Saturation and Magnetic Field Strength

The energy difference AE between gronnd-state and excited-state nnclei is very small. The nnmber of nnclei in the gronnd state is the nnmber lined np with the magnetic held Bq. The ratio of excited nnclei to nnexcited nnclei is dehned by the Boltzmann distribntion  

Rgure 3.4 (a) The 300 MHz proton NMR spectmm of toluene (bottom) and the C NMR spectmm of toluene (top). (Reprinted with permission of Aldrich Chemical Company, Inc.) (b) The 60 MHz proton NMR spectrum of toluene. The structure of toluene is shown, with Me indicating a methyl group, -CHj. Two groups of absorption peaks are seen, one due to the protons of the methyl group and the other to the aromatic ring protons. The spectrum is discussed in Section 3.4. 

For a sample at 293 K in a 4.69 T magnetic field, the ratio N /Nq = 0.99997. There are almost as many nuclei in the excited state as in the ground state because the difference between the two energy levels is very small. Typically, for every 100,000 nuclei in the excited state, there may be 100,003 in the ground state, as in this case. This is always the case in NMR the Boltzmann ratio is always very close to 1.00. For this reason, NMR is inherently a low-sensitivity technique. 

The excess of unexcited nuclei over excited nuclei is called the Boltzmann excess. When no radiation falls on the sample, the Boltzmann excess is maximum, N. However, when radiation falls on the sample, an increased number of ground-state nuclei become excited and a reduced number remain in the ground state. If the RF field is kept constant, a new equilibrium is reached and the Boltzmann excess decreases to N. When = N, absorption is maximum. When = 0, absorption 

If the applied RF field is too intense, all the excess nuclei will be excited, 0, and absorption 0. The sample is said to be saturated. The saturation factor Zg is 

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