Tuning the probe

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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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... 

Once specific absorption features are assigned, kinetic studies can be performed via tuning the probe laser to a frequency absorbed by the fragment whose reaction kinetics are of interest. Ideally, it is also desirable to measure the rate of formation of the reaction product and to verify that these two rates correlate with each other. This has been done for the Fe(C0)x system with added CO where the reaction can be schematically depicted as... 

Tune the probe to one sample and then use an adapted 90° pulse for other solvents where the tuning is not optimal. This, of course, also leads to sensitivity losses. 

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. 

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 ... 

Tune the probe to the correct frequencies. For low y nuclei, or nuclei with very large chemical shift ranges, make sure the probe is tuned for the resonance frequency of the sample you are using, for example, the chemical shift range of ° Rh can exceed the width of the probe resonance. 

Choose an appropriate frequency for observation of the nucleus of interest and tune the probe for the experiment. 

Chapter 5 is concerned with how the spectrometer works. It is not necessary to understand this is great detail, but it does help to have some basic understanding of what is going on when we shim the magnet or tune the probe . In this chapter we also introduce some important ideas about how the NMR signal is turned into a digital form, and the consequences that this has. 

If you want to use the set-up as is and ignore the problem, tune the probe to maximize the received signal by looking, for example, at the detected signal from a dip meter. The loss in S/N of the received signal due to an improperly tuned probe is irrecoverable while that is not so for the transmitter pulse. In principle, an inadequate amplitude due to inefficient coupling between the transmitter and the coil can be overcome by simply having a more powerful transmitter. 

Calibration of pulse widths is often done with a standard sample containing a copious amount of solute (or one that is enriched). Calibration of N pulse widths also benefits from the use of an iso-topically enriched standard sample. Once we put in our real world sample (with a concentration of perhaps less than 5 mM), often the best we can do is tune the probe and hope the calibration arrived at while using a different sample will be sufficiently accurate. We must assume that pulse calibrations determined using a standard are valid for our sample as well. 

For the next experiments the infrared laser is tuned to a fixed transition to prepare a single rotational state in H2O and the OH product state distribution originating from this one state is measured by tuning the probe laser. 

Instead of keeping the pump laser at a fixed wavelength ki and tuning the probe laser wavelength 2, one may also fix the probe transition /) ) while the... 

Use Delta Version 3.1 software to control the instrument and for data processing. For the H-NMR experiments, (see Note 3) tune the probe to a frequency of 270.17 MHz with acquisition parameters as follows receiver gain 22, pulse width 10.4 is (tc/2), spectral width 4053 Hz, offset of 5 ppm, digital resolution 0.25 Hz, data acquisition time 4.04 s, relaxation time 25 s, and total number of scans 32 (see Note 4). 

Fig.12.15). If the pump laser is chopped, the difference of the detected signals with and without the pump laser just gives the collision rate for the transitions (vV,Jp Tuning the probe or the pump laser to different...

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