Altius FL #1 C/P #18

[akimhaneul]

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If the in vitro GC analysis were conducted at a reduced temperature, how would the spectrum obtained differ from Figure 1?
A
The order of peaks 5 and 6 would be reversed.
B
Some of the earliest peaks would not be present.
C
Retention time would increase and the resolution between peaks would decrease.
D
Retention time would increase and the resolution between peaks would increase.



The answer is D. Why would the resolution between peaks increase if you conduct gas chromatography at a lower temp?

 

[aldol16]

I don't believe that the resolution would increase unless you changed the temperature gradient or the flow rate. Simply reducing the temperature universally by the same amount wouldn't change anything except that you wouldn't get any of the higher boiling point stuff out.

 

[desperadoflakes]

I believe the resolution would increase actually. I might be wrong tho. In GC, the polar molecules move more slowly than the gaseous molecules. Polar molecules have a higher boiling point, so are more likely to condense onto the stationary phase than non-polar molecules. As you lower the temperature, vapor pressure of all the analytes decreases, and solubility increases. I think this would increase the retention times for everything, but my gut tells me that polar molecules will have higher increases than non-polar. I only think this because if you lower the temperature enough for a compound with a very high boiling point, then essentially all of it will be in the liquid phase, so it won't really move at all. But a less polar compound may still have a significant portion in the vapor phase at the same temperature, so there will be a bigger distance between the analytes. But I'm not that well versed in this, so I may be totally wrong

 

[aldol16]

I believe the resolution would increase actually. I might be wrong tho. In GC, the polar molecules move more slowly than the gaseous molecules. Polar molecules have a higher boiling point, so are more likely to condense onto the stationary phase than non-polar molecules. As you lower the temperature, vapor pressure of all the analytes decreases, and solubility increases. I think this would increase the retention times for everything, but my gut tells me that polar molecules will have higher increases than non-polar. I only think this because if you lower the temperature enough for a compound with a very high boiling point, then essentially all of it will be in the liquid phase, so it won't really move at all. But a less polar compound may still have a significant portion in the vapor phase at the same temperature, so there will be a bigger distance between the analytes. But I'm not that well versed in this, so I may be totally wrong

In theory, you are correct in that temperature does play a role. In practice and in practicality, temperature plays a very small role. In fact, you don't want your entire compound to condense onto the column. GC is predicated on the equilibrium between the gas and liquid phases and it's very important that an equilibrium is there. So about 30 degrees below the compound's boiling point is usually ideal for separation of that compound. And obviously you don't want to go too high either. If you go too high, it'll all be moving with the mobile phase and therefore won't interact with the column. So as long as you're within this very large window, you can achieve separation.

The more important factors that we play with on a daily basis is the temperature gradient and the flow rate. You can significantly affect your separation by changing the flow rate.

If you're interested in GC theory, the following equation gives "resolution": [k/(k+1)]*[(a-1)/a]*sqrt(N)/4. The relevant part here is "k," or retention factor. This increases with temperature but as you can see, the increase is only modest even if you double k.