Chapter 4. Cryogenically Cooled NMR Probes: a Revolution for NMR Spectroscopy

Abstract

In the early to mid-1980s, NMR spectroscopists were hampered by the low sensitivity of NMR spectroscopy. Long data collection times were the norm and booking enough NMR time was always a concern. NMR sensitivity gains were mainly accomplished through increases in field strength. Advances in NMR probe technology over the past two decades have revolutionized the capabilities of NMR, including a broad expansion of its applications. The speed of acquisition of spectra has gone from being the rate-limiting step to being so rapid that the time required to analyze data has become the major concern. The most notable advancement has been the development and commercialization of cryogenically cooled NMR probes. During the 1980s, pioneers in cryogenically cooled NMR probes started to experiment with increasing the sensitivity of NMR technology by cooling the rf coils and also the preamplifiers to reduce the noise factor in the NMR electronics. Hoult and Richards a decade earlier had proposed that the sensitivity of an NMR spectrometer, which is defined by the signal-to-noise ratio (S/N), could be enhanced by lowering the coil temperature. Now common in many NMR facilities, cryogenically cooled NMR probes have made it possible to obtain data on low- and mid-range field systems with the sensitivity that was previously reserved for very high-field magnets. By 2013, over 1700 cryogenically cooled probes had been delivered to customers worldwide by two of the major suppliers of NMR equipment. Further advances are expected in this technology to increase capabilities and applications, increase accessibility to users, and increase sensitivity. Expanded capabilities and applications will continue to drive research and development efforts.