Analytical Techniques - NMR Samples and Standards (A-Level Chemistry)
NMR Samples and Standards
Preparing a Sample for NMR
Samples are investigated in dilute solution.
A standard is mixed in with the sample. The purpose of the standard is to provide a reference point to compare the readings in the molecule being detected.
The standard used is tetramethylsilane (TMS) – Si(CHā)ā. TMS is used for the following reasons:
- Silicon has a very low electronegativity. Siliconās low electronegativity means that the nuclei in TMS resonate at a very high field strength above that of any Ā¹H or Ā¹Ā³C atoms in other molecules. It produces a peak far away from other organic molecules.
- It provides a strong signal. As there are lots of C and H atoms in TMS, 12 hydrogen and 4 carbon atoms, all in the same chemical environment, it provides one strong, sharp signal which is easily detected.
- TMS is inert. TMS is inert so does not react with the sample molecules.
- TMS is cheap. TMS is non toxic and cheap.
- TMS is volatile. TMS has a low boiling point and can therefore be easily removed by distillation and reused again.
![NMR Samples and Standards](https://studymind.co.uk/wp-content/uploads/2022/04/1-154.png)
The difference between the standard and the peak produced by the sample is called the chemical shift.
The reading for TMS is set to a chemical shift value of zero.
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