NMR Spectroscopy Questions and Answers

Answer: C

A superconducting solenoid carries a persistent current in a closed loop, so it sustains fields of 9 to 23 tesla indefinitely with no power supply and with exceptional stability. Electromagnets drift and dissipate heat, and permanent magnets cannot approach these fields. They are far from cheap, they are not switched off between samples but left energised for years, and they still need shim coils because no winding is homogeneous enough on its own. Field stability is the property that matters most, since a drift of a few parts per billion would smear the spectrum.

Answer: D

The niobium alloy windings superconduct only below a few kelvin, so liquid helium holds them there and a surrounding jacket of liquid nitrogen reduces the helium boil-off. The sample itself sits at room temperature in a bore that is thermally isolated from the cryostat, so the helium does nothing for its relaxation. Neither inert blanketing nor vibration damping is its role. If the helium is allowed to run out the magnet quenches, dumping its stored energy as heat and boiling off the remaining cryogen, which is both expensive and hazardous.

Answer: A

Shim coils are small electromagnets that trim out the residual variation of field across the sample volume, and getting them right is the difference between a sharp line and a broad or misshapen one. Even the best solenoid is homogeneous only to a part in a million over the active volume, whereas a good spectrum needs a part in a billion. The transmitter coil supplies the pulse, the receiver coil detects the signal, and a separate air flow controls temperature. A distorted lineshape is nearly always a shimming problem before it is a chemistry problem.

Answer: B

The lock monitors the deuterium resonance of the solvent and feeds back to the field so that the field to frequency ratio does not drift during acquisition. Without it a long carbon experiment would blur as the magnet crept. It does not remove deuterium peaks, which are simply absent from a proton spectrum anyway, it does not set the shift scale, which the reference compound does, and it is not a decoupler. This is why samples are dissolved in deuterated solvents rather than ordinary ones, quite apart from the wish to avoid a huge solvent proton peak.

Answer: C

Two reasons run together: a protonated solvent would swamp the analyte with a signal thousands of times larger, and the deuterium resonance is what the lock channel follows. Deuterated and ordinary solvents dissolve compounds equally well, since the chemistry is unchanged, and no solvent threatens the magnet, which never touches the sample. The analyte shifts hardly at all. Residual protonated solvent still gives a small peak at a known position, and those residual peak positions are tabulated precisely so they can be recognised and ignored.

Answer: D

The probe carries a tuned radiofrequency coil that delivers the pulse and then receives the voltage induced by the precessing magnetisation, usually the same coil switched between transmit and receive. A photomultiplier belongs to optical spectroscopy, pole pieces to an electromagnet rather than to the probe, and the crystal oscillator sets the reference frequency in the console. Probe design fixes the sensitivity of the whole instrument, which is why cryogenically cooled probes that reduce coil and preamplifier noise give a fourfold gain.

Answer: A

Spinning about the vertical axis averages the field variations in the horizontal plane, so residual inhomogeneity is partly cancelled and the lines sharpen. The price is a set of spinning sidebands at the spin rate on either side of every strong peak, which can be mistaken for real signals. It does not mix the sample usefully, has nothing to do with freezing, and does not help the lock. Modern high field work with good shims often dispenses with spinning, and two dimensional experiments require it to be off.

Answer: B

Spinning sidebands are weak satellites on both sides of an intense peak, separated from it by exactly the rotation rate in hertz. Changing the spin rate moves them, which is the standard way of proving that a suspicious small peak is an artefact rather than an impurity. A rolling baseline comes from other causes, second order effects distort intensities within a multiplet, and deuterium is not decoupled in ordinary work. Recognising artefacts for what they are matters as much as assigning the genuine signals.

Answer: C

The receiver records the free induction decay, an oscillating voltage that dies away as the magnetisation relaxes, and Fourier transformation converts that time domain record into the familiar frequency domain spectrum. The moving mirror interferogram belongs to Fourier transform infrared, and a chromatogram to separation science. Working in the time domain lets every frequency be excited at once by a single short pulse, which is the whole advantage of the pulsed method over the older continuous wave scan.

Answer: D

A short hard pulse excites the whole spectral width simultaneously, so a complete spectrum arrives every few seconds and hundreds of scans can be co-added in the time one continuous wave sweep would take. That is what made carbon-13 practical. A magnet is still essential, a reference is still needed for the shift scale, and solutions are still the normal sample form for high resolution work. Signal to noise improves as the square root of the number of scans, so the multiplex advantage translates directly into sensitivity.