Biomedical subjects

H W Spiess

Publications and source records attributed to H W Spiess.

28 records · Page 2Linked to original sources

PubMed1998

Resolution enhancement in multiple-quantum MAS NMR spectroscopy.

Two techniques for resolution and sensitivity enhancement are introduced in multiple-quantum (MQ) MAS spectroscopy of rigid solids. The first makes use of ultrafast MAS with spinning frequencies of up to 35 kHz, while the second combines MAS at moderately fast spinning frequencies of about 13 kHz with multiple-pulse (MP) dipolar decoupling. For the latter approach, a semiwindowless WHH-4 sequence is applied during the MQ evolution period (MQ dimension) and/or detection period (single-quantum dimension). In the MQ dimension, the MP sequence has to be supplemented by two bracketing pulses in order to preserve the order and the intensities of the evolving MQ coherences. Double-quantum 1H NMR spectra of l-alanine recorded using both decoupling techniques are shown and compared to each other. Triple-quantum 1H NMR spectra under ultrafast MAS conditions are also presented.

Alanine↗

PubMed1998

2D CP/MAS 13C isotropic chemical shift correlation established by 1H spin diffusion.

A new 2D solid-state CP/MAS 13C NMR exchange experiment for through-space isotropic chemical shift correlation is proposed and demonstrated. Through-space correlation is established via a second cross polarization from 13C to 1H and subsequent 1H spin diffusion. A third cross polarization results in the final 13C-13C isotropic chemical shift correlation. The 1H spin diffusion time is a variable parameter allowing different mean square magnetization displacements to be probed. Experimental results on mixtures of differently 13C-labeled alanine and polyethylene indicate that this site-selective 2D technique can be used to characterize domain sizes and proximities over a wide range of length scales (1-200 nm) in solids such as polymers or biological materials.

Alanine↗

PubMed1998

Selective residual dipolar couplings in cross-linked elastomers by 1H double-quantum NMR spectroscopy.

1H double-quantum (DQ) solid-state NMR spectroscopy under fast magic-angle spinning (MAS) is introduced as a new spectroscopic tool for the investigation of the structure and local chain dynamics of elastomers. Dipolar connectivities between the protons of the various functional groups can be directly established from the highly resolved DQ solid-state NMR spectra as is shown for a series of cross-linked poly(styrene-co-butadiene). More quantitatively, residual dipolar couplings within and between the functional groups are evaluated selectively from the build-up curves of the double-quantum signals in the limit of the spin-pair approximation. In particular, the CH-CH and the CH2-CH couplings of butadiene, which both act predominantly along the chain-segment direction, have been measured relative to the CH2 coupling. The total build-up intensity is correlated with the cross-link density.

Cross-Linking Reagents↗

PubMed1997

Multiple-pulse assisted line-narrowing by fast magic-angle spinning.

Combined rotation and multiple-pulse experiments (CRAMPS) are reported that are performed under the conditions of fast magic-angle spinning. Quasi-static conditions, as are required for CRAMPS experiments, can be fulfilled approximately also for fast sample spinning conditions when windowless or semiwindowless sequences are applied. In order to allow direct detection for these cases also, appropriately timed detection windows are introduced without loss of resolution. In contrast to conventional CRAMPS experiments, high-speed MAS was found to play an important role also in the averaging of residual dipolar contributions. The resolution achieved in these first experiments is comparable to that of conventional CRAMPS experiments and the demands with respect to spectrometer hardware and tuning are much lower.

Magnetic Resonance Spectroscopy↗

PubMed1996

Spectral parameters for quantitative mobility contrast in NMR imaging of solid polymers.

Different procedures based on parameters of the wideline NMR absorption spectrum are presented to obtain localized molecular mobility contrast for imaging of solid polymers. For this purpose a 1H-NMR imaging technique with magic sandwich echoes is used for acquiring localized wideline spectra. With samples composed of polystyrene and high impact strength polystyrene, and polycarbonate and low density polyethylene a spatial difference in NMR absorption spectrum lineshape and linewidth is displayed. Furthermore, the spatial distribution of rigid and mobile domains in a heterogeneous polymer can be derived from the NMR spectral components. It is demonstrated that a van Vleck moment analysis can be performed from spatially resolved magic echo decays. The second (M2) and fourth (M4) moments of the rigid components show considerable variation with the spatial composition of the investigated samples.

