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PULSED NMR

Computer memory. Time domain. DIGITIZE. 15. Analogue to Digital Converter A.D.C. 11. 0. Computer output. FFT from FID. Next Slide. Frequency Domain Spectrum. Computer input. PULSED NMR. Acquire F.I.D. Free Induction Decay

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PULSED NMR

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  1. Computer memory Time domain DIGITIZE 15 Analogue to Digital Converter A.D.C. 11 0 Computer output FFT from FID Next Slide Frequency Domain Spectrum Computer input PULSED NMR Acquire F.I.D. Free Induction Decay NMR detection soon after a strong pulse: precessing nuclear magnetization induces a signal in coil when it is free of the perturbing EM radiation Acquisition is automatically in the digitized form F.I.D. This one-dimensional FT NMR spectrum is the same information as the C.W. NMR spectrum

  2. dimension A(50),B(50),Y(50),X(50) K=32 open (unit=1, file="output") Print 10,K DO 11 N=1,K X(N)=(N-1)*3.5/K X(N)=EXP(-1.0*X(N)) Y(N)=X(N)*(COS(2*3.14*(N-1)*10.0/K)+ 1 COS(2*3.14*(N-1)*4.0/K)) 11 write (1,20) N,Y(N) DO 12 M=1,K A(M)=0 B(M)=0 DO 13 N=1,K-1 A(M)=A(M)+Y(N)*COS(2*3.14*(M-1)*(N-1)/K) 13 B(M)=B(M)+Y(N)*SIN(2*3.14*(M-1)*(N-1)/K) A(M)=A(M)/K B(M)=B(M)/K M2=M/2 12 write (1,30) M2,A(M2),B(M2) 10 FORMAT(1x,I2) 20 FORMAT(1x,I2,2x,F10.5) 30 FORMAT(1x,I2,2x,F10.5,2x,F10.5) close (unit=1) STOP END A program in Fortran for“Fast Fourier Transform” Digitized FID Signal Digital Computer ---------------------------------------------------------------------- ---------------------- ------------ - FFT Program run OUTPUT

  3. Time domain FID data: 32 points Real Imaginary 16 data16datapoints points Frequency domainspectrum

  4. Value between +1 & 0 0 +1 F.T F.T COS Real SIN Imaginary Imaginary Real F.T Arbitrary Phase Real Imaginary t=0 Provision is made in the data processing system, for routinely applying phase corrections fc cos(2πνt) + fssin (2πνt) with fc2 +fs2 =1

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