NMR Spectrometer

Overview

A nuclear magnetic resonance spectrometer reads the structure of molecules by listening to their atomic nuclei. Placed in a strong magnetic field, nuclei such as hydrogen and carbon behave like tiny magnets that precess at a frequency set by the field and by the electrons immediately around them. Pulse them with radio waves and they emit a faint signal back; the spread of frequencies in that signal, the chemical shift, maps the atoms and bonds of the molecule. It is the workhorse of organic chemistry and structural biology.

Everything hinges on the field being both enormous and exquisitely uniform. A stronger field spreads the signals further apart and lifts weak peaks out of the noise, which is why these instruments are built around superconducting magnets cooled to a few degrees above absolute zero.

How it works

The field comes from the Superconducting Magnet & Cryostat: a niobium-titanium solenoid carrying a persistent current, immersed in liquid helium inside a nested dewar with a liquid-nitrogen jacket. Its Superconducting Main Coil loses all resistance once cold, so the current, and the field, circulate for years without a power supply. The Shim Coil Set then trims away the last traces of inhomogeneity until the field varies by less than a part per billion across the sample.

The sample sits in the NMR Probe, lowered down the magnet bore. Its RF coils both deliver the excitation pulse and pick up the nuclei's response, tuned to the right nucleus by a Tune & Match Network. The RF Console does the electrical work: a synthesizer sets the frequency, a power amplifier delivers the pulse, and a low-noise receiver digitizes the microvolt free-induction decay that follows.

While a run proceeds the Gradient & Lock System watches the solvent's deuterium resonance and nudges the field to cancel drift. For unattended throughput the Sample Changer & Spinner floats and spins each tube on air and loads the next from a carousel. The Vibration Isolation keeps building tremors out of the magnet, and the Operator Workstation runs the pulse sequences and Fourier-transforms the decay into the spectrum the chemist reads.