Preamplifier coil temperature

The most important factors in determining the s/n ratio or sensitivity of an NMR experiment in a cryogenic NMR probe, insofar as the hardware itself is concerned, are the temperature of the coil, 7/ the resistance of the coil, Rc the temperature of the sample, Ts and the resistance added to the coil by the presence of the sample or the sample resistance, Rs. The temperature of the rf coil in a cryogenic NMR probe is typically in the range of 15-30K commercial examples of the Varian Cold-probe operate at 25 K. The preamplifier noise temperature is generally in the range of 10-15 K and the coil resistance is small compared to a conventional room temperature NMR probe. The first two terms in the expression below provide the basis for the vast improvement in the performance of a cryogenic relative to a conventional NMR probe. The sample temperature, Ts, and sample resistance, Rs,... 

Sensitivity can furthermore be increased up to fourfold by using cryogenic probe technology, in which the radio frequency (rf) transmitter and preamplifier coils are super-cooled to 20K by helium gas that ensures higher signal-to-noise of the electrical signal. Vacuum insulation around the rf coil allows the NMR sample situated only millimeters away from the coil to be measured at ambient temperature. 

Figure 12.4. Block diagram of a modem NMR spectrometer. These systems use superconducting magnets that are based on a solenoid of a suitable alloy (e.g., niobium/titanium or niobium/tin) immersed in a dewar of liquid helium. The extremely low temperature of the magnet itself (4.2 K) is well insulated from the sample chamber in the center of the magnet bore. The probe in which the sample is housed usually incorporates accurate temperature control over the range typically of 4 to 40°C for biological samples. The rf coil in the probe is connected in turn to a preamplifier, receiver circuitry, analog-to-digital converter (ADC), and a computer for data collection.

In addition to the sample concentration and specific parameters related to the pulse sequence being used, the spectral signal-to-noise ratio (SNR) depends on various components of the spectrometer hardware, in particular the sensitivity of the probe and preamplifier. The SNR can be related to the temperature of the receiver coil ( Tq), its resistance (Rc), the temperature of the sample (Tg), the resistance added to the coil by the sample (i s), and the noise temperature of the amplifier (TA) by the following equation 7... 

The move to all-digital instrument components and digital oversampling (as used in CD players, for example) added further benefits. Commercial systems now offer cryogenic probes, which improve signal-to-noise (S/N) by reducing the operating temperature of the coil and the preamplifier. The increase in the S/N ratio by a factor of 4, as compared to conventional probes, leads to a possible reduction in experiment time of 16-fold or a reduction in required sample concentration by a factor of... 

The probe uses a single-coil for 20 mm spinning samples at room temperature. There are no tuning elements in the probe -- rather the tuning is by a variable inductor at the end of a A./4 cable as described in the discussion of quarter wave cables in V.C.8. The transformer at the end of the second quarter wave cable matches the impedance between the tank circuit and the broadband preamplifier. The output... 

Probe 5 mm or 3 mm CH dual probe, QNP probe, TXI probe, Broadband probe. Cryocooled probe in which the probe rf coil and the preamplifier are close to liquid helium temperature The sensitivity of H and is three to four times higher than that of a conventional probe. Significant reduction in time for 2D NMR experimentslD H can be obtained with as little as 10 ng of compound... 

Strength and the sample concentration, po is the permeability of free space, Q is the quality factor of the coil, coo is the Larmor angular frequency, K is the volume of the coil, F is the noise figure of the preamplifier, k is Boltzmann s constant, is the probe (as opposed to sample) temperature, and A/is the bandwidth (in Hz) of the receiver. It can be seen that the concentration sensitivity 5c (SIN per pM concentration of analyte) is poor for microcoils. This is due to the fact that microcoil probes have very small observation volumes and therefore contain a very small amount of analyte. However, if the sample can be concentrated into a small volume, then the microcoil can more easily detect the signal. This high mass sensitivity 5m (SIN per pmol of analyte) is characteristic of microcoil NMR probes. In essence, the use of microcoil probes enhances the mass sensitivity 5m at the expense of the concentration sensitivity 5c. To better understand the relationship between sensitivity and coil diameter, a detailed analysis was reported by Peck et Their results showed that mass sensitivity increases monotonically with decreasing coil diameter within the 1mm to 50 pm range they studied. However, the concentration sensitivity decreases, and therefore there is a trade-off between Sc and 5m that depends on coil diameter. 

A major advance in detection of NMR signals has been the development of probes in which the RF coil and the preamplifier are cooled close to the temperature of liquid helium, but with the sample remaining at ambient temperature. These so-called cryoprobes have a S/N ratio improvement of 500% over conventional probes of the same sample diameter. This is because the thermal noise level in the circuitry scales approximately as the square root of the ratio of the absolute temperatures. There are some limitations to this improvement for highly conducting... 

Cooling the RF coils of a probe to cryogenic temperature improves the RF efficiency and reduces the noise generated by the coils. Further improvements can be achieved if the preamplifier is also cooled to cryogenic temperatures, as in this way the noise generated in the circuit is reduced. These objectives, despite the formidable challenges presented by the need to keep the sample temperature stable at close... 

Finally, it is worthy to note attempts to adopt methodology which is known in the liquid-state NMR based on improving S/N ratio by decreasing temperature of sample, coil or preamplifier. Very recently, Doty Company presented cryocooled probeheads for solid-state CP/MAS [33]. 

CryoProbes improve the signal-to-noise ratio by three to four times, as compared with noncryogenicaUy cooled probes. The coil and preamplifier temperatures are lowered to about 30 K, and the coil resistance can also be reduced. As a result, they can measure lower concentrations and enable experiments to be completed even 16 times faster. The 5-nun, general-purpose CryoProbe enables multi-nuclear NMR measurements and provides quantitative information in addition to small-molecule structural data. 

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