Z-direction 100 MHz Electro- magnet/Permanent magnet Systems Supercon magnet systems above 100MHz up to 900MHz as known currently Magnet Current source. - презентация

1 Z-direction 100 MHz Electro- magnet/Permanent magnet Systems Supercon magnet systems above 100MHz up to 900MHz as known currently Magnet Current source Superconduc ting current carrying coils Pulse widths could be 10-2 μs

2 CW RF Oscillator mw power RF Crystal Detector Display/Record Pulsed RF Transmitter Probe with sample coil and sample in Magnetic Field High gain RF receiver/detector Time domain signal to computer for FFT Display monitor/Plotter Signal generation Signal receiving and detection CW Mode of detection Pulsed Mode of Detection

3 Flowing Current and Induced Magnetic Fields in a Solenoid Electrons (Blue Circles) Move and the Conventional Current Flows in the Opposite direction. The MOTION of the (red) isolated (?) northpoles indicate the induced field distributions and in reality there are no lines existing for the LINES of FORCES (as drawn in the previous slide. It is a virtual line and LOCUS of the point North pole.)

4 PULSE WIDTH PEAK to PEAK PULSE Amplitude

5 Free Induction Digitized FID analog signal FID digitized points Received Analog Signal Analog to Digital Converter A D C Digitized Signal

6 FID FT Real FT Imag. Integration All the signal shapes have been calculated in MS EXCEL In practice FFT program calculates Frequency domain spectra from the time domain signal Time Domain Signal Frequency Domain Spectra after FT Similar to the CW mode Spectra Pulsed detection mode

7 ADC CW RF Source Gate Pulse Programmer DC Pulse Power Amplifier Rectangular RF Pulse Sample coil in the Probe with sample Low noise RF Preamplifier FID High Gain Signal Amplifier Phase Sensitive Detector Reference Signal computer [FFT] Spectrum to display monitor/Plotter High Power RF Pulses to Probe HR NMR in Liquids 100W NMR of Solids 3KW PP Matched 50 Ω Time Domain signal Transmitter Receiver

8 CW Mode Sample tube with sample RF Bridge (Hybrid Junction) RF Source (sweep generator) Scope 50Ω RF Signal receiver- detector Basic Probe unit is a Resonance Circuit with tunable split capacitors configuration for matching. CW Mode High-Power pulse transmitter Receiver off time Low noise pre-amplifier /high gain receiver/PSD/ Digital computer/Plo tter Pulsed RF mode

9 Transmitter ON time Probe & sample Crossed Diodes Receiver Receiver OFF time Receiver Silent or dead time DATA acquisition starts at this time After the RF pulse, the FID is the impulse response from the sample spin system. The pulsing and FID can be repeated and added to acquire the averaged signal for better signal to noise ratio

10 Long T 2 Short T 2 Achieving a Sharp signal depends on the homogeneity of the magnetic filed: Shimmimng the magnetic field using gradient correction coils and sample spinning are the provisions in the spectrometer system for improving the homogeneity Signal level Noise Level

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12 Moderate Resolution HR PMR Spectrum CH3CH3 CH2CH2 OHOH δ= 1.13 ppm TMS δ= 0 ppm Acidic medium: spin coupling for OH protons do not show up This calculated and simulated [60 MHz] spectrum has the chemical shift and frequency values as obtained from a real NMR spectrum of alcohol. The above figure plotted using MS Excel application and line drawing from MS WORD drawing tools. High Resolution spectrum as shown above would be possible with good homogeneity of the magnetic field ppm 3.61 ppm 1.13 ppm 0 OH-CH 2 -CH 3

13 Equal number of lines pass through fixed area of cross section along the length 3 lines inside 6 lines inside Linear Field Gradient along z-axis. Along the length same nmr frequency for the sample NMR frequency will vary linearly along the length. In the Pulsed Field Gradient PFG and Magnetic Resonance Imaging MRI techniques calculated field gradient are externally superposed. The inherent unwanted and incidental inhomogeneities are reduced by Shimming and sample spinning