Sensitivity enhancement methods

Development of Mg solid-state NMR Sensitivity Enhancement Methods... 

The choice of sensitivity enhancement method depends on a variety of factors. The NOE is the most routinely applied since it does not require specialized equipment or specific knowledge of energy levels or coupling constants. However, when y, < 0 (as for N or Si) the NOE gives inverted signals, as in Figure 4 of Chapter 5, or loss of... 

Nitrogen is closely concerned with many or most vital processes. With 95%-99% bioenrichment, which can be achieved with relatively cheap materials such as NH salts, N is several times more NMR sensitive than naturally-abundant C, and can give useful labeling information. Sensitivity enhancement methods such as DEPT or INEPT are of great value, with or without N-enrichment. ° ° Also useful, particularly for in vivo work, is indirect detection of N by double quantum proton NMR. Applications of solid state techniques to biomolecules in N or N resonance were described in Section 1.3. 

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 ... 

Following are some common methods for sensitivity enhancement ... 

Wang, F., The sensitive fluorimetric method for the determination of curcumin using the enhancement of mixed micelle, J. Fluoresc., 16, 53, 2006. 

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...

When the influence was studied of different surfactants on the CL intensity of the reaction of lucigenin with isoprenaline, it was found that while cationic surfactants such as HTAH and HTAB and anionic surfactants such as SDS decrease the CL signal, the presence of Brij-35 increases the signal by a factor of 2.1 compared to that obtained in an aqueous medium [61]. As a result, a quite sensitive analytical method has been established for determination of isoprenaline, using Brij-35 as a CL enhancer. Application of the method has been satisfactorily verified with the determination of isoprenaline in pharmaceutical preparations. 

There are a number of other tests which may be added to a behavioral screen to enhance its sensitivity. These methods tend to require more equipment and effort, so if several compounds are to be screened, their inclusion in a screen must be carefully considered. An example of such methods is the narrowing bridge technique. All these methods require some degree of training of animals prior to their actual use in test systems. 

Detection If a small capillary diameter is desired for efficiency purposes, the detection part of the capillary can be adapted for better detection sensitivity. Examples are the bubble cell capillary and the Z-cell. In the bubble cell capillary, the capillary diameter is enlarged at the detection window so that better concentration sensitivity is obtained. If you implement a bubble cell capillary in your pharmaceutical analysis method, it is important to test different batches. Test also whether you need a bubble cell capillary or whether you can gain similar sensitivity increase with a proper injection procedure. Also, check the effect of the bubble cell on band broadening. An approximately three-times sensitivity enhancement is possible. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

We first review the essentials of the phase distribution of the electric fields at the focus of a high numerical aperture lens in Section II. After discussing the phase properties of the emitted signal, in Section HI we zoom in on how the information carried by the emitted held can be detected with phase-sensitive detection methods. Interferometric CARS imaging is presented as a useful technique for background suppression and signal enhancement. In Section IV, the principles of spatial interferometry in coherent microscopy are laid out and applications are discussed. The influence of phase distortions in turbid samples on phase-sensitive nonlinear microscopy is considered in Section V. Finally, in Section VI, we conclude this chapter with a brief discussion on the utility of phase-sensitive approaches to coherent microscopy. 

Continuous Sampling and Determination. There are no truly continuous techniques for the direct determination of sulfuric acid or other strong acid species in atmospheric aerosols. The closest candidate method is a further modification of the sensitivity-enhanced, flame photometric detector, in which two detectors are used, one with a room-temperature de-nuder and one with a denuder tube heated to about 120 °C. Sulfuric acid is potentially determined as the difference between the two channels. In fact, a device based on this approach did not perform well in ambient air sampling (Tanner and Springston, unpublished data, 1990). Even with the SF6-doped H.2 fuel gas for enhanced sensitivity, the limit of detection is unsuitably high (5 xg/m3 or greater) because of the difficulty in calibrating the two separate FPD channels with aerosol sulfates. 

The most economical and efficient method of sensitivity enhancement in 13C NMR of organic molecules is the pulse Fourier transform technique (PFT) in combination with decoupling methods such as proton broad band decoupling and polarization transfer. These methods will be described in the following sections. 

Positive values of a t improve the resolution at the expense of sensitivity. Another method of resolution enhancement without significant reduction of signal noise is referred to as Gauss multiplication [21 b]. This involves multiplication of the FID signal with an exponential of second order, e- "- 2 where a < 0 and b > 0. The best value of a is the negative digital resolution (Section 2.5.6.2), while the optimum of b is related to the time after which the FID is practically zero. 

Due to its greatly enhanced sensitivity in comparison to CW NMR, the PFT method has made 13C NMR into a routine method of structure analysis for all molecules having the natural 13C abundance of 1.1%. Additionally, phase-corrected PFT NMR spectra contain all spectral details without the lineskewing and ringing observed in CW spectra. Finally, short-lived molecules can be measured by PFT NMR, and sensitivity enhancement by accumulation of interferograms before Fourier transformation requires much less time than the accumulation of CW NMR spectra, due to the short time required for acquisition of FID signals. 

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