The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis

Carbon isotope fractionation in plants

he efficiency of photosynthesis continues to interest biochemists, biologists, and plant physiologists. Scientists interested in CO2 uptake are concerned about the extent to which the uptake rate is limited by such factors as stomatal diffusion and the chemistry of the CO2 absorption process. The fractionation of carbon isotopes that occurs during photosynthesis is one of the most useful techniques for investigating the efficiency of CO2 uptake. Atmospheric carbon dioxide contains approximately 1.1% of the nonradioactive isotope carbon-13 and 98.9% of carbon-12. During photosynthesis, plants discriminate against C because of small differences in chemical and physical properties imparted by the difference in mass. This discrimination can be used to assign plants to various photosynthetic groups. The isotope fractionation also reflects limitations on photosynthetic efficiency imposed by the various diffusional and chemical components of CO2 uptake. When analyzed in detail, this fractionation provides information .about water use efficiency and indicates that different strategies are needed for improving wateruse efficiency in different kinds of plants. Isotope fractionation in simple physical and chemical processes is well understood and is commonly Current studies include

Carbon Isotopes in PhotosynthesisFractionation techniques may reveal new aspects of carbon dynamics in plants

▪ Abstract The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestructive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At the center of nearly all plant ecological research which has made use of stable isotope methods are the notions of interactions and the resources that mediate or influence them. Our review, therefore, highlights recent advances in plant ecology that have embraced these notions, particularly at different spatial and temporal scales. Specifically, we review how isotope measurements associated with the critical plant resources carbon, water, and nitrogen have helped deepen our understanding of plant-resource acquisition, plant interactions with other organisms, and the role of plants in ecosystem studies. Where possible we also...

Stable Isotopes in Plant Ecology

The isotopic composition of reduced organic carbon

The previously discussed principles involved in electrocyclic reactions of openshell ions are exemplified for a type C system, the radical cation couple quadricyclane+ (Q+)/norbornadiene+ (NBD+). Exhaustive calculations (MINDO/3) for the various states of the ions support the general predictions for such types of systems. The barrier of interconversion Q+ NBD+ is estimated to be significantly lower than that for the corresponding neutrals. Experimental support to this prediction is obtained by γ-irradiation of Q or NBD in an electron scavenging matrix. In both cases only NBD+ is observed, indicating that Q+ is not stable at 77 K. Previous observations and propositions referring to the system Q+/NBD+ are discussed in view of the present findings.

Studies on Radical Cations, III. The Type C Valence-Isomeric System Quadricyclane Radical Cation/Norbornadiene Radical Cation†

Es wird experimentell festgestellt, dass bei der Absorption von CO2 aus einem Gasgemisch durch Losungen von Alkalien die Aufnahme von 12CO2 um den Faktor 1,014 schneller erfolgt als diejenige von 13CO2. Ein ahnlicher Fraktionierungsfaktor von 1,011 ergibt sich auch bei der Absorption reiner CO2. Der Effekt lasst sich am besten auf die um den Faktor 1,011 voneinander verschiedenen gaskinetischen Auftreffwahrscheinliehkeiten der 12CO2- und 13CO2-Molekeln bezuglich der absorbierenden Flussigkeitsoberflache zuruckfuhren.

Die Fraktionierung der Kohlenstoffisotopen bei der Absorption von Kohlendioxyd

Die Fraktionierung der naturlichen Kohlenstoffisotopen bei der Atmung keimender Bohnen sowie bei der Assimilation von CO2 durch wachsende Bohnen-Pflanzen wird quantitativ untersucht.

Die Fraktionierung der natürlichen Kohlenstoffisotopen im Kohlendioxydstoffwechsel grüner Pflanzen

Die Untersuchung des Beitrages der im Infraroten liegenden innermolekularen Schwingungen zu den von der Dispersionswechselwirkung herruhrenden Van der Waals'schen Anziehungskraften zeigt, dass diese Infrarotwechselwirkung stets eine grossere Fluchtigkeit der die schweren Isotopen enthaltenden Molekelspezies begunstigt. Der Effekt kommt dadurch zustande, dass der von den Infrarotabsorptionsbanden herruhrende Anteil der Anziehungskrafte fur die leichte Molekelspezies einen grosseren Betrag als fur die schwere Molekelspezies besitzt. Es ist als Folge dieses Effektes die Energie als Funktion des Abstandes zweier (oder mehrerer) Molekeln fur die leichte und schwere Molekelspezies nicht dieselbe.

Dampfdruckunterschiede isotoper Verbindungen. (Infrarot-Anteil der Dispersionswechselwirkung als Ursache für grössere Flüchtigkeit der schweren Molekelspezies)

The vapour-liquid separation factors of the isotopic systems HO/HO and DO/DO have been determined by means of Rayleigh-distillation of H2O and D2O respectively. The results obtained by mass-spectrometric evaluation of the isotopic composition of oxygen in the water samples show that the separation-parameter (δ = ln α) of the system DO/DO is some 10–15% lower than that of the system HO/HO. The temperature dependence of the separation-parameters of both systems is given, showing generally poor agreement with the results of other authors concerning the system HO/HO. This investigation leads to a general principle stating that in compounds containing two species of isotopic elements, the isotope effect of the first element upon the separation-factor of the isotopes of the second element has the same magnitude as the isotope effect of the second element upon the separation-factor of the isotopes of the first element.

Peter Baertschi

Papers

Studies on Radical Cations, III. The Type C Valence-Isomeric System Quadricyclane Radical Cation/Norbornadiene Radical Cation†

Die Fraktionierung der Kohlenstoffisotopen bei der Absorption von Kohlendioxyd

Die Fraktionierung der natürlichen Kohlenstoffisotopen im Kohlendioxydstoffwechsel grüner Pflanzen

Dampfdruckunterschiede isotoper Verbindungen. (Infrarot-Anteil der Dispersionswechselwirkung als Ursache für grössere Flüchtigkeit der schweren Molekelspezies)

Isotopie-Effekt für die Trennung der Sauerstoff-Isotopen 16O und 18O bei der Rektifikation von leichtem und schwerem Wasser