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I am running some fluorine NMR and would like to make a bulk solution of $\ce{CCl3F}$ in $\ce{CDCl3}$ to aid in assessing the amount of fluorinated product present by having my NMR solvent act as a reference. Can anyone suggest a good ratio of the $\ce{CCl3F}$ standard to $\ce{CDCl3}$ NMR solvent?

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    $\begingroup$ The amount of standard should be in the same order of magnitude as the amount of compound in the NMR sample (i.e. not as a v/v% ratio of chloroform-d). This is because unlike in H-NMR where the solvent peak is just residual chloroform-h, the fluorine standard is all fluorine, so if you add too much the reference signal will swamp your actual signal and it'll be difficult to get the peak shape of your compound to come out right (it also screws with the integration but this isn't such an issue with 19F NMR as you probably dont have that much fluorine to worry about) $\endgroup$ – NotEvans. Nov 22 '16 at 19:42
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    $\begingroup$ @NotWoodward -you should expand this into an answer, as it essentially answers the question. $\endgroup$ – long Nov 22 '16 at 20:03
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You need the concentration of your reference peak to be the same order of magnitude as your solute. Accuracy of your integration depends on something called dynamic range, which refers to the relative intensities of the peaks being integrated. The greater the dynamic range; the greater the difference in peak intensities, the bigger the error.

You might want consider that for most medium-high field NMR applications, CDCl3 is used with TMS at concentrations of about 0.03%v/v. Low field and bench top applications use TMS at 0.5-1%v/v concentration.

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  • $\begingroup$ While your answer is certainly correct, the dynamic range of modern spectrometers is large. $\endgroup$ – Karl Nov 22 '16 at 22:00
  • $\begingroup$ On second thought, it is actually not correct. Only when the concentration difference comes close to the dynamic range of the instrument, you suddenly get a big error. $\endgroup$ – Karl Nov 22 '16 at 22:34
  • $\begingroup$ @Karl, this is not DR in terms of limit of detection but rather limit of quantification - very different. Error in measurement is determined by the error of your minor component; the error increases with decreasing S/N. A S/N decrease by an order of magnitude gives an increase in uncertainty by an order of magnitude. For S/N of 1000, uncertainty is ~0.1%; for S/N 100, uncertainty ~1%. So, as per the question, it is very important to get the relative concentrations correct. There are many other things to consider for quantitative measurements also, and depends on level of accuracy needed. $\endgroup$ – long Nov 22 '16 at 22:51
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    $\begingroup$ And FIW, selection of correct relaxation delay for 19F is critical, depending on the types of systems you are analysing. 19F have a big range in T1 - something like CCl3F will have a T1 of a few seconds. Many F-containing molecules will have much shorter T1s. This will compound error across multiple scans if not taken into account, when comparing multiple samples. $\endgroup$ – long Nov 22 '16 at 23:16
  • $\begingroup$ But that is not a matter of dynamic range, but only of sensitivity. The uncertainty is the sum of the inverses of the S/N ratios of each component to be compared if I'm not very mistaken. $\endgroup$ – Karl Nov 23 '16 at 21:02
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There are three points to take into account, imo:

  • Dynamic range: If the concentration of the reference peak is very large, your analyte vanishes in the digital noise. Modern spectrometers have a large dynamic range, which ameliorates the problem, but you should adjust the gain.
  • Signal overlap: A huge reference peak (which has a Poisson lineshape, i.e. broad "feet") distorts the baseline to a great distance. Less of a problem with 19F, because the CS range is large.
  • Sensitivity: It'd be bad if you had to measure four or eight times as many scans, just to get a proper signal from your reference peak.

So I'd recommend a reference with as many fluorine atoms as your analyte in the smallest expected concentration (per peak, of course!), and perhaps not more than one order of magnitude more.

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