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NMR-Spektroscopy |
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NMR guarantees reproducible spectra
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Various investigations have shown that the specifications of NMR tubes that have been available in the past were often not capable of ensuring a good quality of spectrum. A new range of NMR tubes has now been developed which caters for all the quality criteria for obtaining optimum spectra that were only conceivable before.
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What is so interesting about NMR tubes that it is worth writing about? Now all NMR users know how very much the quality of their spectra depend on the quality of the sample vessel they are using. There are initially two reasons for this:
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- The sample tube is also measured as part of the measuring process. This means that its material and geometry properties should have preferably no influence on the measurement or at least an influence which can be eliminated by calculation.
- The sample vessel itself is in an inaccessible position in the measuring instrument during the measurement process, namely in the middle of the magnet. It should not be disturbed there under any circumstances since it is very difficult to interfere with it in this position.
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The material and dimensions of NMR tubes are subject, therefore, to very stringent requirements – and they are accordingly expensive to manufacture.
Before a new range of NMR sample tubes comes on the market, a large number of tests have to be carried out before the products agree with the specifications laid down. Then, when they are in full production, measures have to be implemented to ensure that the specifications laid down are adhered to.
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NMR quality criteria
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The decisive factor for the quality of a spectrum is how accurately the magnetic field is adjusted to the sample in the NMR tube. As this adjustment (or shimming) can be a complicated process, both the absolute geometry of the tube and the comparability of different tubes are a important quality criteria. For this reason, the adjustments between the different tubes, e.g. in the sample changer, should be as identical as possible. Shimming is then only required once and not for each tube. From this the various quality criteria for a good NMR tube can be derived:
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- Identical sample quantities should come to the same level in several tubes in the sample changer. Otherwise the magnetic field must be readjusted.
- All tubes should have the same diameter as far as possible.
- The wall thickness should be the same at every point.
- The tube must be straight.
- All the tubes must sit firmly in the spinner without moving and must be a good fit.
- The tubes should be robust so that they cannot be damaged in the NMR unit.
- The glass should be chemically resistant and inert to all samples.
- The glass may not contain any paramagnetic impurities that can result in problems in the spectrum.
- The tubes must be labeled so that different qualities can be clearly distinguished.
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Nowadays, many samples are measured without rotating them as in former times. The reason here too can be found in the homogeneity of the magnetic field. If they are not rotated completely evenly the result is a worse spectrum. It is, therefore, better in many cases to compensate for any irregularities by shimming. However, it is even better to ensure that as few irregularities as possible occur with an accurately specified quality.
Most NMR spectroscopy workers have got used to rolling NMR tubes on a flat surface as a rough test. This way of sorting out the “bananas” makes sense as a preliminary test in practice, but of course it is not adequate for production development and production control purposes.
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11.05.-15.05.2009
ACHEMA Frankfurt, Germany
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