Sensitivity enhancement

Multiple quantum methods are being developed which involve a double transfer of magnetization H - N - H, greatly increasing the enhancement, and with the additional advantage of obtaining N NMR information in two-dimensional spectra at proton frequencies. 3  

Because the signal decays as a function of time, while the noise is stationary, data points at the beginning of the FID contribute more to the resonance intensity than do points near the end of the FID. One common way to discriminate against later points with poor S/N without abruptly truncating the FID is to use an exponential filter, with 

The exponential factor in the time domain function then becomes 

FIGURE 3.10 Use of an exponential filter to improve signal/noise ratio, (a) Acquisition time T = 4T2, with no filter. (6) T = 4T2, with a matched filter, showing better S/N but a broader line. (c) Relative S/N as a function of acquisition time and without a matched filter. 

Gaussian multiplication is also an attractive alternative to increase sensitivity, with some concomitant alteration in line shape, as well as width. In this case 

In 2D spectroscopy the Gaussian function is often preferred over exponential multiplication because little broadening at the base of the resonance is induced by this filter, and significant sensitivity improvement can still be obtained. 

Some important nuclei, including C and N, have low natural abundances and sensitivities. Pulse sequences have been devised to improve the observability of these nuclei when they are coupled to another nucleus of high receptivity, usually a proton. Pulses are applied in such a way that the favorable population of the sensitive nucleus S is transferred to the insensitive nucleus I. A common sequence developed by Freeman for this purpose is called INEPT, for Insensitive Nuclei Enhanced by Polariz.ation Transfer, as follows  

For C H), maximum increased intensities are 3.98 for INEPT and 2.99 for NOE. When the gyromagnetic ratio of the insensitive nucleus is negative, INEPT has an even greater advantage because of the subtractive factor present in the NOE expression. For N H), the INEPT and NOE factors are 9.87 and -3.94, respectively for Si H, 5.03 and -1.52 and for Sn H, 2.68 and —0.34. Clearly, INEPT is significantly more effective in each case and is always positive. 

Modern NMR Techniques for Chemistry Research. Pergamon Press, Oxford, UK, 1987, p. 137.) 

Because the choice of A2 — 1/2J for the decoupled, refocused INEPT experiment leads to completely refocused doublets, but antiphase triplets and quartets, this particular experiment with decoupling produces a subspectrum that contains only methinyl resonances. Values of A2 also can be selected to optimize the intensities of methylene and methyl resonances. The idea can be depicted graphically by defining an imaginary angle B = 

The INEPT and DEPT sequences assume that coupling between the I and S nuclei is dominant, so that other couplings must be negligibly small. For one-bond couplings, this assumption holds, as all couplings are much smaller. If polarization is to 

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The sensitivity enhancement achieved by VPD is determined by the ratio of the substrate area to the area of the detector aperture (analyzed area), provided there is fiill collection of the impurities. This has been demonstrated for Fe and Zn. For Cu and Au, however, only a small percentage can be collected using this technique, due to electrochemical plating. An example comparing direct TXRF with VPD-TXRF on the same substrate is shown in F%ure 4. 

Figure 4 Direct TXRF (upper spectrum, recording time 3000 s) and VPD-TXRF (lower spectrum, recording time 300 s) on a silicon wafer surface. The sensitivity enhancement for Zn and Fe is two orders of magnitude. The measurements were made with a nonmonochromatized instrument.

In analytical LC there are two primary reasons why chemical derivatization of the sample constituents would be necessary, and they are 1) to enhance the separation and 2) to increase the sensitivity of detection. Under certain circumstances, derivatization can also be used to reduce peak asymmetry, i.e. to reduce tailing, or to improve the stability of labile components so that they do not re-arrange or decompose during the chromatographic process. However, sensitivity enhancement is the most common goal of derivatization. For example, aliphatic alcohols that contain no UV chromaphore can be reacted with benzoyl chloride to form a benzoic ester. 

