The Probe
The Probe
Since it is the probe that must receive the very weak NMR signals, it is perhaps the most critical part of the NMR spectrometer, and its particular design and construction will influence not only the types of experiments it is able to perform but also its overall performance. There has been considerable progress over the years in the design of probes, resulting in ever-increasing performance and a greater array of available probe technologies. The simplest design is a probe containing a single coil, which is designed for the observation of only one nucleus. In fact, it would be ‘doubly tuned’ to enable the simultaneous observation of deuterium for the field-frequency regulation via the lock system, although the presence of a deuterium channel is usually implicit when discussing probe configurations. However, many modern NMR experiments require pulses to be applied to two (or more) different nuclides, of which one is most often proton, for which two rf coils are necessary. The traditional two-coil design is optimised for the observation of the X-nucleus, with the X-coil as the inner of the two allowing it to sit closer to the sample, so offering the best possible sensitivity for X-observation; it is said to have the greatest ‘filling factor ‘. This configuration can be described by the shorthand notation X{1H}. Multipulse experiments tend to utilize the higher sensitivity offered by proton observation wherever possible and benefit from probes in which the proton coil sits closest to the sample with the X-coil now the outer most; 1H{X}. It is this design of probe that is widely referred to as having the inverse configuration because of this switch in geometry. In either case, the X-coil circuitry can be designed to operate at only a single frequency or can be tuneable over a wide frequency range, such probes being known as broadband observe or broadband inverse probes. A further feature offered is the addition of magnetic field gradient coils to the probe head. These surround the usual rf coils and are designed to destroy the static magnetic field homogeneity throughout the sample for short periods of time in a very reproducible manner.
Tuning the probe
The NMR probe is a rather specialized (and expensive) piece of instrumentation whose primary purpose is to hold the transmit and receive coils as close as possible to the sample to enable the detection of the weak NMR signals. For the coils to be able to transmit the rf pulses to the sample and to pick up the NMR signals efficiently, the electrical properties of the coil circuit should be optimized for each sample. The adjustments are made via variable capacitors that sit in the probe head a short distance from the coil(s) and comprise the tuning circuitry. There are two aspects to this optimization procedure known as tuning and matching, although the whole process is more usually referred to as ‘tuning the probe’ .The first of these, as the name implies, tunes the coil to the rf of the relevant nucleus and is analogous to the tuning of a radio receiver to the desired radio station. A poorly tuned probe will lead to a severe degradation in sensitivity, just as a radio broadcast becomes swamped with hissing noise. The second aspect aims to equalize (or match) the impedance (the total effective resistance to alternating current) of the coil/sample combination with that of the transmitter and receiver so that the maximum possible rf energy can pass from the transmitter into the sample and subsequently from the sample into the receiver. As electrical properties differ between samples, the optimum tune and match conditions will also vary and so require checking for each new sample.
