Cryoprobe

From NMR Wiki

Cryoprobe or Cold probe is a technology where the probe coil and/or built-in signal preamplifier are cooled with a stream of He gas at ~20K. This increases the sensitivity of the probe coil and reduces the level of thermal noise generated by electronic circuits and components of the signal receiver itself.

The result is that the signal to noise ratio can increase by about 4X vs. a room temperature probe. In principle experiments can be run in 1/16 the amount of time as on a conventional probe, all else being equal. Or experiments can be run on samples with 1/4 the concentration in the same amount of time as on a conventional probe.

Typical parts included in the cryo-probe system

• Main cooling system - contains a coldhead which cools the helium gas to ~10K
• Helium compressor (provides highly compressed, room temperature helium gas)
• Cooling system for the compressor (compressor can be either air-cooled or water-cooled)
• Cylinder of grade 6 helium gas
• Cryoprobe - electronics are bathed in ~20K helium gas

Maintenance

Because cold probes have several interacting systems, special care should be taken to optimally maintain all of the parts.

The helium compressor operates at high temperatures. Even under the best conditions, compressors can be expected to last 3-5 years before requiring expensive maintenance. In addition, operating the compressor at higher temperatures may cause the capsule oil to break down and leak contaminants into the cryo cooling unit. This can cause the whole system to fail or to develop chronic problems. The compressor should therefore be operated at the lowest temperature possible. Also, if the compressor cooling system fails briefly (ca 30 seconds), the compressor will shut off and the probe will warm up. Because of this, the cooling system for the helium compressor should be carefully maintained and monitored often.

Contamination of the helium is a big problem because any contaminants will freeze out in the coldhead and can disrupt the whole cooling system. Therefore only very high grade helium should be used. Most grade 6 helium is batch tested - the helium is tested and then put into cylinders. The problem is that at that level of purity, the main contaminants come from the inner walls of the cylinder itself. Some grade 6 helium is tested after it's been put into the cylinder and then certified to be grade 6. This type of grade six helium comes with the test results showing the contaminant level for that individual cylinder. Optimally, only grade 6 helium cylinders which have been individually tested and certified should be used.

Vibrations are always an issue because the movement of the coldhead creates mechanical vibrations throughout the system. These vibrations can show up as noise in NMR spectra. In principle these vibrations are damped before reaching the probe, but this sometimes doesn't work perfectly. Great care must be taken in placing the antivibration stands. The position of the stands and the angle of the helium transfer line are very important. Also important is making sure that the stands are flat on the floor. If the stands can move at all, the proble will feel vibrations.

The numerous procedures and recommendations from the manufacturer should be followed exactly - even those that seem trivial or unnecessary.

Expenses

• Around $12,000/year for required maintenance (although it can vary with the vendor or the individual circumstance). Full service contract in the ballpark of $25,000/year. If no service contract, annual maintenance could be quite high depending how unlucky you are (i.e. could be significantly higher than price of service contract in some years). Service contract is highly recommended if you can afford it.