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De-mystifying Probes

Probe construction and performance can seem like black magic because there are many details requiring close attention. Some of the details can be mutually exclusive, requiring some creative compromise. Since many of the “details” are VERY important, getting any one of them wrong may make the probe’s performance suboptimal. Probe Q is discussed in a separate Note. Listed in the table below are other important probe performance parameters.

90o Pulse Width
Resolution and Lineshape
RF Pulse Homogeneity
VT Range and Regulation
Decoupler Gamma H2

The most important details which affect a probes sensitivity are:

Coil Length
Coil Filling Factor
Probe Q
Dielectric value of sample
Percentage of the overall coils inductance in the coil leads

The first and second items in the Sensitivity Parameters list reduce to how much sample can the probe coil see. It is not the first thing that comes to an NMR operator’s mind when thinking of sensitivity, but it is one of the most commonly varied probe parameters in the vendors’ war of sensitivity numbers. For a 5mm 1H probe, the typical coil is between 10 and 20 mm long. Coils as long as 25 mm have been produced. At least three things limit the possible coil length:

1. Length of magnet’s high homogeneity magnetic field
In general, the higher the magnetic field, the shorter the useful magnetic length. Also the more narrow the bore, the shorter the useful magnetic field. A perfect probe can never perform better than the magnet homogeneity allows. Many vendors use long coil lengths, which make magnet to magnet variations more important. Some vendors optimize probe coil lengths based on each magnet. If you are in the market to buy an instrument, this can be one of the potential reasons why the sales demonstrations can be better than the performance you get on delivery.

2. Length of the room temperature shims’ active region
When the main magnetic field is long enough, the probe designer still needs to be concerned with the useful area over which the RT shims operate. If the main field is long enough and the shims don’t operate with the necessary purity over the coil’s length, the probe’s performance will be suboptimal. The first symptom that the coil is too long for the magnet and shim set is a tendency for the NMR signals to “split” or become doublets with VERY small changes in Z2 or with small changes in room temperature.

3. RF Homogeneity
Solenoid coils can have good RF homogeneity over long lengths. However, most superconducting magnet probes must use some type of Helmholtz coils. As these coils get longer, they have lower RF homogeneity. One way to see this low RF homogeneity is to measure the ratio of the signal from a 90 degree pulse and a 180o or 360o pulse. The larger the ratio, the better the probe’s RF homogeneity. This parameter can be very important in many 2D experiments even with compensated pulses.

The other dimension determing coil filling factor is off-axis. The filling factor in this dimension is limited by how close to the sample the coil can be placed. As the sample insert is brought in closer to the sample tube the tolerances on probe construction become tighter. The first symptom of these tolerances being wrong is trouble with sample spinning. As these tolerances get tighter, higher quality tubes are required. Another way to get a better filling factor is to use thinner wall tubes. The thinner walls allow more sample to be inside the probe’s coil and therefore the probe gives higher sensitivity numbers. The sensitivity difference from very thin wall tubes is much larger with 5mm probes than larger diameter probes. When looking at probe performance specifications from NMR vendors be sure to know what sample tubes they are using.

Last updated: 01/22/03