The NMR probes

Both the electrochemical NMR cell and the mini-cell require a custom-built probe (Fig. 3) in order to be loaded into the NMR spectrometer. The probe is designed for an 8.45 (360 MHz) wide bore (89 mm) superconducting magnet, and is loaded into the top of the magnet with the center of the sample positioned at the magnetic center. The electrochemical leads are connected to 0.141-inch diameter semi-rigid coaxial cable, which is part of the probe, by BNC connectors. The semi- rigid coax runs the length of the probe and enters into the probehead where the electrochemical NMR cell/mini-cell is located. LC low-pass filters are installed at this point (Figs. 1(B) and 4). These filters serve the dual purpose of (i) minimizing the introduction of environmental noise, which can be transmitted through the electrochemical leads, and (ii) also acting as RF chokes in the reverse direction, preventing the leakage of high-power excitation pulses out into the potentiostat. The optimal arrangement of the LC circuit is determined experimentally, and can be done on a benchtop with an oscilloscope and a frequency sweeper. Care must be taken to insure that the filter's inductor is not self-resonant at the Larmor frequency of the nuclei under investigation, or tuning problems arise due to multiple resonances in the probes'LC circuit. In the case of 13C NMR at 8.45 T, the inductors are tuned to resonate at around 150 MHz. This prevents multiple tuning resonances at 90 MHz, yet provides sufficient impedance to block the RF power from shunting out of the electrochemical leads.

Fig. 3. Schematics of the electrochemical NMR probe and circuitry. The probes' casing and cap are made of brass. All the internal pieces are made of copper.

Probe

Probe

Fig. 4. Schematic of the low-pass filters used to enhance signal-to-noise ratio and prevent excitation power from shunting out the electrochemical leads. Table 1 lists the configuration of the tuning elements
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