The present invention relates to probes for nuclear magnetic resonance (NMR) spectrometers and specifically to a capacitance tuned probe employing a single sample coil suitable for multiple radio frequency resonance experiments, wherein the tuning elements of the probe are remotely disposed from the magnetic field, connected to the sample coil by means of coaxial transmission lines.
In nuclear magnetic resonance machines, a high-intensity uniform magnetic field is generated within an extremely strong magnet. Inserted into the axial bore of the magnet is the sample to be analyzed, and the combination radio frequency transmitting and receiving sample coil. The sample coil is situated to generate an oscillating field with a component at right angles to the main magnetic field. The oscillating radio frequency field causes an oscillation in the alignment of the nuclear spins present in the sample undergoing analysis. The oscillation of the various chemical species within the magnet causes the emission of radio frequency signals, which are received by the sample coil and associated probe circuits.
For some NMR analyses, particularly those involving solid sample materials, it is often desired to irradiate the sample with radio frequency fields of multiple frequencies at relatively high power levels, for example, in the 300 to 1000 watt range. It is important that a good coupling be achieved at all of the frequencies used. The usefulness and efficiency of prior art probes for this application is limited due to the size and magnetic restrictions for probe components, and the limited number of frequencies (usually only two or three) at which the samples may be irradiated without the operator removing and adjusting the probe settings. Each such removal and adjustment operation extends the time required to complete the sample analysis, and increases the chances of calibration and operator error.
The efficiency of prior art probes is additionally limited by their design characteristics. Traditionally, the desired radio frequencies were impedance matched to a particular value using standard, well known inductor techniques. However, while employing inductors is well known to generate impedance minima, inductors are characterized by high radio frequency losses, lowering the overall probe efficiency. Similar radio frequency losses result from each of the numerous interconnecting leads required between prior art probe components.