Static Low-E Probes Version 2.0:
Modular RF Coil Plugs

Changing Sample Coils

The static low-E probes feature replaceable sample coil assemblies for different sample geometry. The low-E assembly can be made for large and small rectangular samples (Figs. 1A, 1C), and it can also be oriented at 90° to static magnetic field (Fig. 1B). Other shapes such as round coil for bicelle preparations can be arranged (Fig. 1D). Version 2.0 upgrades to the NHMFL static low-E probes include a removable RF plug which simplifies changing sample coils. Rather than removing the brass screws and handling the delicate rectangular resonators, users can remove the entire platform on which the coil assembly sits.

Fig. 1. (A) Standard 470 uL flat coil; (B) Same flat coil rotated 90 degrees; (C) Smaller flat coil for limited sample volumes; (D) Round 5 mm bicelle coil.

Fig. 2.  Locations of RF plug mounting screws and leverage slots. The rear slot is hidden in this view.

To change a coil assembly, fully unscrew the three white plastic slotted flathead screws on the periphery of the tan colored RF plug. Next, inserting an appropriately sized flathead screwdriver alternatively in the front and rear slots shown in Fig. 2, gently leverage the plug upward until comes loose from the probehead. If the plug does not come off, check that the three white screws are fully loosened. Installation of a coil assembly is the reverse of removal. First line up the two front capacitor sockets which are underneath chip capacitor C8A (Fig. 3) with the two plugs residing on top of the probehead. Next lower the RF plug until it clicks into place on the probehead. Be sure to tighten the three white plastic screws to fully secure the platform, being careful not to over torque. Attach correct tuning chips and REMEMBER to adjust your power levels for the newly installed sample coil assembly!!! Wearing gloves is recommended when handling conductive parts inside the probe to prevent unnecessary oxidation.

Changing Observe Nucleus

Fig. 3.  Typical layout of chip capacitors that determine observe frequency. Not all chips require replacement. Actual probe may have smaller number of chips.

Our static low-E probes were designed with the ¹⁵N detection in mind but its observe channel X can be re-tuned to other nuclei. Re-tuning is done by replacing the detachable chip capacitors C8A, C8B, C7A, C5A, C2A, and C3A (where applicable). Typical locations of these chips are indicated in Fig. 3. To switch the observe frequency, use tuning table (go to individual probe's page) to determine which of these chip capacitors require replacement. Loosen the screws holding them in place enough to slide their leads out. Replace capacitors according to tuning table for each probe. For any unused screw terminal, make sure the screw is turned all the way in. When fastened, screws should be tight but be careful not to over torque. People with large fingers should be careful due to their inherent ability to apply more torque. For capacitors C2A and C3A, first the RF plug must be removed from the probehead as described in the section above. Once the platform is removed, gently grip the C2A or C3A capacitor with your fingers and pull until it disconnects from the socket. Installation of a replacement chip is accomplished by simply plugging it in to the vacant socket. Be sure to check the correct probe tuning table since the capacitor values of C2A is often not the same as that of C3A. When viewing the RF plug right side up with the resonator on top and C8A facing you, C3A will be on to your left and C2A will be to your right, as shown in Fig. 3. Again, we also suggest using gloves when handling conductive parts inside the probe to prevent unnecessary oxidation.

How to read the capacitance value on a chip?

Fixed value chip capacitors are normally marked in picoFarads (pF), using 2 significant digits. Another, third digit indicates the power of 10, and the letter after it normally specifies tolerance. Example: 150G means 15 x 10⁰ = 15 pF +/- 2%. If capacitor value is less than 10 pF, letter "R" is introduced in place of a floating dot. Example: 2R2B means 2.2 pF +/- 0.1 pF.