Probe Setup, Operations, Probe Modifiers
User Manual Version 002 BRUKER BIOSPIN 43 (327)
In a wideline probe the NMR coil is easily replaced. So with a few coils of different
inductance one can extend the tuning range determined by the tuning capacitor
inside the probe. This cannot easily be done in a MAS probe for two reasons:
1. The coil must be carefully aligned such that it does not touch the spinning rotor
2. There are two frequency ranges to be set- the X-tuning range and the proton
tuning frequency.
In such a probe, changing the coil would throw off the proton tuning totally, so a
coil change is not possible.
Extending the tuning range of a CP/MAS probe can be done in the following ways
(Figure 3.34.
):
1. Switch the proton transmission line between λ/4 (low range) and λ/2 mode
(high range). The proton transmission line is also part of the X-circuit and is
higher (lower) in capacitance in λ/4 (λ/2) mode (only 400 MHz and up).
2. Add a parallel capacitance to the X-tuning capacitance, which makes the ca-
pacitance bigger (tunes to lower frequency). This is normally done to shift the
tuning range to or below
15
N.
3. Add a capacitance in series to another capacitance. This reduces the total ca-
pacitance and shifts to higher frequency. A capacitance in series to the λ/4 line
will reduce its total capacitance and shift the X tuning to higher frequency.
4. Add a parallel coil to the NMR coil. This reduces the total inductance and shifts
to higher frequency, however at the cost of filling factor. The bigger the parallel
inductance, the smaller the high frequency shift and the loss.
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