Absorption mode

Fig. la-c. Theoretical 2H NMR line shapes for axially symmetric FGT (r = 0) in rigid solids, cf. Equ. (1). a Line shapes for the two NMR transitions b 2H spectrum (Pake diagram) in absorption mode as obtained by Fourier transform methods c 2H spectrum in derivative mode as obtained by wide line methods... 

It is curious that the chair- boat problem, which is most associated with small, liquid-state molecules, arises in the context of solid-state research (B3, II). Although the paucity of useful experiments militates against a definitive solution here E3), the frequency independence of the NMR second moment (E2), the absence of an observable free-induc-tion decay (Tj <25 fis) in the pulsed NMR spectrum (El), and the smoothness of the absorption mode itself (SI), all argue against the... 

Although this eliminates negative contributions, since the imaginary part of the spectrum is also incorporated in the absolute-value mode, it produces broad dispersive components. This leads to the broadening of the base of the peaks ( tailing ), so lines recorded in the absolute-value mode are usually broader and show more tailing than those recorded in the pure absorption mode. 

There are actually two independent time periods involved, t and t. The time period ti after the application of the first pulse is incremented systematically, and separate FIDs are obtained at each value of t. The second time period, represents the detection period and it is kept constant. The first set of Fourier transformations (of rows) yields frequency-domain spectra, as in the ID experiment. When these frequency-domain spectra are stacked together (data transposition), a new data matrix, or pseudo-FID, is obtained, S(second Fourier transformation to convert this pseudo FID to frequency domain spectra. The second set of Fourier transformations (across columns) on S (/j, F. produces a two-dimensional spectrum S F, F ). This represents a general procedure for obtaining 2D spectra. 

There are generally three types of peaks pure 2D absorption peaks, pure negative 2D dispersion peaks, and phase-twisted absorption-dispersion peaks. Since the prime purpose of apodization is to enhance resolution and optimize sensitivity, it is necessary to know the peak shape on which apodization is planned. For example, absorption-mode lines, which display protruding ridges from top to bottom, can be dealt with by applying Lorentz-Gauss window functions, while phase-twisted absorption-dispersion peaks will need some special apodization operations, such as muliplication by sine-bell or phase-shifted sine-bell functions. 

Figure 5.44 (a) Phase-sensitive absorption-mode NOESY spectrum of bovine phos-... 

Absorption-mode spectrum The spectrum in which the peaks appear with Lorentzian line shapes. NMR spectra are normally displayed in absolute-value mode. 

Dispersion mode A Lorentzian line shape that arises from a phase-sensitive detector (which is 90 out of phase with one that gives a pure-absorption-mode line). Dispersion-mode signals are dipolar in shape and produce long tails. They are not readily integrable, and we need to avoid them in a 2D spectrum. 

Phasing A process of phase correction that is carried out by a linear combination of the real and imaginary sections of a 1D spectrum to produce signals with pure absorption-mode peak shapes. 

Phase-sensitive data acquisition NMR data are acquired in this manner so that peaks are recorded with pure absorption-mode or pure dispersionmode line shapes. 

In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

FIG. 1 FT-IR spectra in midfrequency region. DNA-treated gold substrate measured in reflection-absorption mode (a) and transmittance spectrum of DNA cast on Cap2 (b). 

Summarizing, infrared spectroscopy measures, in principle, force constants of chemical bonds. It is a powerful tool in the identification of adsorbed species and their bonding mode. Infrared spectroscopy is an in situ technique, which is applicable in transmission or diffuse reflection mode on real catalysts, and in reflection-absorption mode on single crystal surfaces. Sum frequency generation is a speciality... 

There are two components of the transverse nuclear magnetization one in phase with the field H, and one ir/2 out of phase with Hi. The former is known as the dispersion mode or u mode magnetization, and the latter as the absorption mode or v mode magnetization. That is, referring to Fig. 2, will lag or lead Hi as the resonance is traversed. The magnetic radiofrequency susceptibilities are defined by... 

Until recently, previous studies for continuous monitoring of hepatic function with ICG utilized the absorption mode. However, new studies demonstrate that the highly sensitive fluorescence technique can equally be used [148-150]. In addition to high sensitivity, in-depth analysis of the emission, excitation and polarization properties of fluorescence spectroscopy furnishes additional functional information about the dye molecule. In this system, the fluorescence profile emanating from the clearance of injected biocompatible dye is monitored with a small photodetector. Fig. 8 shows the in vivo fluorescence detection apparatus developed for continuous monitoring of organ functions [147,148]. 

It may be useful to make some explanations on data processing to prepare good D-HMBC spectral data. Sine-bell window is usually employed for processing of HMBC data to give power-mode spectra as shown in fig. 6(a), because they consist of absorption-mode cosine) and dispersion mode sine) signals for both the t and t2 axes. This procedure causes a considerable loss of signal to noise ratio when cross peaks appear as broad ones and when digital resolution is poor as used for ordinary HMBC measurement. 

In order to overcome this problem, the half-absorption mode has been proposed by Bax [14] to take advantage of the phase-mode data processing. In this method, of Sx F, F2) and 7c,s of Sy F, F2) prepared from the data acquired by the States method are exchanged, and the final t2 axis data containing dispersion component sine) are presented in the power-mode, and the t axis data in the pure absorption-mode as follows ... 

This half-absorption mode is also employed for data processing of the D-HMBC spectra with a modification that t2 and t axes data are treated in... 

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