Biomedical subjects

J M Dereppe

Publications and source records attributed to J M Dereppe.

4 recordsLinked to original sources

PubMed2000

Water peak suppression: time-frequency vs time-scale approach.

Wavelets are the most popular time-scale analysis tool. A well-known application of wavelets in nuclear magnetic resonance spectroscopy is water peak extraction/suppression. However, spectroscopists are more familiar with frequency than scale. So, from a spectroscopist point of view, a time-scale analysis tool (i.e., wavelets) is not natural and a time-frequency approach would be much more satisfactory. We explain a time-frequency solution to this problem based on Gabor analysis. As the two formalisms are closely linked together we continuously emphasize their similarities and differences. In particular we show that, here, the Gabor method is as efficient as the wavelet approach, and we give some examples. Those remarks also apply to other NMR problems solved previously with the continuous wavelet transform, such as quantification or dynamical phase correction.

Algorithms↗

PubMed1996

Micro-imaging by magnetic resonance on flexible polyurethane foams.

Flexible polyurethane foams are mainly used for their particular mechanical properties such as hardness and resilience. The correlation of the foam properties with the material structure is believed to be important but it has always been difficult to identify the individual contributions of the cellular structure and of the polymer to the foam physical properties.

Image Processing, Computer-Assisted↗

PubMed1996

Drying of a white porous limestone monitored by NMR imaging.

Staining of porous limestone is a frequent problem observed when this kind of stone is used as floor pavement. From a chemical point of view, this staining originates from a reaction between alkalies (NaOH and KOH) released by the fresh mortar and absorbed by the stone and the organic elements always present in this kind of stone. Up to now, no other explantation of this phenomena has been given. CSTC and UCL showed that, in addition of the chemical potentiality characterised by the organics content of the stone, another factor had to be taken into account: the drying process of the material. A French limestone (Tercé Sculpture) sample has been saturated by capillarity, and its drying has been monitored by NMR imaging. NMR results were compared with the weight loss curve. The results have clearly shown the existence of two different stages in the drying process. The first stage corresponds to a constant drying rate period, and the second is characterised by a falling drying rate. NMR measurements led us to a better understanding of the physical meaning of these two stages. During the first one, evaporation occurs on the external faces of the sample. During the second one, evaporation occurs inside the sample. We concluded that the physical key factor in the problem of staining was the amount of water evaporated during the first stage of drying.

Calcium Carbonate↗

PubMed1984

Substituent effects on reactivity and spectral parameters of cephalosporins.

The chemical reactivity of a series of cephalosporins is examined as a function of the substituents at positions 3 and 7. In most cases, the nature of the C7 side chain has a minor influence on the beta-lactam reactivity. But in the case of amino-containing C7 substituents, when intramolecular nucleophilic attack may occur, the reactivity may be greatly increased. The spectroscopic and structural characteristics of the beta-lactam linkage do not correlate with the chemical reactivity of studied compounds. The hydrolysis rates are linked neither with the IR frequency or 13C NMR chemical shift of the carbonyl beta-lactam nor with the geometry of the beta-lactam ring. However, a relationship is confirmed between the beta-lactam ring opening rate and the polarity of the C3-C4 double bond, reflected in the different 13C NMR chemical shifts of those atoms. The results are an experimental verification of the theoretical calculations of Boyd et al. on cephalosporin model compounds, which foresee that a C3 substituent could favor the opening of the beta-lactam cycle by stabilizing a transition state involved in alkaline hydrolysis.

Cephalosporins↗