Filtered experiments

Predictions on the effectiveness of a fluid loss additive formulation can be made on a laboratory scale by characterizing the properties of the filter-cake formed by appropriate experiments. Most of the fluids containing fluid loss additives are thixotropic. Therefore the apparent viscosity will change when a shear stress in a vertical direction is applied, as is very normal in a circulating drilling fluid. For this reason, the results from static filtering experiments are expected to be different in comparison with dynamic experiments. 

Figure 26 13C CPMAS (A) and DDMAS (B) NMR spectra of [l- 3C]Val-labelled bR from PM. From Ref. 19. The assigned peaks is based on the data of site-directed mutagenesis 85- 87 and REDOR filter experiment.188...

This diffusion-filtered approach has been extended in application to macroscopic supports, specifically a Synphase lantern.77 The advantage of the diffusion-filtered experiment was extended to suppression of the macroscopic support signals, not just those of the protonated solvents. Although, the lanterns required some modification to fit in the MAS rotors, it is proposed that manufacturers account for this size restriction in future formats of these supports. 

Traditional protein NMR spectroscopy of smaller proteins relies of 15N-filtered experiments, due to the relatively low expense of introducing 15N labels into proteins (compared to 13C) and the concomitant ability to use heteronuclear filtering to improve resolution in the H NMR dimension. Jelinek et al. were the first to demonstrate the ability to transfer this approach to peptides on TantaGel.80 They also showed the ability to detect pronounced peptide structure through the appearance of strong NOE correlations in 3H NOESY HRMAS spectra as shown in Fig. 8. This had important implications for the search of biological activity in peptides attached to supports, as the structure on the support may be different or more pronounced than in solution, if present at all in solution in peptides of this small size. 

Figure 21. Emission in the 1800-2800 cm-1 region following the IRMPD of CH2F2 (25 mTorr) in the presence of NO (lOOmTorr) and Ar (lOTorr). Emission from N20 (001) and C02 (001) are seen at long times following the vibrational cascade of highly excited species their presence at early times [together with CO(v = 1)] was established by cold gas filter experiments. N20 is the dominant emitter, and is believed to originate from the reaction sequence (20), (17).

Fig. 37. Power spectrum of the excitation, filter function, and rate distribution for the filtered experiment in Figs. 35 and 36...

Dalvit C, Cottens S, Ramage P, Hommel U, Half-filter experiments for assignment, structure determination and hydration analysis of unlabelled ligands bound to 13C/15N labelled proteins, J. Biomol. NMR, 13 43-50, 1999. 

Heidenreich S and Scheibner B. Hot gas filtration with ceramic filters Experiences and new developments. Filtr. Sep. 2002 May 22-25. Heidenreich S and Wolters C. Hot gas filter contributes to IGCC power plant s reliable operation. Filtr. Sep. 2004 June 22-25. Larbot A, Bertrand M, Marre S, and Prouzet E. Performances of ceramic filters for air purification. Sep. Purif. Technol. 2003 32 81-85. DeFriend KA and Barron AR. A simple approach to hierarchical ceramic ultrafiltration membranes. J. Membr. Sci. 2003 212 29-38. Endo Y, Chen D-R, and Pui DYH. Collection efficiency of sintered ceramic filters made of submicron spheres. Filtr. Sep. 2002 March 43-47. Sakol D and Konieczny K. Application of coagulation and conventional filtration in raw water pre-treatment before microfiltration membranes. Desalination 2004 162 61-73. 

This is confirmed by comparing ID double-quantum filtered experiments with increasing recouphng times. Extension of this to 2D SQ/DQ correlation identifies the nature of the dipolar coupled pair, since the couphng partner of the directly detected spin can be identified in the indirect dimension, where the chemical shift is the sum of the shift of the two protons involved. This spectrum (Fig. 4b) exhibits intense and well-resolved cross peaks between CD and PDMS. The dipolar couphng constant between the guest and host must therefore be of the order of, or exceed, a few himdred Hz, proving that whatever fast motion the polymer chain performs, it must be a local one. 

To overcome these difficulties, the z-filter experiment was adapted to MQMAS by Amoureux et al. [24]. In this three-pulse scheme the two hard pulses (excitation of the MQ coherences and conversion into OQ coherence) are followed by a short delay during which the magnetisation is stored along the z-axis as zero-quantum coherences and then transferred into observable IQ coherences using a selective n/2 pulse (Fig. 5a). The symmetrisation of the echo and antiecho pathways during the two hard pulses (p=0—> 3—>0) forces an equal intensity of the echo and antiecho signals, leading to amplitude-modulated FIDs and, thus, to pure absorption spectra. This is a robust method, easy to optimise. 

In the following, we discuss the -filter experiment and the shifted-echo spl it-r, MQMAS schemes. Although the latter scheme of whole-echo acquisition has been shown to be the most efficient in terms of signal-to-noise and is especially suited for sensitivity enhancement schemes, which are discussed extensively in Section 5, for samples with short T2 the z-filter scheme may prove to be more beneficial. 

Isotope filtered experiments 2D WURST-based C-filtered NOESY... 

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