- 2. Hajira Mahmood Sub topics of NMR spectroscopy 1. Oversampling 2. Digital filtration 3. Decoupling for X nuclei 4. Excitation modes 5. Inverse detection 6. Data processing
- 3. Oversampling in NMR Sampling faster than required by the sampling theorem â˘ effectively increases the dynamic range by a factor of n â˘ prevents certain sources of noise that are NOT band-limited to same extent as the systematic NMR signals from being aliased into the spectral window. â˘ Especially used to alleviate problem while studying large proteins to know whether typical 16-bit analog-to-digital converter (ADC) has sufficient dynamic range to digitize the sum of signals as well as a small signal of interest, even in the presence of excellent solvent suppression. â˘ Noise in an NMR spectrum consists of analog receiver noise, analog noise from the analog-to-digital converter components and "quantization noise" due to the limited precision of vertical digitization decreases by oversampling.
- 4. Diagrammatic representation of oversampling
- 5. Digital filtration â˘ In signal processing, digital filter is a system that performs mathematical operations on a sample discrete-time signal to reduce or to enhance certain aspects of that signal. â˘ largely used in signal processing and differs from an analog filter, which is an electronic circuit working with continuous signals. â˘ Expensive compared to analog ones, but they can turn many impractical or impossible designs into possibilities..0 â˘ Digital filters are used for two general purposes 1. For the Separation of signals that have been combined 2. For the restoration of signals that have been distorted in some way. Analog (electronic) filters can be used for these same tasks; however, digital filters can achieve far superior results.
- 6. Purpose to use digital filters â˘ A digital filter contains an analog-to-digital converter (ADC), which samples the signal coming in as input, a microprocessor and some other components for storing filter coefficients and data. â˘ There is also a digital-to-analog converter that is present just before the output stage. â˘ The software that runs on the microprocessor implements a digital filter by acting on a number from the ADC and perform mathematical operations. â˘ can perform several effects such as amplification & delay on the sample signal
- 7. Spin decoupling Special technique used in NMR spectroscopy to avoid splitting of signals by eliminating partially of fully observed coupling. Involves irradiation of proton with sufficiently intense radio frequency energy, so it prevents the coupling with neighboring proton & gives spectral line as a singlet. Helps to determine structure of chemical compounds Used to simplify a complex NMR spectrum Possible to irradiate each coupled protons in molecule to produce less complex spectrum. It causes multiples to collapse to a doublet or singlet to give easily interrupt spectra Important phenomenon used to enhance spectral signals to get clear distinctions between certain signals
- 8. Coupling & decoupling effect
- 9. Excitation modes in NMR â˘ Sample is irradiated with radiofrequency waves of appropriate frequency â˘ will excite transitions from the ground to the excited state due to the interaction of magnetic dipole with the oscillating magnetic field component of the electromagnetic radiation. â˘ This excitation field must be orthogonal to the direction of the magnetic dipoles to generate transitions of the nuclear spin state.
- 10. Inverse detection â˘ This technique become increasingly popular due to software improvements of spectrometers from one hand and to the increase of sensitivity of the detection of a X nucleus by the indirect way. â˘ In fact, it solves the problem of low concentrations. â˘ Moreover, it allows to reach the NMR parameters such as chemical shifts, coupling constants & relaxation time spin lattice for nuclei that are impossible to study by the direct detection. â˘ arrows figure the magnetization transfer from proton towards 13C and then from 13C towards the proton.
- 11. Excitation modes
- 12. Data Acquisition ďNMR experiments generally require the co-addition of a number of the nuclear spin response. ďRadiofrequency pulse and receiver phases plays role in it. ďThe data are recorded as a set of complex numbers which sample NMR signals as a function of time. ďEarly Fourier transform spectrometers had limited word length and required some scaling of the data before co addition, but this is no longer necessary. ďThe use of DSP gives better filtration of unwanted noise and signals, better effective ADC resolution, and less baseline distortion. ďIn two-dimensional (2D) NMR a series of FIDs is acquired using a pulse sequence. ď3D and 4D NMR extend the principle to two and three evolution periods respectively, but time constraints limit the number of samples in each dimension more severely. ďAlso increase the pressure to use non-uniform sampling of the signal. A very effective alternative way is to use methods where high resolution is not required
- 13. Spectrum Generation ď To make the raw experimental data interpretable they must be converted into a frequency domain spectrum. The classical, and commonest, method is discrete Fourier transformation. This is a linear operation. the information content of the data is unchanged here. ďIn Fourier processing a weighting function is usually applied to the time domain data before transformation. After weighting and any zero-filling, the fast Fourier transform (FFT) algorithm is used to produce the frequency spectrum. The resultant spectrum consists of a set of complex numbers sampling. ďIn 2D NMR, weighting and zero-filling are carried out on the FIDs as normal, but after Fourier transformation the data matrix is transposed to give a set of âinter ferograms.
- 14. â˘