Comprehensive determination of trace elements in human saliva by ETA-AAS

A method is described for the comprehensive determination of a series of trace and ultratrace elements (Fe, Mn, Zn, Cu, Sr, Cr, Al and Si) in human saliva by atomic absorption spectrometry with electrothermal atomization and Zeeman-effect background correction. Procedures for sampling of total mixed saliva and its treatment including centrifugation and mineralization by HNO₃ are described in detail. To avoid the use of matrix modifiers, element standards with an artificial saliva matrix were used to construct calibration curves. Instrumental precision, tested by repeated analyses of natural salivas, is good (error less than 5%) for most elements. Lower precision was obtained for elements like Al and Si present in concentrations close to the detection limit. The analysis of some natural salivas clearly shows the influence of the sampling and treatment procedures on the reliability of the data. An accurate standardization of these procedures is thus recommended.     

Equilibrium and congruence on a rotation surface

Summary Equilibrium and congruence are analyzed for surfaces applicable on a rotation surface: congruence conditions are shown to be in an essential number of two, linked by a differential identity, and the displacement field induced by a congruent strain is obtained. Then the solution of the homogeneous equilibrium equation is determined, when the equilibrium problem is the adjoint problem of the congruence.

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Sunto Si analizzano i problemi dell'equilibrio e della congruenza per superfici applicabili su superfici di rotazione: si mostra che le condizioni di congruenza essenziali sono due, legate da una identità differenziale, e si determina lo spostamento indotto da una deformazione congruente. A partire dalle condizioni di congruenza si determina poi la soluzione dell'equazione di equilibrio omogenea, nella ipotesi che il problema dell'equilibrio sia il problema aggiunto della congruenza.

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This work was done in the sphere of activity of the C.N.R. — Group for mathematical research.

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Istituto di Matematica del Politecnico — Via Bonardi, 9 — 20133 Milano.

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Entrata in Redazione il 3 Marzo 1973.     

Ab initio study of β-lactam antibiotics. I. Potential energy surface for the amidic CN bond breaking in the β-lactam + OH− reaction

The potential energy surface of the β-lactam + OH^? reaction, related to the mode of action of β-lactam antibiotics, was investigated using the ab initio Hartree—Fock method with the STO-3G basis set. Three possible reaction paths for the B_AC2 breaking of the amidic CN bond were obtained and discussed. The minimum-energy reaction path is characterized by the following processes: (1) the formation of a tetrahedral intermediate, ≈ 121 kcal mol^?1 more stable than the reagents; (2) a barrier, ≈ 15 kcal mol^?1 above the intermediate, which is mainly due to the partial breaking of the amidic bond; (3) the complete breaking of the amidic bond concerted with a proton transfer till the formation of the final product, ≈ 34 kcal mol^?1 more stable than the intermediate. The evolution of some molecular orbitals and of the electron population along the reaction path was also discussed.