Total dissolved inorganic carbons, sampling

Sampling sites are also referred to as station locations. For water column work, depth profiles are constructed from seawater samples collected at representative depths. Temperature and salinity are measured in situ with sensors. Remote-closing sampling bottles deployed from a hydrowire are used to collect water for later chemical analysis, either on the ship or in a land-based laboratory. The standard chemical measurements made on the water samples include nutrients (nitrate, phosphate, and silicate), dissolved O2, and total dissolved inorganic carbon (TDIC) concentrations. 

The major disadvantage of solid phase extraction is that it provides a chemically biased view of the DOM reservoir, and seems to preferentially extract compounds with relatively low amounts of N. Other problems include sample contamination by various resins and incomplete elution of adsorbed DOM. SoHd phase extraction remains, however, the only technique capable of reproducibly isolating DOM compounds within the entire range of MWs. Furthermore, the radiocarbon content (A " C) of DOC isolated by XAD resins is depleted relative to the total dissolved inorganic carbon (DIG) reservoir values for XAD isolates are between —300... 

Samples of the culture system water were collected bi-weekly for determination of total alkalinity and total dissolved inorganic carbon (DIG) concentrations, the carbon isotopic composition of DIG (8 G(DIG)), and the oxygen isotopic composition... 

In both TOC and dissolved organic carbon (DOC) determinations, OC in the water sample is oxidized to produce carbon dioxide (C02), which is then measured by a detection system. Inorganic carbon (IC) is removed prior to the analysis by acidifying the sample. Alternatively, TOC can be determined indirectly through the measurement of total carbon (TC) and IC. TOC in the indirect method is calculated as the difference between the two. 

Microscopic examination of seafloor sediments (if shallow enough that the CaCOs does not dissolve) and of material caught in sediment traps has revealed that much of the calcium carbonate in the samples consists of coccoliths. The flux of coccoliths probably accounts for c. 50% of the total vertical CaCOs flux in open ocean waters (in other words, about 50% of the inorganic carbon pump), with foraminifera shells responsible for most of the rest. It is usually not the most numerous species (E. huxleyi) but rather larger species (e.g., Calci-discus quadriperforatus and Coccolithus pelagicus) that make the greatest contributions to the total coccolith flux. 

Total carbon includes dissolved organic material (called total organic carbon, TOC) and dissolved COf and HCOJ (called inorganic carbon, IC). By definition, TC = TOC + IC. To distinguish TOC from IC, the pH of a fresh sample is lowered below 2 to convert COl and HCO J into CO2, which is purged from (bubbled out of) the solution with N2. After IC has been removed, combustion analysis of the remaining material measures TOC. IC is the difference between the two experiments. 

As summarised in Table 3.4, some electroanalytical methods have been certified by standardisation bodies for the chemical characterisation of ambient water samples, mostly in the class of inorganic substances. Conductometric detection is used in direct method for ionic constituents and also as chromatographic detector for individual cations and anions. Total, inorganic and organic carbon in water can be also assayed by conductometric detection. Amperometric detection has been certified for dissolved oxygen and cyanide. ISE potentiometry is used for standardised measurements of a set of ions and also for the evaluation of water oxidation-reduction potential. Voltammetric detection is the base for diverse methodologies oriented to the determination of trace elements including the most relevant elemental pollutants. 

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