TAILIEUCHUNG - Understanding NMR Spectroscopy phần 4

tích cực cho độ rộng xung của 5 và 10 ms, như xung đã được kéo dài thêm cường độ giảm đi qua một vô giá trị 20,5 ms và sau đó chuyển sang tiêu cực. Giải thích những gì đang xảy ra trong thí nghiệm này và sử dụng dữ liệu để xác định cường độ trường RF trong Hz và độ dài của một xung ◦ 90. | 5-2 How the spectrometer works Of course we do not want the sample to be at liquid helium temperatures so a room temperature region - accessible to the outside world - has to be engineered as part of the design of the magnet. Usually this room temperature zone takes the form of a vertical tube passing through the magnet called the bore tube of the magnet the magnetic field is in the direction of this tube. Shims The lines in NMR spectra are very narrow - linewidths of 1 Hz or less are not uncommon - so the magnetic field has to be very homogeneous meaning that it must not vary very much over space. The reason for this is easily demonstrated by an example. Consider a proton spectrum recorded at 500 MHz which corresponds to a magnetic field of T. Recall that the Larmor frequency is given by 1 V0 Y Bo 2n where Y is the gyromagnetic ratio X 108 rad s-1 T-1 for protons . We need to limit the variation in the magnetic field across the sample so that the corresponding variation in the Larmor frequency is much less that the width of the line say by a factor of 10. With this condition the maximum acceptable change in Larmor frequency is Hz and so using Eq. we can compute the change in the magnetic field as y X 10-9 T. Expressed as a fraction of the main magnetic field this is about 2 X 10 10. We can see that we need to have an extremely homogeneous magnetic field for work at this resolution. On its own no superconducting magnet can produce such a homogeneous field. What we have to do is to surround the sample with a set of shim coils each of which produces a tiny magnetic field with a particular spatial profile. The current through each of these coils is adjusted until the magnetic field has the required homogeneity something we can easily assess by recording the spectrum of a sample which has a sharp line. Essentially how this works is that the magnetic fields produced by the shims are cancelling out the small residual inhomogeneities in .

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