Recommendations for NMR Tubes
While a good nmr tube will not guarantee a good spectrum, a bad nmr tube will always result in a "poor" spectrum. A bad NMR tube will introduce large spinning side bands and bad line shape that cannot be compensated for with instrumental adjustments. No matter how long you shim or the number of transients acquired a spectrum obtained with a bad tube will have problems.
The factors that dictate what is a good quality tube are primarily, how straight the tube is (Camber), how uniform is the wall thickness, and how close are the inner and outer surfaces of the tube to perfect cylinders running parallel to each other. The last two factors relate to the "Concentricity". Wilmad defines this as "A measure of the lack of wall uniformity, ...the degree to which the cylinders defined by the inner and outer surfaces of the tube are parallel and overlap". See http://www.wilmad.com/NMR001.html for a full description of these properties. The outer diameter of a tube has to meet exact tolerances. If it is too large it can contact the glass insert protecting the probe coils. If it is too small the tube can move in the spinner turbine when the sample is lowered into the probe, resulting in the tube hitting the bottom of the probe. Either way the probe can be damaged, and the nmr tube can break with loss of the sample.
The first step is to buy good nmr tubes and avoid the bad nmr tubes. The cheap disposable tubes are adequate on a 200 MHz nmr, but will show limitations when run on higher level instruments. Can you use the cheapest tubes at the higher fields? Yes, BUT you shouldn't. We strongly recommend against the use of the "disposable" tubes on any instrument over 200 MHz. The cheap tubes may fit in the spinner turbine, but they could damage the probe.
These parameters apply to most of the instruments and probes. For almost all work you will use a 5mm outer diameter tube. Work on the Varian consoles should be done with tubes 8 inches in length. If working only with the Bruker consoles a 7 inch length is good. Obviously use the 8 inch tubes if working with both Varian and Bruker and you don't want to keep two sets of tubes.
You will see advertisements for thin wall tubes. Unless you have a dramatic need to really maximize your sensitivity, these thin wall tubes are very easy to break and are therefore not recommended.
For some "X-nucleus" work on the Bruker AM360 and Bruker/Tecmag200 10 mm probes are available, in which either a 5mm or 10mm outer diameter tube will work. If sufficient sample is available you may want to use a 10mm outer diameter tube, 7 inches long. This will provide a large increase in sensitivity, noticeably for low sensitivity nuclei like 15N.
Because the company Wilmad has been marketing nmr tubes so long it's tube nomenclature has become somewhat of a standard reference. For this note the tube designations are from Wilmad, unless noted otherwise. Each company lists which tubes it recommends for a given magnetic field strength (listed as proton spectrum frequency). The better tubes listed at each class will usually work with routine samples at the next class. Thus, the 528pp listed for 400 MHz work is also fine for most 500 MHz experiments; and the 526pp at 360 MHz is also good on a 400 MHz instrument for most experiments. And, of course, you can always use the higher quality tubes at the lower magnetic fields.
So, what tube(s) should be purchased? The following table represents our recommendations.
It is important to take good care of your nmr tubes after purchase. The glass used for nmr tubes is a "soft" glass. Avoid storing the tubes leaning in a beaker or flask. Instead, store them horizontally on a flat surface. Also, avoid exposing the tube to extreme heat. The best way to dry tubes is in a vacuum oven at a temperature below 105 ĄC, with the tube lying flat on a glass plate. If a vacuum oven is unavailable, use a regular oven set to 125 ĄC, place the tubes on a flat glass plate, and remove the tubes as soon as they are dry, usually 30-45 minutes.
See http://www.wilmad.com/NMR010.html for a full description of cleaning procedures.
-Chris Kojiro May 2001
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