Probe Coil Leads
One very critical part of a probe is the probe coil itself. Great care is taken to design probe coils for the best RF homogeneity and highest Q as well as many other important parameters. In this design process, the leads between the active probe coil and the probe’s tuning and matching capacitors are often overlooked. These leads can play a very important role in the overall probe performance.
The first area of consideration is the length of the probe coil’s leads. The longer the leads, the farther removed the capacitors and other probe components can be from the active area of interest. This is good because these components have a magnetic susceptibility which affects probe resolution, lineshape and shimability. It is bad because the leads add inductance to the total probe coil inductance AND are not surrounded by sample. In effect, this condition reduces the probe coil’s filling factor. The maximum sensitivity would occur when all of the sample coil inductance was filled with sample. In probe design, the probe sensitivity can be improved when the ratio of active coil inductance to coil lead inductance is increased. This is one reason why multiple turn coils give better results than single turn coils. This holds true whether the multiple turns are in parallel or series.
One way to minimize the effect of the probe coil’s leads is to get as much of the tuning capacitance near the active part of the coil as possible. Since the circulating current in a probe is proportional to the Q and impedance level of the coil, having most of the capacitance at the bottom of the coil and before the leads would reduce the ability of the leads to pick up signal. However, it also reduces the tuning range from the variable capacitor at the end of the leads. This range is not only needed for tuning the probe to other nuclei but for samples of different ionic strength on the same nucleus.
Where the leads are located relative to the sample is also very important. If the probe coil’s leads are run up the sample insert, then they will be near the sample in areas removed from the main active region of the probe coil. These removed areas have at least two major problems. The first is that they do not have good B1 homogeneity. Possible consequences of poor B1 homogeneity are poor inversion and poor water saturation. The second major problem area is that the sample near the leads experiences a different magnetic field value, resulting in poor spectral lineshape (hump). This phenomenon gives rise to a strange set of conditions with unexpected results. If the magnetic field were perfect in all areas where there is sample and coil inductance, then the lineshape would be perfect. If the magnetic field is perfect in the active area of the coil but of an extremely different value (ie., would give rise to an NMR signal outside the spectral window) in the area of the leads the lineshape would be okay. This means that often a very good magnet or a very poor magnet can give good lineshape results, but an in between magnet would give poor results.
To get around the “lead pickup” problem, the coil’s leads must either not pick up sample signal or be shielded from the sample signal. This can be done in many ways:
Geometrically position the leads to minimize differential signal pickup by each lead. This works because if both leads see the same signal there is no induced voltage across the coil’s output and therefore no signal from the leads.
- Shield the leads with a band of copper or another material (plating) in the area of the leads.
- Arrange a virtual ground area of the coil to shield the leads.
- Get as much of the coil’s tune capacitance as close to the active coil as possible. This lowers the current in the leads and thereby the lead pickup.
One way to test your current 1H probe’s lead response is to take a very small drop of water in the bottom of a 5mm tube and place it in the center of the probe coil. Run a spectrum using a very wide sweep width, process and integrate the spectrum. Set the integral to a value such as 100. Now repeat the process changing the sample depth in 2 mm increments using the same normalization and processing constants. Move in both directions until the signal goes to zero. Be certain to go far enough since the signal often goes to zero and returns as the sample is moved from the coil to the lead area. Use these data to create a plot of integrated intensity versus sample depth. This plot has a wealth of information. It shows the proper sample depth, the length of the coil, and any lead pickup taking place. Sometimes lead pickup can be minimized by pulling the bottom of the sample up above the area of lead pickup. This makes shimming more difficult due to end effects of the sample, but should reduce lead pickup.
Last updated: 01/22/03