To our surprise and satisfaction, the general approach worked the CBI derivatives did chemiluminescence, and the sensitivity enhancement was 30- to 50-fold over fluorescence With this success, we embarked on a more thorough study of chemiluminescence with the goal of optimizing the method. Identifiable parameters that affected the efficiency of light emission from a chemically generated fluorescent molecule included ... 

Semen sinapis 187 Sennosides 166,167 Sensitivity enhancement by derivatization 56... 

Following are some common methods for sensitivity enhancement ... 

The sensitivity enhancement in the INEPT experiment is particularly marked when nuclei of low magnetogyric ratios are being detected. A comparison of the signal intensities obtained by polarization transfer against those obtained by full NOE for various nuclei is presented in Table 2.1. 

Nonselective polarization transfer, as implied by the term, represents a process that allows simultaneous polarization transfer from all protons to a//X nuclei. In sefectmepolarization transfer, however, the population of just one nucleus is inverted at any one time. The selective polarization transfer sequence therefore cannot be used to generate a proton-decoupled C-spectrum containing all sensitivity-enhanced C resonances. 

The APT pulse sequence provides limited information about the number of hydrogens bonded to the carbons in a molecule, since it does not readily allow us to distinguish between the CH, and CH carbons or between CH and quaternary carbons. The INEPT spectrum not only can yield information about the multiplicity of all the carbons, but also affords sensitivity-enhanced C signals due to polarization transfer. 

INEPT (insensitive nuclei enhanced by polarization transfer) Polarization transfer pulse sequence used to record the NMR spectra of insensitive nuclei, e.g., C, with sensitivity enhancement may be used for spectral editing. 

Matched filter The multiplication of the free induction decay with a sensitivity enhancement function that matches exactly the decay of the raw signal. This results in enhancement of resolution, but broadens the Lorentzian line by a factor of 2 and a Gaussian line by a factor of 2.5. 

The main implications of the new remote detection methodology are the sensitivity enhancement, which can be several orders of magnitude in certain applications, the option to study flow transiently with time-of-flight techniques and the broadening of the application range of NMR and MRI to samples that were previously... 

Cross-polarisation (CP) in the rotating frame has been introduced as a means of transferring polarisation between different nuclear species in solids [168], and has become of central importance for obtaining spectra of rare spins with low gyromagnetic ratios such as 13C, since a significant sensitivity enhancement may be achieved. Cross-polarisation can be used either for direct observation of low-sensitivity nuclei or for indirect detection of such nuclear species via high-sensitivity nuclei such as protons [169]. 

In keeping with earlier observations (19,98), the nonoxidative thermal dehydrochlorination of PVC has been shown recently to be facilitated by preliminary photodegradation of the polymer (10,99). The thermal sensitivity enhancement increases with decreasing wavelength of irradiation (10) and undoubtedly results from the photolytic formation of thermally labile defect sites (10). 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

This sensitivity enhancement arises through two mechanisms ... 

Figure 18A shows the overlaid multiplicity-edited GHSQC and 60 Hz 1,1-ADEQUATE spectra of posaconazole (47). As will be noted from an inspection of the overlaid spectra, there is an overlap of the C46 and C47 resonances of the aliphatic side chain attached to the triazolone ring that can be seen more clearly in the expansion shown in Figure 18B. In contrast, when the data are subjected to GIC processing with power = 0.5, the overlap between the C46 and C47 resonances is clearly resolved (Figure 18C). In addition, the weak correlation between the C3 and C4 resonances of the tetrahydrofuryl moiety in the structure is also observed despite the fact that this correlation was not visible in the overlaid spectrum shown in A. This feature of the spectrum can be attributed to the sensitivity enhancement inherent to the covariance processing method.50...

For those purposes, the authors used constant-time version of the sensitivity-enhanced HMBC sequence,79 combined with a two-step low-pass J filter. Constant-time experiments have no coupling structures in the carbon dimension making it easy to identify the centre of signals in... 

Figure 1. Pulse sequence for 27Al-29Si HETCOR experiment with sensitivity enhancement by RAPT and CPMG data acquisition.

This application of sensitivity enhanced RAPT-CP-CPMG HETCOR, to microporous aluminosilicates illustrates the potential of... 

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