Nuclear Magnetic Resonance Facility (NMR) | Tuning And Matching The Probe

Tuning And Matching The Probe
In general, once the sample is inserted, the probe should be tuned and matched using the knobs. Note that the knobs have a limited range and attempting to turn them beyond this range will damage the probe. The correct tuning and matching is especially important for higher frequencies. The purpose of this operation is to increase the signal-to-noise ratio. For proton and carbon observations in CDCl₃ it is not necessary to tune and match probe, because the Probe is already tuned?only minor adjustments are necessary. However, if a strongly ionic sample is used then you need to tune the probe. Using the polar solvents may cause a lengthening in the 90^o pulse width by a factor of two or three.
In a probe there is a resonant circuit for each observed and decoupled nucleus on the Probe (e.g., one for ¹H and one for X (X = ¹³C, ³¹P, ¹⁵N etc). Each of these circuits has a frequency at which it is most sensitive (the resonant frequency).
Tuning is the process of adjusting this frequency until it coincides with the frequency of the pulses transmitted to the circuit. For example, the frequency at which the ¹H resonant circuit is most sensitive must be set to the carrier frequency of the ¹H pulses.
Matching is the process of adjusting the impedance of the resonant circuit until it corresponds with the impedance of the transmission line connected to it. This impedance is 50Ω;. Correct matching minimizes the power that is transmitted to the coil.
Remember that you should get training on the tuning of the probe. Do not attempt to adjust anything unless you have been trained.
For the INOVA 600MHz NMR machine 2 probes are available:
Direct detection Probe (SW/PFG): in normal configuration, the Proton channel is tuned to proton, X channel is tuned to carbon and the Lock channel is tuned to deuterium. The X channel can be tuned also from nitrogen to phosphorus frequency.
Indirect detection Probe (H{¹³C/X}PFG: in normal configuration, the Proton channel is tuned to proton, Carbon channel is tuned to carbon, Lock channel is tuned to deuterium, the X channel can be broad band tuned from nitrogen to phosphorus frequency. This probe is designed for inverse detection triple resonance experiments. The probe has optimal proton sensitivity and capability for simultaneous or single irradiation at ¹³C and X frequencies.