Transmission-Line Magic Angle Spinning NMR Probes

Transmission-Line Probes for Solid-State NMR.

There are no commercial equivalents to our transmission-line probes. The strategy of inserting radiofrequencies only at specific impedance minima along the transmission line results in isolation between channels of over 80 dB (compared to 40 dB in standard probes). The isolation means that small REDOR and 2D CODEX difference peaks are completely trustworthy. We have performed tests on 13C-labeled solids that contained no fluorine in which millions of 19F 180-degree pulses were used and obtained a zero REDOR difference. That is, there was no measurable leakage between channels.

A HFPN four-frequency transmission-line probe for operation in a 500-MHz spectrometer. The probe sits on a pallet supported on the legs of the magnet dewar. The support points are connected to worm gears which are synchronized by universal joints. The probe is raised and lowered by driving a single shaft (top of photo) with a hand drill. The yellow cable is part of the active control circuit for the power amplifiers. Capacitive coupling to the transmission line monitors the rf voltage at the coil.

The ratio of inner to outer pipe diameters is determined by impedance matching to the power amplifiers. Large pipes are used because the transmission lines are part of the tuned circuit. High currents are carried on the surface of the inner conductors, and the larger the surface area, the greater the efficiency.

The transmission line permits the tune-and-match circuits of all channels to be remote from the coil and external to the magnet. This results in temperature stability (no proximity to spinning gas) and allows the use of a non-perturbative capacitive pickup near the coil of the rf voltages generated by the final-stage amplifiers. Test pulses fired on each channel after each data acquisition period generate the voltages which are diode detected. The test voltages are compared to a calibration voltage and the difference is used to correct the drive to the power amplifiers. In effect, we recalibrate the spectrometer every one or two seconds which makes the long-term data acquisition periods practical. Short-term fluctuations are suppressed by REDOR and CODEX differencing and long-term drifts are removed by real-time calibration and active control of the power amplifiers.

Schematic drawing of the HFPN probe. Point A is the proton-frequency impedance minimum and is the insertion point for the next lowest frequency in the cascade (fluorine). Adjustments of the tune-and-match tuning circuits for F, P, and N have no effect on higher-frequency tuning. C3 and C5 are part of a fluorine trap. The C1 capacitor in the probehead ensures a homogeneous rf field. The capacitor C2 allows fine tuning of the transmission-line length and accurate positioning of point A.

There are a number of copper and plastic components required for assembly of the transmission-line probe. The high-frequency input components also contain specially machined capacitors to handle high frequency and high power. The transmission line itself is made from standard copper plumbing pipe.







(Left) Machined copper and delrin parts for transmission lines and high-voltage capacitors. (Right) Probehead assemby for air lines and stator support.

However each pipe has to be adapted to fit with precision machined copper components and to locate impedance minima accurately (Figure below, right). Final assembly is by clamps so that disassembly does not require removing solder welds.







(Left) Section of the transmission-line probe showing the four-way junction on the left and the proton-fluorine trap in the center. (Right) Adjustment of phosphorous and fluorine isolation by G. Potter.