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Warning: extreme pedantry. I am performing Gutmann-Beckett experiments, basically utilizing an NMR-tube with my sample and a 31P-lewis base (e.g. triethylphosphine oxide) inside another one with a standard/reference (e.g. trimethylphosphate). This is classified as an internal reference, but I am unsure about that. Wouldn't an internal reference be in direct contact with the sampled solution?

Since we need a constant 31P signal to reference, I understand that the reference has to be isolated, and wether this is "physically" by glassware or by an extensive shielding residue is irrelevant (when the signal is constant); but somehow this is not what I would call an internal reference.

Buttonwood
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Katjuscha
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    Is there an error in your wording? I'm used the other way around (the sealed capillary of e.g., trifluoro acetic acid is plunged into the greater NMR tube containing the sample solution). In the example of TFA, the separation prevents reaction with the analyte. In other cases ($\ce{CDCl3}$ plus the added [an option!] TMS), standard and analyte share the same solution in one sample in common (then per se an inner standard). Really external would be instruct the spectrometer software «with operational frequency x, use frequency y (outside the spectrometer) to compare with». – Buttonwood Jan 20 '22 at 08:26
  • We basically use the original method Beckett used (Beckett, M.A. 1996. Polymer 37, 4629–4631) where the sample-tube is inside the reference containing tube. It makes sense the way you explain it, but here we use two different solvents for the two tubes. I am inclined to call that an external reference, since in analytical chem., iirc, a separate HPLC run with the standard in the same matrix as the sample is an external standard, e.g. Maybe calling it internal is due to the constraints of NMR-methods? – Katjuscha Jan 20 '22 at 08:52
  • Though your sample and standard are recorded simultaneously, the two experience slightly different magnetic field (separate containers shielding each other). But (more importantly:) molecules of one don't interfere with those of the other as they would for a homogeneous solution of TMS in $\ce{CDCl3}$ in one tube. Thus my take for a separate analysis, and hence an external reference. (cf. Orange Book, internal standard/section 9.2.3.3). – Buttonwood Jan 20 '22 at 10:09
  • The other Colour Books of IUPAC are mentioned briefly in section «nomenclature» in the chemistry.se resource page here. – Buttonwood Jan 20 '22 at 10:13
  • Thank you very much, that clarified my issue! – Katjuscha Jan 21 '22 at 14:07

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