Evaluation of the sealed-tube low-temperature combustion method for the C-13/C-12 and H-2/H-1 ratio determinations of cellulose nitrate

Traditionally-suggested combustion time of 1 h at 550degreesC with the sealed-tube combustion method for determining the C-13/C-12 ratio of cellulose nitrate or other nitrogen-containing components could produce large negative deviation up to 1%.. Three types of cellulose are used to ascertain possible causes. The presence of nitrous oxide (N2O) formed during combustion is most likely responsible for this deviation. Prolongation of the combustion time (at least 5 h at 550degreesC) and intimate contact between copper oxide and organic matter can greatly improve the analysis precision and effectively reduce this deviation to an acceptable level. Regardless of scattered carbon isotope data, hydrogen isotope data are all reproducible within 2% when this method is coupled with the high temperature uranium reduction method. Thus, care should be taken for determining carbon and nitrogen isotope compositions of nitrogen-containing substances using the low temperature sealed-tube combustion method

An attempt to understand the behavior of dissolved organic carbon in coastal aquifers of Pondicherry region, South India

Groundwaters of the coastal region are significant and serve as a fragile ecosystem in the sedimentary terrain. The Pondicherry region is characterized by different geological formations of distinct geological epochs Cretaceous, Tertiary and Quaternary. Ninety-three samples were collected from specific aquifers with respect to spatial distribution [Alluvium (31), Upper Cuddalore (13), Lower Cuddalore (13), Other Tertiary (7), Cretaceous (24) and Mixed—multiple completion aquifer (5)]. The collected samples were analyzed for major ions Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, PO42−, SO42−, F and DOC. Stables isotopes of δ18O, δD and δ 13C were also analyzed along with heavy metals like Al, Ba, Fe, Sr, Cd and Zn. The δ18O ranges between −6.85 and −2.15 ‰, and δD ranges between −43.94 and −19.41 ‰. The distribution of these environmental isotopes in groundwater of this layered aquifer sequence was also attempted. A comparison of the isotopic data with the rainfall, Local Meteoric Water Line with a equation of δD = 7.398 * δ18O + 5.067, Indian Meteoric Water Line and Global Meteoric Water Line was performed. The δ13C values for groundwater ranges from −5.3 to −18.1 ‰. The result indicates that the groundwater in the study area is mainly meteoric in origin, and few samples show evidence of evaporation. The dissolved organic carbon (DOC) is a very important component in biogeochemical cycling of elements characterized by high susceptibility to leaching. The range of the DOC in the study area is 0–16 mg/L. Factor analysis was applied to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry. The Alluvium aquifers show a complex hydrogeochemistry than the older aquifers. The major factor influencing the hydrogeochemistry of the region is rock–water interaction and anthropogenic processes. Hence, metal mobility of the groundwater shows the correlation of metal bonding and DOC. The relation between the DOC and heavy metal concentration in the aquifers was also identified

Human impact on the historical change of CO₂degassing flux in River Changjiang

<p>Abstract</p> <p>The impact of water quality changes in River Changjiang (formally known as the Yangtze River) on dissolved CO₂and silicate concentrations and seasonal carbon flux in the past several decades (1960s–2000) was evaluated, based on monitoring data from hydrographic gauge. It was found that dissolved CO₂and silicate in Changjiang decreased dramatically during this decades, as opposed to a marked increase in nutrient (<it>e.g</it>. NO₃^-) concentrations. Our analyses revealed that dissolved CO₂in Changjiang was over-saturated with the atmosphere CO₂, and its concentration had showed a declining trend since the 1960s, despite that fluvial DIC flux had maintained stable. Analysis results also suggested that the decrease in dissolved CO₂concentration was attributed to changes on the riverine trophic level and river damming activities in the Changjiang drainage basin. Due to the economic innovation (<it>e.g</it>. agriculture and industry development) across the Changjiang watershed, fertilizers application and river regulations have significantly altered the original state of the river. Its ecosystem and hydrological condition have been evolving toward the "lacustrine/reservoir" autotrophic type prevailing with plankton. Accordingly, average CO₂diffusing flux to the atmosphere from the river had been reduced by three-fourth from the 1960s to 1990s, with the flux value being down to 14.2 mol.m^-2.yr^-1in the 1990s. For a rough estimate, approximately 15.3 Mt of carbon was degassed annually into the atmosphere from the entire Changjiang drainage basin in the 1990s.</p