Pulse repetition time

Fig. 6. The CP/MAS spectra of cellulose acetate-butyrate (CAB) and of cellulose acetate (CA, degree of substitution = 1.97), 20. The observation frequency was 50.1 MHz and the irradiation frequency 199.5 MHz. The pulse repetition time was 5 s and the contact time 2 ms. For CAB 400 scans and for CA 60 scans were collected...

Fig. 9. Solid-state NMR spectra of stiff chain aromatic polyesters containing sulfur bonds and tentative assignements of their signals, 401. A contact time of 2 ms and a pulse repetition time of 10 s were used...

Proton NMR and deuteron NMR spectra of soluble fractions and spent solvent mixtures were obtained by using a JE0L FX60Q FT NMR Spectrometer. A flip angle of 45° was used which corresponds to 14 ms for and 75 ms for 2H. The pulse repetition times were 6.0 and 9.0 s, respectively. Chloroform-d was used as the NMR solvent, and chloroform was used as the 2H NMR solvent. 

Record four pulses (averaged to reduce signal noise) with a pulse repetition time of 2 sec for nonstabilizing fats. For stabilizing fats, use a single 6-sec pulse. 

In a net effect, the pulse repetition times in flowing systems can be reduced to the decrease in the apparent spin-lattice relaxation times Tifiow, whereas at a given detection volume an increase in flow rate leads to an increase in the signal half-width W due to the decrease of r2fi0w... 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

Figure 2. I3C NMR spectra of curdlan(DPn 540) in water suspension (A) and alkaline solution (90° pulse, repetition time 0.6 s) (8)...

The envelope function defines the pulse repetition time T = 27r/u>r by demanding A(t) = A(t — T). Inside the laser cavity the difference between the group velocity and the phase velocity shifts the carrier with respect to the envelope after each round trip. The electric field is therefore in general not periodic with T. To obtain the spectrum of E(t) the Fourier integral has to be calculated ... 

The pulse width is an important factor in the measurement of pulsed spectra. The optimal pulse-width may be estimated21 from the equation cos a = exp(— TJT), in which a is the pulse width (in degrees), Tt the spin-lattice relaxation-time (in s), and T the pulse-repetition time (in s). For monosaccharides in 20% aqueous solution, values of the protonated carbon atoms are22 1 s at 30°. Using 8 k of computer memory for the acquisition, and a sweep width of 5-6 kHz, T becomes 0.6-0.8 s, and the equation gives an optimum pulse-width of 60°. In Fig. 1 is shown a series of spectra measured at different pulse-widths, all other variables being kept constant. The best s/n is seen to correspond to a 63° pulse. If, 3C-n.m.r. spectra are recorded for very concentrated solutions, or impure samples, the Tj values may become small, and, in such cases, a 90° sample pulse will be optimal. 

Figure 18.2 13C NMR of syndiotactic polystyrene produced by highly stereocon-trolled catalyst system. Apparatus JEOL Lambda 500 (13C 125.65 MHz). Frequency 25000 Hz. pulse 9.0 xs (45° pulse). Repetition time 4 s. Scans 10000...

TABLE 7. Comparison of DHA Content from the Interlaboratory NMR Analysis Between Japan and Norway Together with GC Data. Ethylene Glycol Dimethyl Ether was Used as Internal Standard CH NMR Analysis with 30s Pulse Repetition Time). 

Uronic Acid Composition and Block Length. A sample containing 75 mg alginic acid was first suspended in a small volume of D2O, then fully neutralized by titration with 1.0 M NaOD. D2O was added to achieve a final concentration of 25 mg alginic acid/mL D2O. Equilibration of at least one hour was allowed prior to NMR acquisition. The uronic acid composition was determined by NN spectroscopy, with a 2 s pulse repetition time and 16,000 scans. The reported composition and block lengths were the average of two determinations. 

Figure 20. C-NMR spectra from a perfused mouse liver at SS C. (c) C natural abundance background of this liver, accumulated before the substrate was added. The substrate, 8 mM [3- C]alanine and 20 mM unlabelled ethanol, was then added at 0 minutes and again at 120 minutes, and a series of C-NMR spectra were taken, (b) Spectrum measured during the period 150-180 minutes (a) C-NMR spectrum of the perfusate after the perfusion was terminated, at 240 minutes this spectrum consisted of 5000 scans. The pulse repetition times were 0.5 seconds for b and c and 2 seconds for a. Abbreviations pci, oCl, ]9C3.5, aC4, pC6, aC6, PC2, C2.5 and aC3, the carbons of the glucose anomers Glu C2, glutamate C-2 Gin C2, glutamine C-2 Asp C2, aspartate C-2 Ala C2, alanine C-2 LacC3, lactate C-3 CB, cell background peak W, X, Y and Z, unknowns AA Ca, acetoacetate CHj and / -HB Ca, -hydroxybutyrate CHj (from [31]).

Nowadays almost all the NMR data are acquired by the pulsed Fourier transform (FT) method. In this method, the consideration of spin-lattice relaxation times (Tj) for the signals of interest, that is, proper selection of the observed pulse width (or flip angle) and of pulse repetition time (or pulse interval) is of primary importance for quantitative analysis.5-8... 

Fig. 1. Relative intensities of OCH3 and CH2 proton signals against a-CH3 signals in PMMA at various pulse repetition times and flip angles.

Table 11. Effect of pulse repetition time on relative peak intensity and tacticity determination for radically prepared PMMA by 500-MHz 1H NMR. ...

Fig. 21.2. 67.9 MHz C CP/MAS NMR spectra of a series of P(3HB-co-3HV) samples (2-ms contact time, 5-s pulse repetition time, 1000 FID accumulations, s indicates spinning side band). (Reproduced from Ref. [53] with permission.)...

Under conditions where there is complete decay of transverse magnetisation between scans (i.e. the FID decays to zero) the optimum tip angle for a pulse repetition time h, known as the Ernst angle, Oe, is given by [1,2] ... 

Figure 4.2. The Ernst angle for optimum sensitivity when signal averaging. The pulse tip angle is dictated by the pulse repetition time, t relative to the longitudinal relaxation time, Ti.

Figure 4.5. Optimum pulse repetition time when using 90 pulses. The curve has been calculated from the equation in [31.

Figure 4. Aliphatic carbon signals in cis-polyisoprene from Solidago altissima observed with a pulse repetition time of 7 s for 45° pulse ( denotes signals due to impurities). Reproduced with permission from Ref. 13. Copyright 1983, The American Chemical Society.

NMR spectra at 80 MHz were recorded on the Bruker WM200 spectrometer at the MRC Biomedical NMR Centre, Mill Hill, using 15 mm tubes and ca. 4 ml of sample. Typically, 3 ml of packed red cells, previously washed twice with saline solution, were resuspended in 1 ml of D20-saline. Compounds which were insoluble in water were added in ca. 5-20 il of MeOH. Spectra are usually the result of ca. 1 hour accumulations 2000 pulses, 1.8 sec pulse repetition time, 16 K computer points, 45 pulse, and peaks are referenced to external H3PO4 (85% in H20)/15% D2O. 

P. W. Alexander and U. Akapongkul, Differential Pulse Voltammetry with Fast Pulse Repetition Times in a Flow-Injection System with a Copper-Amalgam Electrode. Anal. Chim. Acta, 166 (1984) 119. 

---