gel layer, then one would expect a slowing down of the processes which determine the transient signal (16,19). This can partly be attributed to the increase of the diffusional layer thickness and partly to the decrease in value of the diffusion coefficient compared to that in the solution phase.
Americium / Curium / Mass spectrometry / Thermal ionization / Isotopie dilution / Resin bead
SummaryPicogram quantities of curium and americium have been measured in solutions of natural elements and fission products by thermal ionization mass spectrometry using the resin bead method combined with isotopie dilution.The fixation yields of americium on a resin bead vary from 6%, when the element is extracted from a few ml solution, to 2% for an aliquot 15 times larger. For curium the yields vary from 6%, when it is extracted from a few ml of solution, to 1 % for an aliquot 5 times larger.The ionization yields with CAMECA ΤΗΝ 206 spectrometers are 1 ion for 3000 to 5000 atoms for the resin bead method and 1 ion for 25000 to 30000 atoms in the case of direct loading on a triple filament.The reliability is determined by comparison of the mass spectrometric results with those obtained by alpha spectrometry. The relative standard deviation on Cm and Am ratios measurements is ±0.2%.
A new couloamperometric apparatus has been designed to extend the range of this kinetic technique to the measurement of very high rate constants, l@M-'s-', by using TFCR-EXSEL conditions (TFCR-very low reactant concentration; EXSEL-salt excess), which give half-lives of a few seconds for very fast second-order reactions. Very low faradaic currents, in the nanoampere range for halogens, corresponding to very low reactant concentrations of 10-s-10-9M, are measured selectively by compensating the eddy currents, principally the residual and the induced currents. When the electroactive species is bromine, the concentration is demonstrated to be linearly related to the limiting reduction current in the very low concentration range. The upper limit of this technique for bromination is at present 3The method is applied to the kinetic study of highly reactive enol ethers EtO-C(R)=CH-R', where Rand R' are H or Me. A value of 2.2 X 108M-ls-l is obtained for k~~~, the rate constant for free bromine addition to EtO-CH=CH2, by extrapolating the kinetic bromide ion effects to 1Br-l = 0. An a-methyl effect ( k a -~e / k~)~t~ of 15 is found; this is a small decrease in the methyl effect compared to the marked increase in the double bond reactivity. For the enol acetate MeCOO-CH=CHz, whose rate constant is 6 X 102M-'s-', ( k a .~e / k~)~~~e is 21. The dependence of substituent effects on reactivity is discussed in terms of the Hammond effect on the transition state position and of charge delocalization by group G of olefins G-CH=CH2.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.