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981.
982.
983.
Diacetyl and 2,3-pentanedione react with isoniazide in acidified solutions to give the corresponding hydrazones which, by reacting with a zirconium (IV) salt, form fluorescent complexes. Both reactions have been studied separately. Variables such as pH, heating time, and excess of reactives are discussed. The method may be used for the determination of these compounds in foods. 相似文献
984.
E. España P. Beneitez V. Soler T. Calderón 《Journal of Radioanalytical and Nuclear Chemistry》1993,173(1):55-66
A technique for quantitative analysis of different soils, ceramics, feldspars and natural halide samples is presented, based on the measurement of -activities using a Geiger-Müller system. The system was calibrated with KCl, KC8H5O4, KNO3 and K2SO4 standards and a potassium content of 1% yields a net -count rate (background subtracted) of about 55 cph. Precision values of less than ±0.4% K can be achieved in counting times of about 4 hours. The results agree with those obtained by means of other more common analytical methods such as flame photometry, atomic absorption and -spectrometry. In comparison with these methods, this approach is direct, precise and non-destructive, because the samples do not require prior treatment. 相似文献
985.
J. Lesný J. Tölgyessy M. Harangozó G. Čík 《Journal of Radioanalytical and Nuclear Chemistry》1993,175(1):37-39
The subject of this paper was to determine some radionuclides in fly-ashes of Czechoslovak origin.226Ra,228Ac and40K were chosen because of their high information value. The radiometric measurements were carried out by means of a semiconductor detector. 相似文献
986.
Summary The aim of this work was to clarify the process of evaporation of impurity elements from silicon carbide powder of industrial purity under the action of thermochemical additives, as a step towards the development of a method for the direct determination of all impurities. Considering the influence of classical additives — (C2F4)n, AgCl, CuCl, PdCl2 on both evaporation and excitation processes, the idea was developed of a modification of the matrix itself to more easy volatile compounds, allowing the impurities to be released. A complex modifier CoF2+Ba(NO3)2 was used for that purpose. The evidence for its effective action is a complete evaporation of the elements Al, Fe, Cu, Ti, Va, Mo and B during the first 10–20 s of arcing. The mechanism of the action of a modifier was studied using a thermodynamic evaluation of thermochemical processes that take place in the electrode crater. The mechanism supposed is supported by scanning electron microscopy images of residues in the crater.This paper is prepared in connection with the common research project IV-31 of the protocol of scientific and technical cooperation between FRG and SFR 相似文献
987.
Zoltán Füredi 《Journal of Geometry》1993,46(1-2):55-65
Let s1 (n) denote the largest possible minimal distance amongn distinct points on the unit sphere
. In general, let sk(n) denote the supremum of thek-th minimal distance. In this paper we prove and disprove the following conjecture of A. Bezdek and K. Bezdek: s2(n) = s1([n/3]). This equality holds forn > n0 however s2(12) > s1(4).We set up a conjecture for sk(n), that one can always reduce the problem of thek-th minimum distance to the function s1. We prove this conjecture in the casek=3 as well, obtaining that s3(n) = s1([n/5]) for sufficiently largen.The optimal construction for the largest second distance is obtained from a point set of size [n/3] with the largest possible minimal distance by replacing each point by three vertices of an equilateral triangle of the same size . If 0, then s2 tends to s1([n/3]). In the case of the third minimal distance, we start with a point set of size [n/5] and replace each point by a regular pentagon. 相似文献
988.
Jorge A. León 《Applied Mathematics and Optimization》1993,27(3):313-327
Let (X,l,) be a measure space, letW be a cylindrical Hilbert-Wiener process, and let be an anticipating integrable process-valued function onX. We prove, under natural assumptions on, that there exists a measurable version Yx,x X, of the anticipating integral of(x) such that the integral x Yx(dx) is a version of the anticipating integral of X
(x)(dx). We apply this anticipating Fubini theorem to study solutions of a class of stochastic evolution equations in Hilbert space. 相似文献
989.
We consider overdetermined nonlinear systems of equationsF(x)=0, whereF:
n
m
,mn. For this type of systems we define weighted least square distance (WLSD) solutions, which represent an alternative to classical least squares solutions and to other solutions based on residual normas. We introduce a generalization of the classical method of Cimmino for linear systems and we prove local convergence results. We introduce a practical strategy for improving the global convergence properties of the method. Finally, numerical experiments are presented.Work supported by FAPESP (Grant 90/3724/6), FINEP, CNPq and FAEP-UNICAMP. 相似文献
990.
Nora Bretón 《General Relativity and Gravitation》1993,25(6):567-578
We derive Ernst potentials for vacuum Bianchi I through VII models. A scheme to find inhomogeneous generalizations of such models by using generating techniques which incorporate electromagnetic fields or gravitational wave perturbations to a seed Bianchi solution is presented.On leave of absence from: Departamento de Fisica, CINVESTAV del IPN, Mexico 相似文献