Cryogenically cooled probes

As of this writing (2007), cryogenically cooled probes are commercially available but not yet financially viable for all but those institutions and firms with the deepest pockets. 

The high cost to play the cooled probe game arises from the myriad problems associated with the probe s technical requirements. Requirements and weaknesses include  

The high costs of frequently repairing cryogenically cooled probes have prompted vendors to require or at least strongly suggest that their cooled probe customers purchase an annual probe plus heat exchanger maintenance contract for sums in excess of 25,000. 

When we consider that the price of this accessory starts around 125,000 (if we drive a hard bargain 200,000 if we do not) we may conclude that liquids NMR experimentation employing cooled receiver coil probes has to evolve more before it becomes accessible to those with more limited finances. 

It is expected that high maintenance costs will lessen and poor reliability will diminish in the next five to ten years. As a result of the extensive engineering required, the cost of the cooled receiver coil probe is expected to remain more than double the cost of a conventional probe for longer than that. 

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NOM and in turn provide key information as to the transformation and reactivity of NOM in the environment. Increased detection limits afforded by cryogenically cooled probes will be especially beneficial in this area. 

The development of cryogenically cooled probes has significantly decreased sample amount requirements for H NMR. Thermal noise in the probe and the first-stage receiver electronics dominate noise in NMR experiments. These new probes have built in first-stage receivers and rf coils that are cryogenically cooled ( 20°K), and have S/N improvements of 4 X standard probes. It is obvious to users that the highest field instrument available provides the best sensitivity. For fixed concentration (N), we would need 2.8 times as much material with a 300 MHz as on a 600 MHz system to obtain spectra with identical S/N ... 

The incredible natural abundance double quantum transfer experiment (INADEQUATE) was proposed by Ray Freeman in 1980.1 2 The acronym of this remarkable technique sat high among the many NMR acronyms since, at the time of its birth, it accurately described all its attributes. INADEQUATE was seen as an incredible experiment with vast potential its widespread applications are dwarfed only by its inadequate sensitivity. Depending on the equipment available, an ovemight-to-weekend INADEQUATE experiment on a medium-sized molecule would typically require hundreds of milligrams of sample. Only recently have such unfavourable requirements been addressed with advent of cryogenically cooled probes. Now, INADEQUATE experiments using samples of 5-10 mg have become realistic propositions. 

INADEQUATE in the field of high-resolution NMR is still mainly applied to the establishment of carbon-carbon connectivities, but its application to other nuclei continue to appear in the literature sporadically. Improvements in the sensitivity comparable to those offered by 1H- or 13C-optimised cryogenically cooled probes would undoubtedly increase the usage of INADEQUATE for non-carbon nuclei significantly. 

Initial thermal equilibrium, often based on the lock level stability, can be reached quickly taking as little as a few minutes (e.g. a room temperature sample inserted into a regulated magnet at 25 °C). Increasing the airflow to the variable temperature controller can speed this equilibrium process but unfortunately can easily introduce microphonics in the resulting spectra due to minute vibrations (i.e. wiggling of the sample in the probe). This is of course deleterious, and a compromise between temperature stability and positional stability must be reached for each spectrometer. For cryogenically cooled probes this becomes... 

The solvent relaxes not with the expected Ti time constant (i.e. small rate constant), but instead with a much smaller observed time constant Tird that is proportional to the quality factor (Q), filing factor (77, ratio of sample volume over coil volume), ° and the magnetization (equilibrium being Mq). Room temperature probes typically have a Q of perhaps a few hundred units but cryogenically cooled probes can push this value to 1500 (or even upwards of 40 000 is possible though not applied in biomolecular probes). The increased rate of relaxation due to radiation dampening is immediately evident. 

Figure 5 Cold probe demagnetization field effects. (A) shows the lock responses of a 90/10 H2O/D2O sample while (B) shows the lock of a 98% D2O sample. The marked intervals correlate to (i) equilibrium unlocked as a control, (ii) equilibrium with lock circuit active, (iii) locked while repeatedly pulsing with a 2.5 s 90 Hz presaturation pulse and a 4 s recycle time, and (iv) the same repeating presaturation sequence as shown in (iii) but with an inactive lock circuit. Results are from an 800 MHz spectrometer with a 5 mm HCN cryogenically cooled probe.

Probes usually have variable temperature control to run experiments at temperatures selected by the analyst. Cryogenically cooled probes can improve the resolution of a system, so that a 600 MHz spectrometer equipped with such a probe can provide resolution equivalent to a 700-800 MHz instrument. New probe designs with flow-through sample holders are commercially available, for use in coupled HPLC-NMR instruments and HPLC-NMR-MS instruments. These hyphenated instruments are discussed under applications later in the chapter. 

Kovacs H, Moskau D, Spraul M (2005) Cryogenically cooled probes - a leap in NMR technology. Prog Nucl Magn Reson Spectrosc 46 131-155... 

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