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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Fluctuations of atmospheric radiocarbon

Grey, Donald Carson, 1927- January 1972 (has links)
No description available.
2

The excited states of carbon twelve

Wayland, James Robert, 1937- January 1962 (has links)
No description available.
3

The effect of vinblastine on the incorporation of C14-formate into the acid-soluble fraction of thymus cell suspensions.

Jones, Richard Gareth Wyn January 1963 (has links)
In earlier studies by Beer¹ the alkadloid vinblastine, VLB, has been shown to cause an almost complete inhibition in the incorporation of C¹⁴-formate into the deoxyribonucleic acid (DNA) of rat bone marrow in vivo. Also the incorporation of the same isotope into the DNA of suspensions of rat bone marrow and thymus cells in vitro was substantially depressed within one hour by the presence of the alkaloid² . In this work, the effect of VLB on the incorporation of C¹⁴-formate into the purine and pynmidine bases isolated from the hydrolysed acid-soluble fraction of thymus cell preparations was studied. A method for the separation of the constituents of this fraction was standardized and some of the major ultraviolet absorbing and radio-active compounds were identified. In general, VLB did not cause a marked depression in the incorporation of C¹⁴ -formate into the bases isolated from the hydrolysed acid-soluble fraction of either washed or unwashed cell suspensions. It was therefore considered probable that vinblastine did not inhibit the activation of 1-carbon units by tetrahydrofolic acid or their transfer. Also the results indicated that the de novo synthesis of the purine and pyrimidine compounds was not inhibited by the alkaloid. It is tentatively suggested that VLB inhibits a step in the incorporation of the mononucleotides into the nucleic acids. Comparison of the compounds isolated from the hydrolysed acid-soluble fractions of washed and unwashed cell preparations showed that considerably less hypoxanthine and xanthine were recovered from the former. VLB was found to markedly increase the total radio-activity of serine in washed cell preparations in the third hour of incubation but this effect was not pronounced in unwashed cell preparations. The total count of adenine decreased in the third hour of washed cell incubations in the presence of VLB. Neither the effect on serine nor on adenine is considered to be directly related to the inhibition of DNA synthesis as the latter is apparent within only 1 hour's incubation. Washing appears to increase the sensitivity of certain biochemical changes in the cell. It is tentatively suggested that the appearances of changes in the total radio-activity of adenine may be related to the loss of a substrate on washing. 1 Beer, C.T., Canadian Cancer Conference 4, 355 (1961) Academic Press, Inc., New York. 2 Richards, J.F., and Beer, C.T., personal communication. / Medicine, Faculty of / Biochemistry and Molecular Biology, Department of / Graduate
4

