TAILIEUCHUNG - Understanding NMR Spectroscopy phần 8

Hiện tượng này, mà magnetizations của hai quay khác nhau được kết nối, được gọi là thư giãn chéo. Tốc độ từ hóa S được chuyển giao cho tôi từ hóa được đưa ra bởi thời hạnđược gọi là tỷ lệ qua thư giãn không đổi, và đôi khi cho σIS biểu tượng. Rõ ràng rằng trong trường hợp không có các con đường thư giãn giữa các tiểu bang αα | that the cosine and sine modulated signals are generated by using two mixers fed with reference signals which differ in phase by n 2. If the phase of each of these reference signals is advanced by ộrx usually called the receiver phase the output of the two mixers becomes cos Qt - ộx and sin Qt - 0rx . In the complex notation the overall signal thus acquires another phase factor exp iQt exp i0sig exp -i0rx Overall then the phase of the final signal depends on the difference between the phase introduced by the pulse sequence and the phase introduced by the receiver reference. Lineshapes Let us suppose that the signal can be written S t B exp iQt exp i0 exp -t T2 where 0 is the overall phase 0 . - 0rx and B is the amplitude. The term exp -t T2 has been added to impose a decay on the signal. Fourier transformation of S t gives the spectrum S a S rn B A rn iD rn exp i0 1 where A à is an absorption mode lorentzian lineshape centred at I Q and D à is the corresponding dispersion mode lorentzian A ffl r Q Normally we display just the real part of S a which is in this case Re S rn B cos0 A rn - sin 0 D rn In general this is a mixture of the absorption and dispersion lineshape. If we want just the absorption lineshape we need to somehow set 0 to zero which is easily done by multiplying S à by a phase factor exp i0 . S rn exp i0 B A I iD exp i0 exp i0 B A rn iD rn exp i 0 0 As this is a numerical operation which can be carried out in the computer we are free to choose 0 to be the required value here -0 in order to remove the phase factor entirely and hence give an absorption mode spectrum in the real part. This is what we do when we phase the spectrum . Relative phase and lineshape We have seen that we can alter the phase of the spectrum by altering the phase of the pulse or of the receiver but that what really counts is the difference in these two phases. We will illustrate this with the simple vector diagrams shown below. Here the vector shows the position of .

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