bunch 😳

I doubt (but please someone correct me if I'm wrong) that you'll have to know any more about UV-Vis than the fact that conjugated compounds absorb at above ~200nm, while non-conjugated compounds absorb at a lower wavelength. Compounds with 8+ conjugations absorb in the visual light wavelength range.

 

[bananaboat]

I doubt (but please someone correct me if I'm wrong) that you'll have to know any more about UV-Vis than the fact that conjugated compounds absorb at above ~200nm, while non-conjugated compounds absorb at a lower wavelength. Compounds with 8+ conjugations absorb in the visual light wavelength range.

so does this mean if theres no absorption of uv light, then its a conjugated compound?

 

[DrWanahbe]

I doubt (but please someone correct me if I'm wrong) that you'll have to know any more about UV-Vis than the fact that conjugated compounds absorb at above ~200nm, while non-conjugated compounds absorb at a lower wavelength. Compounds with 8+ conjugations absorb in the visual light wavelength range.

I know nothing about gas chromatography, but I thought that UV light occurred after the visibile violet light, which is at around 700 nm, so wouldn't UV light be above that? Or is it different because it's chromatography/spectroscopy?

 

[Ziggy08]

Gas chromatography seperates compounds based on volatility, not polarity because the interactions between the compound and the substrate is usually similar. So, the most volatile compound would emerge first and the less volatile compounds would emerge later.

 

[87138]

I know nothing about gas chromatography, but I thought that UV light occurred after the visibile violet light, which is at around 700 nm, so wouldn't UV light be above that? Or is it different because it's chromatography/spectroscopy?

Actually, violet is around 400nm (~380 or so I believe). An easy way to remember it (or at least the way I do) is from EK AO. At one point their producer chimes in saying that the way he remembers which end violet and red light are is that UV light has so much energy (compared to infrared light, which flanks the visual spectrum on the other side), it can give you sunburn.

The key there is to realize it has increased energy. Since energy is directly proportional to frequency, UV light must also have higher frequency. Thus, it must have lower wavelength than red/infrared.

Hope that helps!

 

[87138]

so does this mean if theres no absorption of uv light, then its a conjugated compound?

Not necessarily. The UV spectrum continues up to around 380nm or so (that's a rough figure, it's not exact). So in between ~210 and ~380, you're still within the UV range, but you've also got a conjugated compound. Once you reach an increased number of conjugated bonds (the figure given is usually 8), you start to creep into the ~400nm range.

A general rule:

Increased number of conjugated bonds = increased wavelength absorption.

 

[DrWanahbe]

Actually, violet is around 400nm (~380 or so I believe). An easy way to remember it (or at least the way I do) is from EK AO. At one point their producer chimes in saying that the way he remembers which end violet and red light are is that UV light has so much energy (compared to infrared light, which flanks the visual spectrum on the other side), it can give you sunburn.

The key there is to realize it has increased energy. Since energy is directly proportional to frequency, UV light must also have higher frequency. Thus, it must have lower wavelength than red/infrared.

Hope that helps!

Dang it, you're right! I always remember the UV/infrared thing by the Ultraviolet being near the violet and the red near the infrared. But I constantly get mixed up with the wavelenghts. THe HIGHER the number, the LONGER the wavelength. Ack. For some reason, I keep thinking the other way around, which is just silly.

 

[bananaboat]

Gas chromatography seperates compounds based on volatility, not polarity because the interactions between the compound and the substrate is usually similar. So, the most volatile compound would emerge first and the less volatile compounds would emerge later.

okay and are there any general rules for X is more volatile than Y?

 

[xanthomondo]

okay and are there any general rules for X is more volatile than Y?

you need to look at intermolecular forces, molecular weights, etc

lower MW = more volatile (if the intermolecular forces are the same)

example: methane and pentane, methane is gas at RT, pentane is liquid

Intermolecular forces cause volatility to go down

example: ethanol vs ethane, the OH group contributes to hydrogen bonding which makes it a liquid at RT while ethane is a liquid

 

[DrWanahbe]

Another way to think of this:

Volatility is the tendency for molecules to escape into the gas phase. The higher the intermolecular forces and the greater the molecular weight, the more the molecules would rather stay in the liquid phase than leave for the gas phase, because, let's face it, they're happier there.

 

[bananaboat]

Another way to think of this:

Volatility is the tendency for molecules to escape into the gas phase. The higher the intermolecular forces and the greater the molecular weight, the more the molecules would rather stay in the liquid phase than leave for the gas phase, because, let's face it, they're happier there.

thanks guys, here's what i've gathered from you:

increase molecular weight, decrease volatility
increase branching, decrease volatility (since branching decreases London dispersion forces)
increase IMF, decrease volatility

and basically, the ovlatile substances will escape first, and the less volatile will escape last.

the end.

 

[go lakers]

Gas chromatography seperates compounds based on volatility, not polarity because the interactions between the compound and the substrate is usually similar. So, the most volatile compound would emerge first and the less volatile compounds would emerge later.

whoa.. is this 100% certain? i thought i read on a recent solutions to a diag that GC chromatography separates by polarity and not volatility.

 

[pfh]

for gc i thought the more polar compounds would interact more w/ the stationary phase and thus have a longer retention time?

 

[estairella]

No it means that if IT IS CONJUGATED it will absorb UV light. Conjugated double bonds in particular. absorbance ranges for UV light are roughly 200-400nm.

I think you're confusing his question. Highly conjugated systems will absorb in the VISIBLE light range (NOT the UV light range), not-so-highly conjugated systems absorb UV light, and systems with little or no conjugation (below butadiene) do not absorb anything at all (except maybe carbonyls).

To help you remember, carrots contain carotene -> carrot + alkene.. in other words, a long conjugated double bond system. As a result, they absorb in the visible light region just adjacent to UV light (blue and green), resulting in the colour of carrots.. orange!

 

[go lakers]

for gc i thought the more polar compounds would interact more w/ the stationary phase and thus have a longer retention time?

http://www.unsolvedmysteries.oregonstate.edu/GCMS_05.shtml

volatility.

soo.. just remember BP increases with MW and decreases with branching. 🙂

 

[Knickerbocker]

for gc i thought the more polar compounds would interact more w/ the stationary phase and thus have a longer retention time?

HPLC

 

[novawildcat]

GC **usually** separates based on volatility, but it really depends on what your stationary phase is on the column of the GC. Inside the GC you have a coiled, very thin tube, that is several meters long. The tube is internally coated with a high boiling point substance (which is your stationary phase). there is no reason why your stationary phase couldn't be a polar substance, which would then make your separation on GC based on polarity of the molecules in your sample. A stationary phase that has something like polyethylene glycol WOULD separate your sample in the GC based on polarity. Hell, with the right stationary phase in the GC, you could even separate your sample based on the stereochemistry of your compound if you needed to separate something like enatiomers. BTW, polar molecules don't always elute last. Most of the time with modern intruments you will see them elute first since a lot of instruments like GC/MS, LC/MS, LC/MS/MS etc. use reverse phase chromatography.

A regular TLC plate is a cheap way to analyze a sample. Your stationary phase is usually something like alumina or silica gel. More non-polar molecules will rise to the top faster than more polar stuff. Separations rely heavily on your choice of the solvent system you are using to elute the plate. After the plate develops, you can see how the molecules moved sometimes with the naked eye or with a UV lamp. If your compound has any C=C double bonds the UV lamp will most likely be able to see it. UV works by exciting the electrons in the pi bond to an excited state (pi-->pi* transition). When the excited electrons move back down to their lower MO, they emit light which you can then see with your eye.