PubMed · 15416745
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S MIYAKE, G HONJO. 1950-04-22. Anomalous refraction effect in electron diffraction.. https://doi.org/10.1038/165644a0
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Despite the crucial role of "iron(III) citrate systems" in the iron metabolism of living organisms (bacteria as well as plants or mammals), the coordination chemistry of ferric citrate remains poorly defined. Variations in the experimental conditions used for the preparation of so-called ferric citrates (iron salt, Fe:cit molar ratio, base, pH, temperature, solvent) lead to several different species, which are in equilibrium in solution. To date, six different anionic complexes have been structurally characterized in the solid state, by ourselves or others. In the work described herein, we have established the experimental conditions leading to each of them. Five were obtained from aqueous solution. With the exception of a nonanuclear species (of which fragments have been detected), all were identified in aqueous solution on the basis of electrospray ionization mass spectrometry. In addition, the spectra revealed a new trinuclear species, which could not be crystallized. Kinetic studies of iron uptake from citrate species by iron chelators confirmed the results indicated by the ESI-MS studies. These studies also allowed the relative molar fraction of mononuclear versus polynuclear complexes to be determined, which depends on the Fe:cit molar ratio.
Some physical properties of nanogel particles formed by chitosan ionically cross-linked by tripolyphosphate (TPP) have been studied. Electrokinetic properties and colloidal stability were analyzed as a function of pH and ionic strength of the medium. Chitosan particles showed volume phase transitions (swelling/shrinking processes) when the physicochemical conditions of the medium were changed. Experimental data were mainly obtained by electrophoretic mobility measurements and by photon correlation spectroscopy and static light scattering techniques. Chitosan chains possess glucosamine groups that can be deprotonated if the pH increases. Therefore, modification of pH from acid to basic values caused a deswelling process based on a reduction of the intramolecular electric repulsions inside the particle mesh. Electrophoretic mobility data helped to corroborate the above electrical mechanism as responsible for the size changes. Additionally, at those pH values around the isoelectric point of the chitosan-TPP particles, the system became colloidally unstable. Ionic strength variations also induced important structural changes. In this case, the presence of KCl at low and moderate concentrations provoked swelling, which rapidly turned on particle disintegration due to the weakness of chitosan-TPP ionic interactions. These last results were in good agreement with the predictions of gel swelling theory by salt in partially ionized networks.
Linearity and detection thresholds of atmospheric pressure ionization-mass spectrometry (API-MS) were determined for 11 aroma compounds in air at concentrations ranging from 50 ppb to approximately 450 ppm (moles of volatile per mole of air). In most cases, the protonated molecular ion (i.e., m/z = M + 1) was the base peak throughout the range; however, some compounds showed an increase in fragmentation at lower concentrations. Detection limits varied greatly (from 50 ppb to 14 ppm) depending upon the aroma compound being measured. The linear range was also strongly dependent upon the aroma compound, with values ranging from <10-fold change in concentration to >4000-fold change in concentration depending upon the volatile being studied. The two volatiles with poor detection thresholds also exhibited the smallest linear range. Most compounds had linear ranges of >200. There was no apparent relationship between gas-phase basicity and either detection limit or linear range.