Probe tuning

Inside the probe is a wire coil that surrounds the sample tube. This wire transmits the radiofrequency pulses to the sample and then receives 

Normally it is not necessary to adjust these capacitors. But if a very high-quality NMR spectrum is wanted, then it may be necessary to tune the probe by adjusting these two capacitors, since the inductance of the coil will vary from sample to sample. The two capacitors are adjusted in 

How does probe tuning affect the quality of the NMR spectrum  

To maximize the efficiency of the delivery of the RF to the sample via the probe, one or more electrical components may need to be adjusted. This process is called probe tuning or just tuning, and is often a prerequisite for obtaining good data. 

Probe tuning is required because the efficiency of the delivery of the RF power to the sample depends on the complex impedance of the transmitter coil in the NMR probe. NMR probes are normally tuned to a complex impedance (resistance as a function of frequency) of 50 ohms at the NMR frequency of interest. A few basic physics equations, show why probe tuning is important. 

Power (P) is current (1) times voltage drop (V) across a circuit element (fbe probe s transmitter coil), or more simply. 

Probe tuning. The adjustment of the complex impedance of the probe to maximize the delivery of RF power to the sample (forward power), to minimize reflected RF power, and to maximize the sensitivity of the instrument receiver to the NMR signal emanating from the sample following the application of the pulse sequence. 

Complex impedance Electrical resistance as a function of frequency. 

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In order to demonstrate probe tuning using this scheme, a setup shown in Figure 11A was assembled. Here, the X channel of a Varian... 

Figure 23 Calculation of the shape of the actively compensated pulse can be carried out on the software. (A) shows the real (red line) and the imaginary (green line) component of an example of the target pulse shape t>,(f). Its leading and the trailing edges have a cosine shape with a transition time of 1.25 xs in 50 steps, and the width of the plateau is 5 ps. (B) Laplace transformation B(s) multiplied by the Laplace transformed step function U(s). (C) It was then divided by the Laplace transformation Y(s) of the measured step response y(t) of the proton channel of a 3.2-mm Varian T3 probe tuned at 400.244 MHz to obtain V(s). (D) Finally, inverse Laplace transformation was performed on V(s) to obtain the compensated pulse that results in the RF pulse with the target shape. Time resolution was 25 ns, and o = 20 was used for the Laplace and inverse Laplace transformations.

Next, bi(t) was Laplace transformed into B(s), and then multiplied by the Laplace transformation U(s) of the step function u(t). The result B(s)U(s) is displayed in Figure 23B. In this example, the step response y(t) was measured for the 1H channel of a Varian 3.2 mm T3 probe tuned at 400.244 MHz with a time resolution of 25 ns, and Laplace transformed into Y(s). By dividing B(s)U(s) by Y(s), the function plotted in Figure 23C was obtained, from which, by performing inverse Laplace transformation, the programming pulse shape v(t) was finally obtained, as shown in Figure 23D. The amplitude and the phase of the complex function v(t) give the intensity and the phase of the transient-compensated shaped pulse. 

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. 

Robustness Even if the NMR instrument is not properly calibrated (for example, the probe tuning and pulse length calibration are not optimized), as... 

Fig. 6 Bottom-up (i) fluorescence excitation spectrum of the 1 1 diastereomeric complexes between (5)-2-naphthyl-l-ethanol (F ) and 2-butanol (M /M ) (ii) hole-burning spectrum obtained with the probe tuned on the transition located at - 136 cm ([F M ] complex) (iii) (c) hole-burning spectrum obtained with the probe tuned on the transition located at — 69 cm ([F -Ms] complex) (iv) hole-burning spectrum obtained with the probe tuned on the transition located at — 73 cm ([F -M/ ] complex). The probed band is denoted by A. The bands due to the bare chromophore are denoted by 2-NEtOH (reproduced by permission of the American Chemical Society).

Modern instruments usually offer an on-board choice between a quadrature phase detector and some kind of diode or square detector. The latter, however is mostly used just for instrument setup (probe tuning, etc.) while signal acquisition is done almost exclusively by the phase detector. 

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 ... 

Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to H is to crudely shim on H2O. If there is no proton channel most multinuclear probes will tune to D, so D2O can be used. For CP-MAS probes that tune to - C, adamantane is a useful compound which should be shimmed under spinning and H decoupling conditions. A typical resolution for in admantane of 3-4 Hz at 7.05 T and 10 Hz at 11.7 T should be achieveable. 

As was discussed earlier in the chapter, probes typically include two coils H and X nucleus (e.g., or N). The inner (or observation) coil is more sensitive and requires more careful adjustment. Normal NMR spectra usually have such good signal-to-noise ratios that probe tuning is not critical for relatively concentrated samples. Tuning, however, is very important for both one- and two-dimensional X-nucleus-detected experiments and for many two-dimensional H-detected techniques. A surprising number of these experiments have failed simply because the X coil was not tuned to the correct nucleus In addition, if X-nucleus detection is to be conducted with proton broadband decoupling, as is usually the case (Section 1-5), then it is important that the H decoupling coil also be tuned optimally. 

Probe tuning is necessary for a number of reasons. Other than the fundamental requirement for maximising sensitivity, it ensures pulse-widths can be kept short which in turn reduces off-resonance effects and minimises the power required for broadband decoupling. A properly tuned probe is also required if previously calibrated pulse-widths are to be reproducible, an essential feature for the successful execution of multipulse experiments. 

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. 

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