Tuning and Matching the Probe

Tuning the probe assures that the resonant frequency of the probe coil is the same as the RF frequency you will be using and matching the probe matches the probe coil as a load to the impedance (internal electrical resistance) of the amplifiers. This gives maximum efficiency of transfer of RF power from the amplifiers to your sample nuclei and maximum sensitivity in detecting the FID. Each sample modifies the resonant frequency and matching of the probe, so these have to be reoptimized with each new sample. Tuning the probe is not necessary for routine XH spectra, but for advanced experiments it is important if you wish to use standard values for pulse widths without the need to calibrate for each sample. 

Every electrical device that supplies power (such as a battery) has an internal resistance associated with it. For example, if you short out the two terminals of a battery, you will get a very large current but not the infinite current that would result from zero resistance (Ohm s law current = voltage/resistance). This is because the battery s voltage is applied across the total resistance of the circuit the sum of the external load (zero resistance) and the internal load (the internal resistance of the battery). In the case of the short circuit, all of the power (power = current2 x resistance) is delivered to the battery itself since there is no resistance in the load, and the battery heats up. If, on the contrary, the resistance of the load is very high, there will be very little current flow and very little power will be transferred to the load. It turns out that the maximum transfer of power from the source (the battery) to the load (e.g., a light bulb) is obtained if the resistance of the load equals the internal 

The probe tuning rods are long extensions of the variable capacitors located at the top of the probe, near the probe coil. The capacitors are delicate and there are two ends of the travel of the knob If any force at all is applied at the end of the travel, the capacitor will break. This will usually require that the probe be sent back to the manufacturer for repair, a process requiring a week or two and costing many thousands of dollars. For this reason many NMR labs do not allow users to tune the probe  

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Tune and match the probe head s H and 13C channels, respectively. 

By carefully tuning and matching the probe and the various stages in the transmitter amplifier chain and by allowing for a wide rf bandwidth, it is possible to reduce phase transients (for a definition, see Haeberlen, 1976, Appendix D) to a level where their effect upon the m.p. spectrum becomes insignificant. We therefore confine ourselves here to study by simulations phase errors, nonuniform pulsewidths, and a power droop of the transmitter. 

An automatic probe tuning and matching (ATM) accessory allows one to automatically tune the NMR probe to the desired nuclei s resonant frequency and match the resistance of the probe circuit to 50 Q [7]. Traditional NMR instruments are designed so that one must perform these adjustments manually prior to data acquisition on a new sample. The advent of the ATM accessory allows the sampling of many different NMR samples without the need for human intervention. The ATM in conjunction with a sample changer enables NMR experiments to be conducted under complete automation. The sample changers are designed so that once the samples are prepared, they are placed into the instrument s sample holders. Data are then acquired under software control of both the mechanical sample delivery system as well as the electronics of the spectrometer. 

The key development for using probe ions to study mineral systems is site selective laser spectroscopy. A tunable dye laser is tuned to match the absorption line of a particular ion with a particular environment or site within the sample. Only that ion will be excited and only that ion will fluoresce so the resulting fluorescence spectrum is much simpler than the conventionally obtained spectrum. 

The probe head is separately tuned and matched (25) for each sample and experiment. The magnetic field... 

The introduction of gradient shimming (that makes this procedure very easy to automate) together with the possibility of performing automatic tuning and matching (via the use of motors mounted on the probe body) has further improved the whole procedure. 

Figure 3.44. Tuning and matching an NMR probehead. The dark line represents the probe response seen for (a) a mis-tuned and (b) a correctly tuned probe head.

The method for probe tuning on older spectrometers that are unable to produce the frequency sweep display is to place a directional coupler between the transmitter/receiver and the probe and to apply rf as a series of very rapid pulses. The directional coupler provides some form of display, usually a simple meter, which represents the total power being reflected back from the probe. The aim is to minimise this response by the tuning and matching process so that the maximum power is able to enter the sample. Unfortunately with this process, unlike the method described above, there is no display showing errors in tune and match separately, and there is no indication of the direction in which changes need be made, one simply has an indication of the overall response of the system. This method is clearly the inferior of the two, but may be the only option available. 

Fig. 4.24 A schematic drawing of a wide-bore high-pressure, high-resolution multi-nuclear probe used for frequencies up to 400 MHz (a) the upper part, including the tuning and matching electronics (b) the high-pressure vessel and (c) the NMR insert and sample tube. (The dimensions are all in millimetres.) (Reprinted with permission from High Press. Res., 2, 237, (1990).)...

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... 

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