Peak suppression

Melphalan produces less nausea and vomiting than does cyclophosphamide however, its bone marrow suppression tends to be more prolonged and affects both white cells and platelets. Peak suppression of blood counts occurs 14 to 21 days after a 5-day course of drug therapy recovery is generally complete within 3 to 5 weeks. 

Figure 15. H-NMR spectra(2C)0 MHz) of intact banana fruit tissue (A) non-MAS spectrum obtained in a conventional high resolution probe with sample axis parallel to magnetic field (B)MAS spectrum obtained without water peak suppression (C) vertical expansion of (B) (D)MAS spectrum obtained with water peak suppression, the signal-to-noise ratios (S/N) in spectra C and D are 55 and 1137, respectively. The magic angle spinning (MAS) frequency was 1.05 kHz.[Reproduced with permission from Ref.81].

At a pH of 6.0 the fulvic acid was adsorbed at the hanging mercury drop electrode. The resulting peak suppressions made the evaluation of the measured results impossible. 

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

Fig. 46. — Structural-reporter-group Regions of the Resolution-enhanced, 500-MHz, H-N.m.r. Spectrum of Compound 63. [The bold numbers and letters in the spectrum refer to the corresponding residues in the structure. The relative-intensity scale of the IV-acetyl-proton region (see insertion) differs from that of the other parts of the spectrum, as indicated. For solvent-peak suppression, a w.e.F.t. pulse-sequence was used.]...

Scenario (a) transplants acquisition parameters from a typical ID proton spectrum into the second dimension leading to unacceptable time requirements, whereas (b) and (c) use parameters more appropriate to 2D acquisitions. All calculations use phase cycles for f quad-detection and axial peak suppression only and, for (b) and (c), a recovery delay of Is between scans. A single zero-filling in f] was also employed for (b) and (c). 

A simple way of suppressing axial peaks is to select the pathway Ap = 1 on the first pulse this ensures that all signals arise from the first pulse. A two-step cycle in which the first pulse goes 0°, 180° and the receiver goes 0°, 180° selects Ap = 1. It may be that the other phase cycling used in the sequence will also reject axial peaks so that it is not necessary to add an explicit axial peak suppression steps. Adding a two-step cycle for axial peak suppression... 

We now address the question how much atomic physics needs to be included in order to account for ATI In fig. 9.6, we show experimental data for ATI from [493], obtained at several laser intensities. One of the important properties of ATI peaks, referred to as peak suppression, is that the relative intensity of the first ATI peaks above threshold does not increase uniformly with laser field strength, but actually begins to decrease in intensity relative to higher energy peaks as the laser field strength increases. Such behaviour cannot be explained in a perturbative scheme, in which interactions must decrease monotonically order by order as the number of photons involved increases, but can be accounted for in terms of the AC Stark shift of the ionisation potential in the presence of the laser field. In ATI experiments, the ionisation potential appears to shift by an average amount nearly equal to the ponderomotive potential, so that prominent, discrete ATI peaks are seen despite the many different intensities present during the laser pulse. However, ATI peaks closest to the ionisation limit become suppressed as the amplitude of the laser field oscillations increases and the ionisation threshold sweeps past them (a different effect which also suppresses ionisation near threshold is discussed in section 9.24.1). 

Fig. 9.6. Experimental data, showing the first few ATI peaks above threshold obtained at several different laser field strengths. Note the reduction in relative intensity of the lowest energy peaks (peak suppression) which occurs as the laser intensity is increased. This is a nonperturbative effect (after T.J. Mcllrath et al. [493]).

Molecular motions of intermediate frequency, with correlation times of 10 5 s, which can interfere with the proton decoupling frequency (ca 50 kHz), can be very easily detected when certain 13C NMR signals from both the DD-MAS and the CP-MAS NMR spectra of [3-13C]Ala-bR are simultaneously suppressed (blanked area b), as illustrated in Fig. 15A. Such motion was first recognized when the 13C NMR signals of the C terminus were almost completely suppressed both in the CP-MAS and DD-MAS NMR spectra, when the temperature was lowered to between -40 and -110°C.116 This kind of peak suppression is most pronounced at ambient temperature for the whole range of transmembrane a-helices and a part of the loop in the [3-13C] Ala-labeled bleached bacterio-opsin (bO) where retinal was removed from bR (see Fig. 16), as viewed from the suppression of Ala 39, 53, and 84 (B and C helices), Ala 215 (G-helix), the E-F loop (Ala 160), and the F-G loop (Ala 196).98 A similar type of peak suppression... 

Fig. 15. Detection of several types of motion either by observation of dynamics-dependent suppressed peaks (A) or measurement of relaxation parameters (B) as a function of respective motional frequency (Hz) or its timescale or correlation times (s). NMR peak suppressed by fast isotropic motion (a), interference with proton decoupling frequency (b), and magic angle spinning (c).69 Reproduced with permission from Elsevier Science.

Fig. 3. Example of a typical orange juice H NMR spectrum obtained with water peak suppression. Reprinted from J. Agric. Food Chem., Vol. 44, J. T. W. E. Vogels, L. Jerwel, A. C. Tas, F. van den Berg, F. Dukel and J. van der Greef, Detection of adulteration in orange juices , pp. 175-180, Copyright 1996, with permission from the American Chemical Society.

The most obvious use for such selective de-excitation is for solvent peak suppression wherein an overwhelming but undesirable solvent peak must be reduced or eliminated. Redfield (1976) has developed specialized techniques to selectively de-excite the solvent peak in a multiline spectra. 

Ito and Tsukada proposed a method to correct for peak suppression owing to matrix effects. The ionization efficiency of API is, in fact, greatly affected by coeluting compounds contained in a... 

Presaturation Traditionally, the solvent signal is irradiated for a period of time with a continuous wave rf field Can be easily set up Useful for eliminating single solvent signal. Extremely sensitive to spectrometer stability and shimming. Not easy to suppress multiple solvent peaks simultaneously. Also, suppresses exchange peaks Suppresses NMR signals from the compound that overlaps with the solvent. 

E. Special pulse methods in FTNMR (Determination of relaxation times, solvent peak suppression etc.)... 

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