Question

1. What is the purpose of the TMS or DSS which is added to the NMR solvents?

2. What would you do if your NMR sample contained solid particulate matter (as a contaminant)?

3. Why is it necessary to wipe the bottom of an NMR tube before you placed it into the spectrometer?

4. Why do NMR spectrometers spin the tube inside of the magnet?

5. What is shimming?

Answer 1

hi,

1. Tetramethylsilane is the accepted internal standard for calibrating chemical shift for ¹H, ¹³C and ²⁹Si NMR spectroscopy in organic solvents (where TMS is soluble). In water, where it is not soluble, sodium salts of DSS, 2,2-dimethyl-2-silapentane-5-sulfonate, are used instead. Because of its high volatility, TMS can easily be evaporated, which is convenient for recovery of samples analyzed by NMR spectroscopy.

2.Solid particles distort the magnetic field homogeneity because the magnetic susceptibility of a particle is different from that of the solution. A sample containing suspended particles thus has a field homogeneity distortion around every single particle. This causes broad lines and indistinct spectra that cannot be corrected. So that there are no solid particles in your samples, you must filter ALL samples into the nmr tube. You should filter samples through a small plug of glass wool tightly packed into a Pasteur pipette. If the plug is not tight enough, the filtration will be ineffective; if it is too big, some of your sample will remain trapped in it. Do not use cotton wool, since most NMR solvents dissolve material from it which can easily be seen in 1H spectra. After filtration the sample should be as clear as water though, of course, not necessarily colourless.

3. Wiping clean the nmr tubes is necessary because it may contain both dust and sample residue as possible contaminants.Failure to wipe off grease and other chemicals off of the outside of the NMR tube will contaminate the spinner and may even cause the spinner to fail to grip the tube properly. So the nmr tube should be always wiped with kim wipes before introducing into spinner.

4. The NMR magnet is arguably the most important part of the NMR spectrometer. The NMR magnet is one of the most expensive components of the nuclear magnetic resonance spectrometer system. Most NMR magnets today are shielded magnets. These magnets have an additional superconducting coil outside of the main coil which cancel out much of the fringe field from the main coil. As a consequence, the stray field outside the magnet is very small. Having a small fringe field becomes important in higher field magnets where safety concerns become more important. This is the reason for spining the tube inside of the magnet

5. Shimming is adjusting the resolution of the signal by optimizing the homogeneity of the magnetic field. The lines in NMR spectra are very narrow-linewidths of 1 Hz or less are not uncommon-so the magnetic field has to be very homogeneous. What we have to do is to surround the sample with a set of shim coils, each of which produces a tiny magnetic field with a particular spatial profile which can be used to cancel out the inhomogeneities in the main magnetic field. The current through each of these coils is adjusted until the magnetic filed has the required homogeneity, something we can easily assess by recording the spectrum of a sample which has a sharp line.
The shims are labeled according to the filed profiles they generate. So, for example, there are usually shims labeled x, y and z, which generate magnetic fields varying in the corresponding directions.
The axial coils, z1-z4 modify the field profile along the axis of the main field, which is vertical. The radial coils are x and y, and other combinations containing one of the two. The radial inhomogeneities can be averaged out to some extent by spinning. Adjustment of the axial shims should be done with the sample spinning. The radial shims are done without spinning. There are several ways of shimming to optimize the homogeneity of the magnetic field (shimming on the FID, shimming on the spectrum, autoshim, gradient shimming and shimming on the lock). The shimming on the lock is more preferable for a standard user and you will open the lock window, which allows shimming to be performed on the lock. When shimming on the lock, you monitor the intensity of the lock signal. You need to adjust only z1 and z2 shims. Each shim setting (z1-z4) controls the current through shim coils that control magnetic field gradients in different directions. It is important to know that the z direction is parallel to the vertical direction of the probe and it is for this reason that the height (5 cm) of the sample in the NMR tube affects the z shim settings rather dramatically.

Hope this was helpful.