Delay before Data Acquisition

Dipolar dephasing experiments were performed by inserting a delay before data acquisition in which the decoupler was gated off. 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

To reach a steady state before data acquisition, a certain number of dummy scans are usually required. If the relaxation delay between the... 

Pulse sequences used to induce 1H-13C polarization transfer (PT), with suitably chosen delays (A) before data acquisition, provide a simple and reliable way of separating 13C resonances according to the number of attached protons.84 In an illustration for cholesterol, the seven CH carbons are displayed with A = (2J) 1... 

INEPT can be used for determining the multiplicities of C nuclei coupled to protons. This is done by deleting the 180° H and C refocusing pulses and by including a delay A the value of which is adjusted to give a particular angle 0, before data acquisition. The CH, CH2, and CH carbon intensities depend on the delay A (i.e., on the angle 9), as shown in Table 2.2. 

Equation 1 was derived assuming that the nuclear spin magnetizations are at thermal equilibrium values prior to the start of the presaturation. In practice, due to time constraints on the instrument, this condition may not usually be reahzed and the nuclear spin magnetization can generally be in a quasiequilibrium state prior to presaturation. If (tj + t) is the delay between two consecutive 90° observe pulses, where t is the presaturation period and fa is the time delay before presaturation (this includes the data acquisition time for the previous pulse), then the appropriate expressions for STD and for control NMR spectra are given by ... 

Figure 4. 31.94 MHz 13C NMR data for intact lime cutin (bottom) and the solid residue of a depolymerization treatment (top). Both spectra were obtained with a 1H-13C contact time of 1.0 ms, repetition rate of 1.0 s, spinning rate of 3.0 kHz, a H decoupling field of 60 kHz, and a line broadening of 20 Hz. (For the chosen contact time, peak intensities within each spectrum reflect the approximate numbers of each carbon type.) Only the intact cutin spectrum retained signal intensity near 30 ppm when decoupling was delayed before acquisition (13,14).

SPC does require statistical quantities of product and automated data tracking. The best processes for SPC are those which are used to make large quantities of inexpensive parts. SPC is also difficult to apply to processes where the number of independent variables is large. Automated data acquisition is a must for SPC, but this is becoming inexpensive and common in the workplace. SPC is also a delayed control method. Many defective parts may be made before SPC corrects the process. The longer it takes to evaluate the results of the process, the more delay in the ability to react to process changes. Another important requirement of SPC is attention to detail on the part of the operator and/or process engineer. No battery of QC tests will detect every variation either in materials and process or in final quality. 

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 some cases, CP is not necessary to obtain a suitable solid-state NMR spectrum. In these cases, the SPE/MAS sequence may be used and for quantitative analysis only the X-nucleus Ti time needs to be determined. The standard inversion-recovery experiment can be used to measure this value, keeping in mind that MAS and high-power decoupling is still necessary. As before, once the X-nucleus Tl time is determined, 1-5 X T may be inserted as the delay time between successive pulses for quantitative data acquisition. 

NMR-SIM simulation is that the zero- and first-order phase distortions can be investigated separately in contrast to experimental spectra where both errors occur simultaneously. In the first part of Check it 3.2.3.4 the zero-order phase distortion is simulated using a 40° phase shift difference between the excitation pulse and the receiver phase, in the second part the first-order phase distortion is simulated using a delay dlO before the data acquisition. The spin system used in the Check it consists of a number of singlets evenly spaced over a frequency range of 3200 Hz. 

To improve the sensitivity the selective J-resolved pulse sequence may be combined with a refocused INEPT polarization transfer experiment. In contrast to the original heteronuclear J-resolved experiment the first excitation pulse is executed on the F2 channel with the spin-echo sequence sandwiched between two incremental delay. Coupling evolves during the second incremental delay before the refocused INEPT unit creates in-phase coherence for the nuclei which are coupled to the selected proton nucleus allowing decoupling on the F2 channel during data acquisition. 

When a free induction decay is Fourier transformed, some of the peaks may appear distorted or even inverted. They are then said to be out-of-phase with respect to a reference peak. Examples of the quartet of ethylbenzene before and after phase adjustment are given in Figure 3.14. The phase errors may be due to the phase detector setting, delay between pulse and start of data acquisition, or filter settings. The phase is digitally corrected after Fourier transformation. 

The H LC-NMR spectra were obtained on peaks stored in the BPSU-36 storage loops. Data were acquired with WET [41] solvent suppression on the residual water and acetonitrile signals. A composite 90° observe pulse, (tt/2)y—(tt/2) x—(tt/2) y—(tt/2)x, was employed. Spectra were collected into 32K data points over a width of 12 019 Hz, giving an acquisition time of 1.36 s, with an additional relaxation delay of 1.5 s. The data were multiplied by a line-broadening function of 1 Hz to improve the signal-to-noise ratio and zero-filled by a factor of two before Fourier transformation. 

All H NMR spectra were obtained with three signals suppression, i. e., water, methyl and methylene of ethanol signals in a 5 mm inverse-detection probe head. Eight FIDs were collected as 65536 data points using a 8.5 ps pulse (90°) spectral width, 8013 Hz acquisition time, 4.1 s and relaxation delay, 6.4 s. Spectra were processed using 32768 data points by applying an exponential line broadening of 0.3 Hz for sensitivity enhancement before Fourio transformation and were accurately phased, baseline adjusted and converted into JCAMP format to build the data matrix. 

All the 2D-NMR experiments have the same basic format which can be divided into four units preparation, evolution, mixing, and detection periods. For an A-B correlated 2D-NMR experiment, the preparation period usually starts with a delay during which the spins are allowed to return to equilibrium this is followed by a pulse or a cluster of pulses and delays that transfers the magnetization from B to A nuclei, called coherence transfer, just before the evolution period. The evolution period (tj) is a variable time delay, increased in a stepwise manner from an initial value of zero to a final value ni x lAw (related with spectral window swl in the indirectly detected dimension of nucleus A), provides the key to the generation of the second dimension. The mixing period serves to transfer the magnetization back from A to B nuclei. Then the detection ((2) period is needed to collect the FID (Free Induction Decay) data. It produces a spectrum similar to the one obtained from a ID experiment of the detected nucleus. Each value in the 2D sequence is repeated nt times and np data points (FID s) are stored (np is related to the acquisition time at and sw is the direct detection dimension spectral window of nucleus B). 

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