TPR MCAT Demo test B/BC #27

[basophilic]

If size exclusion chromatography (SEC) were used to separate a mixture of ethylene glycol oligomers, as shown below, with fractions further purified by gas chromatography (GC), what might be expected?

A)
The use of nonpolar solvents in SEC will lead to good separation and efficient purification by GC.

B)
The use of ether-type solvents in SEC will lead to poor separation, and subsequently to inefficient purification by GC.

C)
Oligomers with long retention times on the SEC column have long GC retention times.

D)
Oligomers with short retention times on the SEC column will have long GC retention times.

With SEC, I thought the stationary phase is made of hollow beads with small holes in them; hence, I would expect smaller PEG oligomers to have a longer diffusion path length since they would go through the beads, whereas the larger oligomers would avoid the beads and hence have a much shorter diffusion path length. This would mean small oligomers should have a longer retention time and, of course, a shorter GC retention time. Hence I picked D.

But the answer is C. Am I missing something?

---

Members don't see this ad.

 

[basophilic]

If size exclusion chromatography (SEC) were used to separate a mixture of ethylene glycol oligomers, as shown below, with fractions further purified by gas chromatography (GC), what might be expected?

A)
The use of nonpolar solvents in SEC will lead to good separation and efficient purification by GC.

B)
The use of ether-type solvents in SEC will lead to poor separation, and subsequently to inefficient purification by GC.

C)
Oligomers with long retention times on the SEC column have long GC retention times.

D)
Oligomers with short retention times on the SEC column will have long GC retention times.

With SEC, I thought the stationary phase is made of hollow beads with small holes in them; hence, I would expect smaller PEG oligomers to have a longer diffusion path length since they would go through the beads, whereas the larger oligomers would avoid the beads and hence have a much shorter diffusion path length. This would mean small oligomers should have a longer retention time and, of course, a shorter GC retention time. Hence I picked D.

But the answer is C. Am I missing something?

This was TPR's reasoning:
"C. Long retention times on the SEC column mean that the oligomers are large, which also means they are less volatile and have long retention times on GC. The converse is also true; short SEC retention times mean the oligomers are small and volatile, with short GC retention times, so choice D is false. The use of one solvent or another shouldn’t make a large difference in SEC retention times, since it's size, not polarity, that causes separation (eliminate choices A and B)."

 

[geauxmed]

Small oligomers flow faster because they would "bump" into beads less often. Large oligomers on the other hand would have trouble flowing between beads without making contact so they would be slower. Try imagining the SEC column to be packed with uniform beads, so essentially the path length for both types are the same. This is typically held constant when determining retention times.

Oh wait I mixed this up with liquid chromatography. Small molecules elute later on SEC.

 

[chemist16]

I think you're right. The principle of SEC is this: small molecules penetrate into the porous stationary phase and so get eluted later. Large molecules don't penetrate into the pores and so are eluted faster. So large molecules have short retention times on the SEC and large retention times on the GC, as you said.