yeah i shouldve known that.in the proton NMR spectra, they should. http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm
Retention Time (RT)
The retention time, RT, is the time it takes for a compound to travel from the injection port to the detector; it is reported in minutes on our GCs. The retention time is measured by the recorder as the time between the moment you press start and the time the detector sees a peak. If you do not press start at the same time you inject your sample, the RT values will not be consistent from run to run.
Factors which affect GC separations
Efficient separation of compounds in GC is dependent on the compounds traveling through the column at different rates. The rate at which a compound travels through a particular GC system depends on the factors listed below:
Generally the number one factor to consider in separation of compounds on the GCs in the teaching labs is the boiling points of the different components. Differences in polarity of the compounds is only important if you are separating a mixture of compounds which have widely different polarities. Column temperature, the polarity of the column, flow rate, and length of a column are constant in GC runs in the Organic Chemistry Teaching Labs. For each planned GC experiment, these factors have been optimized to separate your compounds and the instrument set up by the staff.
- Volatility of compound: Low boiling (volatile) components will travel faster through the column than will high boiling components
- Polarity of compounds: Polar compounds will move more slowly, especially if the column is polar.
- Column temperature: Raising the column temperature speeds up all the compounds in a mixture.
- Column packing polarity: Usually, all compounds will move slower on polar columns, but polar compounds will show a larger effect.
- Flow rate of the gas through the column: Speeding up the carrier gas flow increases the speed with which all compounds move through the column.
- Length of the column: The longer the column, the longer it will take all compounds to elute. Longer columns are employed to obtain better separation.
The "higher boiling point" is probably meant to mean that it is a polar molecule, for example ethanol vs. propane. The more polar the substance the less it is attracted to the stationary phase, the less time it spends in that phase, the earlier it leaves the column.yeah i shouldve known that.
if you know anything about gas chromatography: why do substances with a higher boiling point have a longer retention time in the apparatus?
yes, i was actually asked this question! who knew?
I stand corrected. I can't believe I didn't learn about this in my analytical chemistry class. Polarity was constantly emphasized with no mention of b.p.the link that crazy bob gave makes it seem like the higher bp is important for retention time in GC for a different reason than polarity. it doesnt really explain why....why would low boiling compounds move faster than high boiling compounds? if both compounds are vaporized, it shouldnt matter what their BP is.
If there is a significant difference in polarity of the two compounds, then polarity will also determine retention time. If there is only a difference in molecular size and little/no difference in polarity, you will still see a difference in retention time--because of size. Decane and propane have the same polarity but they will have very different retention times because of size.ishchayll...are you assuming, then, that compounds with a higher BP have larger dispersion forces? I mean, the polarity thing would overshadow dispersion forces anyway, so i dont see how BP could really be that important if its only contribution is more VDW forces
Yeah, the stationary phase of the GC column is a liquid. So yes, the transitions are to the liquid phase. The transitions are rapid, so all of the molecules of a given analyte are constantly exchanging. However, at any given time, a certain fraction of the molecules are in the mobile (gas) phase and a certain fraction are in the stationary (liquid) phase. This fraction depends primarily on the boiling point of the analyte. Molecules with a higher fraction in the gaseous mobile phase will travel through the column faster.Hm, so are the transitions to the stationary phase essentially transitions back to the liquid phase?
Carbon NMR tells you how many different kinds of carbon there are in a compound. It is especially useful in a case where a carbon does not have a hydrogen directly attached to it (like if it's quaternary), since there will be no signal for that carbon on an H-NMR. It is generally not as useful as H-NMR because there is no signal splitting in C-NMR.So how exactly is carbon NMR different from H-NMR? Ek doesn't really explain it.
Actually, they're not very different. They use the same basic principles, except in 1H NMR, the nucleus under study is a proton, and in 13C NMR, a C-13 nucleus is under study. In 13C spectra, each unique carbon will show up as a peak. Just like proton spectra, 13C (and all other types of NMR) exhibit chemical shift which is affected by electron density.So how exactly is carbon NMR different from H-NMR? Ek doesn't really explain it.