Spectrum acquisition spectral width

Just as when acquiring a chemical shift spectrum, we would set the acquisition spectral width, or sweep width (SW) (in hertz), to the field of view Av (again in hertz). (Actually, we would set the spectral width a bit larger to avoid having the spectrum or image bump into the upfteld and downfteld Nyquist spectral limits.) The spatial resolution obtained is limited by the spectral resolution of the acquisition. If we... 

Figure 4. Proton decoupled 19F-NMR spectrum of pABG5 /7-glucosidase inactivated with 2F/ GluF (conditions as described in text). This spectrum was recorded on a 270 MHz Bruker/Nicolet instrument using gated proton decoupling (decoupler on during acquisition only) and a 90° pulse angle with a repetition delay of 2s. A spectral width of 40,000 Hz was employed and signal accumulated over 10,000 transients.

Continuous-flow 19F LC-NMR spectra were acquired for 16 transients using 60° pulses into 8192 data points over a spectral width of 11 364 Hz, giving an acquisition time of 0.36 s. A relaxation delay of 0.64 s was added to give a total acquisition time for each spectrum of 16 s. The data were multiplied by a line-broadening function of 3 Hz to improve the signal-to-noise ratio and zero-filled by a factor of two before Fourier transformation. The results are presented as a contour plot with 19F NMR chemical shift on the horizontal axis and chromatographic retention time on the vertical axis. 

For the continuous-flow measurements, the pseudo-2D spectrum was recorded with a spectral width of 9616 Hz and 64 transients with 8K complex data points, thus resulting in an acquisition time of 0.42 s/transient along the 128 t increments. A relaxation delay of 1.2 s was used and the time resolution... 

Figure 8.2.15 One-dimensional spectra acquired with the four-coil probe. Each sample (250 mM in D2O) was loaded into the coil via the attached Teflon tubes 32 scans were acquired for each spectrum, with no delay between excitations of successive coils. Concurrent with the switch position being incremented, the spectral width was optimized for each compound 1 Hz line-broadening was applied before Fourier transformation and baseline correction. The spectral widths were (a) 600 Hz (galactose) (b) 1400 Hz (adenosine triphosphate) (c) 2000 Hz (chloroquine) (d) 500 Hz (fructose). 2048 complex data points were acquired for each spectrum, giving data acquisition times of approximately 1.7, 0.7, 0.5 and 2.0 s, respectively. The delay between successive 90 degree excitations was 4.9 s for each sample. Reprinted with permission From Li, Y., Walters, A., Malaway, P., Sweedler, J. V. and Webb, A. G., Anal. Chem.,l, 4815-4820 (1999). Copyright (1999) American Chemical Society...

In 2D experiments, the precision of the measured values is determined by the precision with which peak positions can be determined in a 2D spectrum. The precision of the values measured along the F2 axis is determined by the acquisition time (as in ID spectra), but the precision of the values measured along the FI axis (i.e. indirectly detected) is determined by the maximum evolution time used in the experiment (assuming it is shorter than the 7V relaxation time of the signal). Hence, if a heteronuclear coupling [e.g. 2/(Si—H)] has to be determined with a precision of 0.1 Hz, it would require a maximum evolution time of the order of 10 s, that is, some 40,000 increments if the spectral width along FI were 4 kHz (in a correlation experiment), which is not very realistic. On the other hand, chemical shifts can be easily determined with the needed precision of 1 Hz along FI. 

Unlike the sample condition, the experimental parameters have only a minor effect on the NMR spectral parameters. Experimental parameters such as spectral width, flip angle, repetition time, number of points in the free induction decay (FID) and in the real spectrum, number of scans, and processing parameters need to be comparable to those used for the acquisition of the database spectrum or spectrum of the authentic... 

The last equation tells us what value of the dwell time we have to use to establish a particular spectral width. In practice, the user enters a value for SW and the computer calculates DW and sets up the ADC to digitize at that rate. It is important to understand that with the simultaneous (Varian-type) acquisition mode, there is a wait of 2 x DW between acquisition of successive pairs of data points. The average time to acquire a data point (DW) is the total time to acquire a data set divided by the number of data points acquired whether they are acquired simultaneously or alternately. The spectral window is fixed once the sampling rate and the reference frequency have been set up. The spectral window must not be confused with the display window, which is simply an expansion of the acquired spectrum displayed on the computer screen or printed on a paper spectrum (Fig. 3.15, bottom). The display window can be changed at will but the spectral window is fixed once the acquisition is started. 

