7/23/2015 1:07:26 AMPulsed NMR by Dr.S.Aravamudhan 1 P -X π/2 At t =0, the end of pulse External Magnetic Field Chemical substance Spin ensemble Z X Y. - презентация

1 7/23/2015 1:07:26 AMPulsed NMR by Dr.S.Aravamudhan 1 P -X π/2 At t =0, the end of pulse External Magnetic Field Chemical substance Spin ensemble Z X Y Magnetization Z X Y x,y-axes Rotating about Lab Z-axis; frequemcy same as the precession frequency Z X Y A rotating RF magnetic field results on application of RF at resonance frequency X Y The rotating magnetic field tilts the magentization away from z-axis by 90º for a π/2 pulse Viewed from within the rotating frame the RF field appears stationary Tilted Magnetization in xy plane viewed from Lab Frame. Precessing at resonance frequency. Magnetization in XY plane appears stationary when viewed in Rotating Frame from within the rotating frame X Y Induced NMR signal at receiver (RF 300 MHz ) Phase Sensitive detector Reference in phase at NMR (300MHz) frequency Output (0 freq) Reference in phase; offset from NMR frequency (300±0.003 MHz) Phase Sensitive detector Output at offset frequency (audio range) ~ 3KHz D.C. A description of the Induced RF signal Characteristics would appear……… After the pulse: at t>0 Transverse Relaxation and magnetization decay in XY plane is not depicted. No more CLICKs. This show has automatic timings from this stage. Free Induction NMR Signal NO F.I.D. yet! Right CLICK mouse And CLICK on option PREVIOUS OR…………. CLICK toTransit. Rotating x,y axes :rotation about Lab z-axis Apply the 90º, -X pulse now, P -X π/2 A BLUE line for z-Axis indicates the view from within the rotating coordinate system. X Y Rotating system viewed from within that system: STATIONARY X Y Z X Y Z Z Y

2 7/23/2015 1:07:26 AMPulsed NMR by Dr.S.Aravamudhan 2 Apply the 90º, -X pulse now, P -X π/2 X Y Z Viewed from within the rotating frame the RF field appears stationary Tilted Magnetization in xy plane viewed from Lab Frame. Precessing at resonance frequency. X Y After the pulse: at t>0 Induced NMR signal at receiver (RF 300 MHz ) Rotating x,y axes :rotation about Lab z-axis A BLUE line for z-Axis indicates the view from within the rotating coordinate system. Z Y Magnetization in XY plane appears stationary when viewed in Rotating Frame from within the rotating frame X Y Z When the XY magnetization decays with transverse relaxation time T 2, immediately after the pulse…… When PSD reference is in phase off set from Resonance frequency; NMR signal at receiver (RF 300 MHz ) If No T 2 …….. Free Induction Decay Signal No More Clicks ! This show has automatic timings The F.I.D. When PSD reference is in phase at Resonance frequency; NMR signal at receiver (RF 300 MHz )

3 7/23/2015 1:07:26 AMPulsed NMR by Dr.S.Aravamudhan 3 No net magnetization CLICK ! Magnetization Sample: Ensemble of spins Magnetization Magnetization Builds up exponentially t Magnetization T1T1 I0I0 e -t/ T1T1 ItIt I0I0 = ItIt External Magnetic Field To repeat the above events: Right Click & Select option previous Z X Y Initially, before the external magnetization is applied, the spins are randomly oriented When the magnetic field is turned on, the spins align at the characteristic longitudinal relaxation time T 1 CLICK ! +1/2 -1/2 Magnetic field +1/2 -1/2 No radiations are present Not stimulated transitions: but spontaneous relaxation transitions Degeneracy removed/Energy levels split On the application of field….. Splitting is instantaneous & population redistribution requires more time called the relaxation time Thermal equilibrium Boltzmann distribution Net magnetization along Z-direction & ZERO XY component random

4 7/23/2015 1:07:26 AMPulsed NMR by Dr.S.Aravamudhan 4 Randomization in XY plane I h NET Magnetization Transverse T 2 Relaxation Longitudinal T 1 Relaxation π/2 pulse Relaxation Longitudinal and transverse