188 5.4  NMR and Other Radio Frequency and Microwave Resonance Spectroscopies

5.4.5  NMR IN PRACTICE

NMR often requires isotope enrichment, which can be technically rate limiting for

investigations, in addition to typically tens of milligrams of purified sample, which can

present a significant challenge in the case of many biomolecules, with liquid-​state NMR

requiring this to be dissolved in a few hundred microliters of pH-​buffered solvent to produce

high millimolar concentrations. To achieve the high B-​fields required for NMR, with sev­

eral machines capable of generating >20 T, magnets are based on a solenoid coil design.

Early NMR magnets had an iron core and could generate fields strength up to ~5 T; how­

ever, most modern NMR machines, which can achieve maximum field strengths of ~6–​24 T,

utilize a superconducting solenoid, and some prototype machines using larger solenoids can

FIGURE 5.4  NMR spectroscopy. (a) Schematic of an NMR spectrum taken on with a “400 MHz”

NMR machine in TMS solvent. (b) Schematic of a typical research NMR machine with double-​

skin Dewar. (c) Biot–​Savart law: the contribution dB to the total circular B-​field around an elec­

trically conducting wire carrying current I can be calculated from the incremental element ds

along the wire length of the length.