Figure 17 Phase sensitive P, Pt H PMG-HMQC spectrum of [Pt(C5H5 N)(PPh3) CI2] in THF at 295 K. A standard gradient experiment could not be used due to the large /(P-Pt). NS = 2, spin-lock pulse duration 5900 xs, 14 dB attenuation, total acquisition time 15 min. Further reductions in experiment time, or improved resolution in FI could be achieved by using a smaller spectral width in the Pt dimension. SW = 125 ppm, with 128 increments was used here. Compare the signal-to-noise achieved in the Pt spectrum with that in Section 3.1...

With the test sample in the magnet, the probe is tuned to H and the magnet homogeneity maximized. Next, a test spectrum is determined in which the tp used is unimportant. This spectrum then serves as a starting point for another spectrum, which has a reduced spectral width. The original sw is now reduced to about 500 Hz, and the transmitter offset is adjusted so that it is in the middle of the reduced sw. Many spectrometers have programs that do both operations with one command. Since sw has been considerably reduced, the number of data points should also be decreased, to approximately 4,000, so as to maintain an acquisition time of around 4 s. 

Because np2 is so much smaller than the number of points in an ordinary 1D spectrum, whereas SW2 is not commensurately smaller than common ID spectral widths, two-dimensional acquisition times typically are in the 100-300-ms range for H-detected, and less than 100 ms for heteronuclear-detected, 2D experiments. Remember that, as in ID experiments, sw2 (in Hz) depends on the magnetic-field strength and, therefore, affects the value of Similarly, is normally set by the spectrometer after np2 and sw2 have been selected. 

As stated in Section 7-4b, digital resolution in the v domain is a function of the number of increments (ni) and the spectral width (swi). Spectral data describing the v dimension can be acquired in the either the phase-sensitive or the absolute-value mode. Real and imaginary ui-domain data sets exist for both types of acquisition, but are treated differently. The imaginary data are discarded in phase-sensitive acquisition, just as with the V2 dimension data previously described. By contrast, with absolute-value data, both the absorptive (real) and dispersive (imaginary) components of the v domain are used to describe the spectrum. The important point is that, for both kinds of data, the acquisition of 2M increments yields M points, after Fourier transformation, to characterize spectra in the ui dimension. Therefore, if swi = 2,100 Hz and ni = 512, then DR = swi/(ni/2) = 2,100 Hz/(512/2) = 8.2 Hz/point. If one level of zero filling is carried out, then the effective ni = 1,024 and it follows that DRi = 2,100 Hz/( 1,024/2) = 4.1 Hz/point. 

The most basic NMR experiment is the one-pulse proton experiment.23-25 Proton chemical shifts typically range from 0 to 10 ppm, so the spectral width should be set at least this large. A good approach is to set the spectral width to a larger value, such as 15 ppm, to identify the actual limits of the resonances observed for a given sample. Then the spectral width can be reset to a smaller value specific to the sample. Acquisition parameter values determined for the 1-D proton spectrum can be used as a guideline for other proton-detected experiments, including the proton dimension of two-dimensional experiments. 

The second fundamental requirement is for the data be sufficiently well digitised for the lineshape to be defined properly. To minimise intensity errors it is necessary to have at least four acquired data points covering the resonance linewidth, although many more than this are preferable, so it is beneficial to use the minimum spectral width compatible with the sample and to adjust the acquisition times accordingly. The spectral width should not be too narrow to ensure the receiver filters do not interfere with resonance intensities at the edges of the spectrum. 

One further difference between the two frequency dimensions of a 2D spectrum should be mentioned. In contrast to direct acquisition in t2 no filters (analogue or digital) can be applied in the tl dimension. As a consequence signals outside of the fl spectral limits are folded back and give rise to "ghost" peaks in the 2D spectrum. To prevent this occurring in either the f 1 spectral width and observation frequency have to be adjusted or a more sophisticated pulse sequence with region selective pulses in fl has to be used. NMR-SIM can also produce folded peaks in the fl dimension as demonstrated in Check it 3.3.1.2. 

Pulse delay Acquisition time Spectral width Data points/spectrum Free Induction Decays (FID s) accumulated ... 

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