6. What turbine caps do I use for extended VT?
Most of our turbine caps are not rated for the full VT range of the probe. GFT caps have the widest VT range. For use at XVT the GFT caps may have to be glued in. Glue the caps into the rotor with a good high temperature epoxy. The caps will then have to be drilled out to remove from the rotor so the caps would definitely be one-time use. (For some of our early probe models, to achieve spinning at ...
5. How can I spin a liquid or semi-liquid in a rotor without leaks?
First some liquids can be very challenging to spin without leaks and there will be a limit for most methods to the speed that can be achieved before leaks occur. The Doty MAS sealing cells for XC and DI type spinners are the best starting point for these types of samples. We have a variety of sample volumes available and while some volume is always lost in the use of these cells, they will seal for most liquids and ...
4. My MRI RF coil does not have the homogeneity I expect. What should I do?
Homogeneity is sometimes hard to evaluate on the bench and we need to have a quantitative indication to use. Look at the discussion on Coil Lengths for guidelines on what typical field profile to expect in small coils. We recommend a spin-echo mapping method (see S. Crozier, K. Luescher, L.K. Forbes, and D.M. Doddrell, "Optimized Small-Bore, High-Pass Resonator Designs," J. Magn. Reson. Ser. B , 1995, 109 , 1-11) in which a series of slice-selective spin-echo images are acquired with ...
3. What is the best procedure to use to spin an MAS rotor to avoid problems?
There are normally instructions in the user manual for the correct method for your probe. However, some general instructions are: • Start with about 10 psi of bearing gas pressure. • Add a couple of psi of drive pressure to get the rotor spinning. • Gradually increase bearing and drive together keeping bearing 10 to 15 psi higher than drive up to about 40 psi on the bearing. • After this the drive can be increased faster than the ...
2. I have spikes in my FID. What is causing this?
"Spikes" in the FID are asynchronous noise bursts at the observe frequency being generated somewhere in the NMR system. There are several possible sources for this noise including arcing in the probe or sample coil. A bad cable or poor quality cable that has recently been put into the system could cause the problem. Devices external to the probe or sample coil can arc causing the noise. Frequently we find that high power filters in-line before the Doty probe or ...
1. What do I check if my MAS probe will not spin up to specification?
The most common problem with spinning is poorly packed or balanced samples. Samples should be finely powdered to avoid more massive lumps positioned asymmetrically on the inside circumference of the rotor. Add the sample to the rotor as evenly as possible and then spin the sample slowly for a couple of minutes to let it “spin pack” before gradually increasing the speed. Also fairly common with probes that have been in the field for a while is excessive turbine ...
