Sensitivity enhancement methods spectrum

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...

The projection-reconstruction approach is a technique unrelated to covariance processing which can provide data typically inaccessible to the natural product chemist. For example, 13C-15N correlation spectra were obtained for vitamin B12 at natural abundance.104 Compared with a conventional three-dimensional 13C-15N correlation experiment, the projection-reconstruction method provides a sensitivity enhancement of two orders of magnitude. The final 13C-15N spectrum was reconstructed from data obtained from ll l5N and H- C correlations acquired using a time-shared evolution pulse sequence that allowed all the information to be obtained in one experiment.104 Martin and co-workers also demonstrated the ability to generate 13C-15N correlation spectra using unsymmetrical indirect covariance NMR with vinblastine as an example.105 In the latter case, 13C-15N correlation spectra were obtained from - C HSQC data and H-1sN HMBC data that were acquired separately. Both methods provide access to correlations that would be inaccessible for most natural products at natural abundance. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

A somewhat similar to the above, but this time with the quadrature detection with sensitivity enhancement, was used by Ding and Gronenborn to measure different coupling involving back-bone nuclei. In one version of the method the J-evolution was introduced into 2D [ H, N]-HSQC-like spectrum by synchronising it with the chemical shift using the accordion principle. The... 

Improvement with respect to these SRM methods was rendered possible by the availability of data-dependent acquisition or information-dependent acquisition (IDA), by which a tandem mass spectrometer can automatically switch from a survey mode to a dependent (or confirmation), full-spectrum MS/MS mode. In addition, the introduction of linear ion-trap-triple quadrupole (LIT-QqQ) hybrid instruments further extended the possibilities of LC-MS/MS in STA or GUS. In this instrument, the second mass analyzer can be used as either a conventional quadrupole mass analyzer or a linear ion trap, which by accumulation of ions provides enhanced full-spectrum sensitivity compared to a conventional quadrupole. The group of Weinmann used targeted SRM with up to 700 transitions as the survey detection mode, and the enhanced product ion (EPI) spectrum mode as the dependent mode (11). Whereas this procedure seems to be a more specific approach to STA as it allows searching rich spectra against those entered in libraries, the use of SRM as the survey mode cannot answer the more general clinical question as to whether an individual has been intoxicated at all, rather than intoxicated with a compound from a predefined list (12). Also, the use of only the positive-ion mode narrows the detection window. 

The interfacial rheologic properties are extremely sensitive parameters toward the chemical composition of immiscible formation liquids [1053]. Therefore comparison and interpretation of the interfacial rheologic properties may contribute significantly to extension of the spectrum of the reservoir characterization, better understanding of the displacement mechanism, development of more profitable enhanced and improved oil-recovery methods, intensification of the surface technologies, optimization of the pipe line transportation, and improvement of the refinery operations [1056]. 

The simplest and most popular experimental method is the well known one-dimensional (ID) NOE difference procedure [3], which is very easily implemented in any spectrometer and which can be routinely set up even by novice spectrometer operators. However, this difference method is based on subtraction of the unperturbed spectrum from the NOE-containing one, both separately recorded, and therefore the required difference information contributes only a small part of the recorded signal. Furthermore, the difference spectrum is very sensitive to subtraction errors, as well as pulse imperfections or missettings, or other spectrometer instabilities, all of which often result in prominent phase distortions or other subtraction artifacts which prevent the accurate measurement of the desired NOE values. Therefore the reliable measurement (or even detection) of enhancements below 1 % is not generally available using this difference method. 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

We demonstrate the Mil method, which couples the sensitivity of multiphoton excitation on the spectral phase of the laser pulses to probe microscopic chemical environment-induced changes in the multiphoton excitation spectrum of sensitive reporter molecules. We carry out the optimization of the required phase functions in solution and provide theoretical simulations. We show experimental images whereby pH-selective two-photon microscopy is achieved and demonstrate how selective excitation can be used to enhance contrast and, consequently, to achieve functional imaging, using fluorescent probes sensitive to changes in their local environment. 

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