Determination of the value of the C¹²/C¹³ ratio in the solar atmosphere

Ganiaris, Nicholas, 1941- January 1966 (has links)
No description available.
5

Environmental control of stable carbon isotope systematics in Emiliania huxleyi

Eek, Magnus 14 February 2018 (has links)
The carbon isotope fractionation in the coccolithophore Emiliania huxleyi constitutes the basis for the paleo-pCO₂ barometry. Under the premise that the carbon isotope fractionation is dependent on the availability of dissolved CO₂, measurements of the carbon isotope ratio of sedimentary alkenones can potentially produce a proxy record of ancient atmospheric CO₂ levels. However, recent studies, including this thesis have suggested that other factors than CO₂ may influence the carbon isotope fractionation in Emiliania huxleyi and hence the validity of the proxy. In this thesis work the effects of irradiance on carbon isotope fractionation were studied in batch cultures of non-calcifying Emiliania huxleyi. It was found that the biomass becomes more ¹³C depleted as the light intensity decreases. This is in agreement with utilization of CO₂ via passive diffusion where fractionation is a function of the rate of diffusion of CO₂ into the cell relative to the rate of carbon utilization. However, results reported in the literature for a calcifying strain show the opposite trend with a ¹³C enrichment of the biomass. These results suggest that the carbon utilization of the calcifying strain of Emiliania huxleyi differ from that of the non-calcifying strain. This is supported by observations in the literature, which indicates a connection between the process of calcification and the supply of carbon for photosynthesis. A mechanism for the effect of calcification on carbon isotope fractionation in light limited cells is presented here. The mechanism is based on the fact that the calcification and photosynthesis respond differently to light limitation. This difference leads to an imbalance in the rate of calcification to the rate of photosynthesis ratio (C/P), which ultimately affects the availability of CO₂ inside the cell. Apart from light, the availability of nutrients has also been shown to affect calcification. Nutrient starved cells will enhance calcification to the degree that the C/P ratio changes, thus affecting the internal concentration of CO₂. To study the effect of these environmental parameters on carbon isotope fractionation, C₃₇:₂-alkenones were extracted from samples of marine particulate organic matter. The particulate organic matter was collected together with information of the environmental conditions during three cruises in the North-East Pacific and during a Pacific transect from Victoria B.C. to Guam. Results from the NE Pacific show a lower carbon isotope fractionation in samples collected at the bottom of the euphotic zone compared to samples collected in the mixed layer. This may be an expression of the effect of light limitation. In this work carbon isotope fractionation shows no correlation with dissolved CO₂. Instead, a correlation with the ratio of phosphate concentration to concentration of dissolved CO₂ ([PO3/4⁻/[CO₂]aq.) was observed. Nitrate availability appears to play an important role in maintaining this relationship as in the absence of nitrate the carbon isotope fractionation is lower than can be predicted from the relationship relating carbon isotope fractionation to [PO3/4⁻/[CO₂]aq. The C₃₇:₂-alkenone based results from the Pacific transect shows a strong correlation between carbon isotope fractionation and phosphate. This correlation is independent of the concentration of dissolved CO₂, implying a nutrient dominated control of isotope fractionation. However, this control may not be typical as the transect passed through waters with very low nutrient levels. Therefore, the results seen here may be a consequence of extreme nutrient conditions. In conclusion, the results presented in this thesis challenge the classical belief that the carbon isotope fractionation in Emiliania huxleyi is a direct function of the availability of dissolved CO₂ by suggesting that the observed isotope fractionation is a result of a complex interaction between environmental factors such as irradiance and the availability of nutrients. In particular, a correlation between phosphate concentration and carbon isotope fractionation has been found. / Graduate
6

Carbon-14-containing compounds produced by the pile-neutron irradiation of cyanoguanidine

Lapp, Thomas William. January 1961 (has links)
Call number: LD2668 .T4 1961 L36
7

Stable carbon isotopic composition of methane from ancient ice samples

Schaefer, Hinrich. 10 April 2008 (has links)
No description available.
8

An experimental approach to the generation of phenylcarbyne anion

Schell, Philip L January 2011 (has links)
Typescript (photocopy). / Digitized by Kansas Correctional Industries
9

The carbon-13 content of atmospheric formaldehyde.

Johnson, Brian James. January 1988 (has links)
A measurement of the stable carbon isotopic composition of atmospheric formaldehyde was undertaken as a means of testing current photochemical theories. Sulfito surfaces were shown to meet the stringent analytical requirements of the project; an extensive characterization of these surfaces was performed for the first time. Models were developed to describe the chemical evolution of the surfaces during sampling. It was established that potassium salts have more favorable properties for an atmospheric collection system than do sodium salts. Considerable selectivity in collection was also demonstrated. A highly selective multistep procedure for the isolation and chemical oxidation of collected formaldehyde was developed expressly for this project. A previously unreported combination of reagents, HgCl₂ and AgClO₄, was used in the final reaction step of the procedure. Through the use of synthetic samples, the method was shown to be isotopically reproducible and highly chemically selective. The first data for the carbon-13 content of atmospheric formaldehyde have been obtained, with an observed mean value of δ¹³C = -17‰. This value is enriched in carbon-13 over the known atmospheric sources of formaldehyde; isotopic fractionation in the atmosphere is therefore indicated. It is believed that fractionation due to photolysis can account for the observed effect.
10

An organic geochemical study of the Cretaceous/Tertiary boundary

Gardner, Adele F. January 1996 (has links)
No description available.

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