Chemical Phenomena↗

PubMed1996

Spatially resolved solid-state MAS-NMR-spectroscopy.

A comprehensive account of spatially resolved solid-state MAS NMR of 13C is given. A device generating field gradients rotating synchronously with the magic angle spinner is described. Spatial resolution and sensitivity are compared for phase and frequency encoding of spatial information. The suppression of spinning sidebands is demonstrated for both cases. Prior knowledge about the involved materials can be used for the reduction of data from spatially resolved spectra to map chemical structure. Indirect detection via 13C NMR gives access to the information about mobility from proton-wideline spectra. Two-dimensional solid-state spectroscopy with spatial resolution is demonstrated for a rotor synchronized MAS experiment which resolves molecular order as a function of space. By comparison of different experiments the factors affecting the spatial resolution are investigated.

Carbon Isotopes↗

PubMed1994

Spatially resolved NMR of rigid polymers and elastomers.

NMR imaging with protons is becoming a more and more versatile tool for material research. Two different approaches are presented to obtain spatial resolution in elastomers and solids. The first, magic-sandwich-echo-imaging, belongs to the group of multiple-pulse techniques which are applied to overcome the strong dipolar couplings in solids. A driver for fast gradient pulses was constructed, and the technique was used to measure one-dimensional projections and spatially resolved spectra of rigid polymers. The second approach uses surface coils. In this way large objects can be investigated from the surface at optimum signal to noise. The drawback of the inhomogeneous B1 field resulting in signal attenuation across the image can be overcome by imaging an NMR parameter like the relaxation time T2. This was applied to image a phantom of different polyurethane foams. Furthermore spin-echo images of signal intensity maps around conducting wires are presented for the analysis of current distributions.

Magnetic Resonance Imaging↗

PubMed1994

Application of nuclear magnetic resonance magic sandwich echo imaging to solid polymers.

A solid-state 1H nuclear magnetic resonance (NMR) imaging technique based on magic sandwich echoes (MSE) is described for obtaining spatial projections of solid polymer samples. The modification of MSE multiple echo detection is combined with short gradient pulses applied during the sandwich windows. This allows proton homonuclear decoupling in solids with dipolar couplings up to 50 kHz. For a rapid gating of the gradients a fast gradient pulse driver was constructed with switching times between 550 and 910 ns giving a maximum gradient strength of 330 mT/m. With our detection technique the spectral width can be doubled. One-dimensional projections and spatially resolved spectra of rigid polymer phantoms are presented. For contrast enhancement the use of the T1 relaxation time and the number of the magic sandwiches functioning as an adjustable magnetization filter discriminating between different strengths of the dipolar coupling is demonstrated.

Echo-Planar Imaging↗

PubMed1994

A new approach to determining homopolymer domain sizes in polycarbonate-polyether dialysis membranes by solid-state NMR.

The size of micro-separated domains of polyether (PEO), -[CH2CH2O]n-, and polycarbonate (PC), -[(C6H4)-C(CH3)2-(C6H4)-OCO2]m-, in the dialysis membrane 'Gambrane' have been determined using an advanced solid-state NMR technique which exploits differences in 1H spin diffusion. The characteristic diameter of a PEO domain is 4.8 +/- 1.4 nm and that of PC is 5.2 +/- 1.4 nm with a mixed phase region of 0.8 +/- 0.5 nm.

Dialysis↗

PubMed1993

Spinning sidebands from chemical shift anisotropy in 13C MAS imaging.

Solid state imaging by 13C MAS imaging is described. The spinning sidebands occurring at moderate spinning speeds, which disturb the images, can be suppressed by TOSS. For rigid solids the spatial resolution that can be achieved in this way is better than that of 1H images at the same spinning speed. Spatially resolved spectra with or without spinning sidebands can likewise be recorded providing information about the isotropic and the anisotropic chemical shifts which can be exploited for the study of structure, order and dynamics. The techniques are demonstrated on a phantom made with 13C-labelled glycine.

Anisotropy↗