TBR: NMR as a Quantitative Technique???

[justadream]

---

Members don't see this ad.

TBR page 206 excerpt (about experimental study of reaction rate):



“Aliquots can be collected at uniform intervals and analyzed using GC….A bigger problem is that the reaction may continue to react in the syringe or pipette after it has been removed from the original flask. To avoid this problem the sample of solution is quenched upon removal (to prevent further reaction). Once the samples are quenched, they are analyzed by a quantitative technique, such as GC or NMR”





I may be wrong but I think GC works as a quantitative technique because you can compare the areas of the peaks (integrate the peaks) corresponding to the reactants and products.



But how does NMR provide relative ratios of products to reactants? I thought NMR was used to identify a compound. Also, wouldn’t the NMR of a mixture (of reactants + products) be so messy that it would be hard to distinguish anything?

 

[JMMTB]

With NMR there are also integration peaks that show the proportions of different protons. You could then use it quantitatively to compare different signal strengths. But to be completely honest it is used more, like you said, to identify a compound or even just show if it has been run to completion.

 

[justadream]

@JMMTB

Oh so are you saying like:

1) Identify which protons belong to which molecule (which would suck if there's like 3+ things in the solution)
2) Take the integrated values for the protons from one molecule and compare them to the protons from another molecule?

 

[JMMTB]

@JMMTB

Oh so are you saying like:

1) Identify which protons belong to which molecule (which would suck if there's like 3+ things in the solution)
2) Take the integrated values for the protons from one molecule and compare them to the protons from another molecule?

Exactly. I've never seen it used quantitatively before and even when I did a quick google search for it I just found academic papers saying that you could try it.

So overall - stick with GC for quantitative analysis.