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Pulse widths could be 10-2 μs Sine arbitrary phae sinuous magnet/Permanent magnet Systems Z-direction Magnet Current source Supercon magnet systems above 100MHz up to 900MHz as known currently Z-direction 100 MHz Electro- cosine Superconduc ting current carrying coils CW RF Oscillator mw power RF Crystal Detector CWPulsed Mode of Mode of detection Detection Pulsed RF Transmitter Signal generation Probe with sample coil and sample in Magnetic Field Display/Record High gain RF receiver/detector Signal receiving and detection Time domain signal to computer for FFT Display monitor/Plotter 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.) PULSE WIDTH PEAK to PEAK PULSE Amplitude 1.50 1.00 0.50 0.00 -0.50 -1.00 -1.50 0.0 5.0 cosine 10.0 15.0 Exponenetial 20.0 25.0 product(cos*exp)=fid Free Induction Digitized Received Analog Signal Analog to Digital Converter FID analog signal FID digitized points ADC Digitized Signal All the signal shapes have been calculated in MS EXCEL In practice FFT program calculates Frequency domain spectra from the time domain signal Integration FID FT Imag. Time Domain Signal FT Real Pulsed detection mode Frequency Domain Spectra after FT Similar to the CW mode Spectra Power Amplifier Rectangular RF Pulse HR NMR in Liquids 100W NMR of Solids 3KW PP Matched 50 Ω High Power RF Pulses to Probe Sample coil in the Probe with sample Spectrum to display monitor/Plotter FID CW RF Source Gate Low noise RF Preamplifier DC Pulse High Gain Signal Amplifier Pulse Programmer Reference Signal Transmitter computer [FFT] Receiver Phase Sensitive Detector Time Domain signal ADC Basic Probe unit is a Resonance Circuit with tunable split capacitors configuration for matching. Sample tube with sample RF Source (sweep generator) High-Power pulse RF Bridge transmitter (Hybrid Junction) Pulsed RF mode 50Ω CW Mode RF Signal receiverReceiver detector off time CW Mode Low noise pre-amplifier Scope /high gain receiver/PSD/ Digital computer/Plo tter 52341 Probe & sample Transmitter ON time Receiver OFF time Crossed Diodes Receiver 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 Receiver Silent or dead time DATA acquisition starts at this time Signal level exp1.9 exp2.8 exp5.8 exp8 exp12 Noise Long T2 Short T2 exp1.9 exp12 Achieving a Sharp signal Level 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 OH-CH2-CH3 HR PMR ch3ch2oh Acidic medium: spin coupling for OH protons do not show up 5 CH3 Intensity/Amplitude 3.61 ppm 4 5.24 ppm OH 3 1.13 ppm CH2 0 2 1 TMS δ= 0 ppm 0 348 323 298 273 Moderate Resolution HR PMR Spectrum 248 223 198 173 148 123 98 73 48 23 frequency from TMS HR PMR ch3ch2oh δ= 1.13 ppm 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. In the Pulsed Field Gradient PFG and Magnetic Resonance Imaging The inherent unwanted and incidental inhomogeneities are reduced by Shimming and sample spinning MRI techniques calculated field gradient are externally superposed. NMR frequency will vary linearly along the 3 lines length. inside 6 lines inside Along the length same nmr frequency for the sample Equal number of lines pass through fixed area of cross section along the length Linear Field Gradient along z-axis.