Dissolved organic carbon measurement

Ogura and Hanya [62-65] investigated the components of the ultraviolet absorption in an attempt to devise a useful method for oceanic dissolved organic carbon measurements. They concluded that while the method might have limited application in coastal waters, most of the absorption in oceanic waters was due to the inorganic components, principally nitrate and bromide ions. 

Fig. 12.2Apparatus layout and flow diagram for dissolved organic carbon measurement Source Reproduced with permission from Blackwell Science Publishers Ltd [39]... 

The early field studies revealed that elevated concentrations of fallout plutonium correlated with Increased concentrations of dissolved organic carbon. Experiments at Argonne National Laboratory corroborate this correlation the explanation Is probably that the organic compounds complex Pu(IV), and, hence, decrease the distribution ratio between water and sedlments(27). In these experiments the distribution ratio (Kj) between sediment and natural waters was measured as a function of DOC. Measurements of Kj In both field and laboratory experiments show an unmistakable effect of DOC upon the distribution ratio. Figure 4 shows the Inverse correlation between the K, of plutonium and concentration of DOC. 

Hedges, J. I. and Lee, C. (1993). Measurement of dissolved organic carbon and nitrogen in natural waters. Mar. Chem. 41, 290pp. 

Fig. 2 Longitudinal changes in nutrient concentrations below the effluent input of a WWTP without tertiary treatment in La Tordera Stream. Values are the average ( SEM) of monthly measurements done over a year (see more details in [47]). In the left panel, note the net decline of ammonium concentration with concomitant net increases in nitrate concentration, suggesting a potential hot spot for nitriflcation. However, in the latest meters downstream, dissolved inorganic nitrogen (DIN) tends to decrease, which indicates net lost of DIN possibly due to denitrification. The right panel shows net changes in phosphate and dissolved organic carbon (DOC) concentrations. While phosphate does not exhibit any clear trend on an annual basis, DOC seems to decline similarly to DIN, which supports the relative dominance of denitrification...

Organic contaminants (as measured by dissolved organic carbon) continue to move through the aquifer even when acidity has been neutralized. 

Figure 3 presents some measurements of the extent of binding of DDT to dissolved organic carbon in secondary sewage effluent. The data is somewhat noisy, partially due to the fact that a portion of the DOC was able to pass through the dialysis bag. Nonetheless, the experiments showed that the DDT was bound to the sewage effluent DOC. 

Overall there is still no really satisfactory method for sampling the surface film. Of the methods now in use, the method favoured by most workers is favoured for practical reasons and not for any inherent superiority as a collector. Also, as long as we have no simple accurate method for measuring total dissolved organic carbon, it will be difficult to estimate the efficiency of any surface film collectors. 

Simply on the basis of the normal composition of marine organisms, we would expect proteins and peptides to be normal constituents of the dissolved organic carbon in seawater. While free amino acids might be expected as products of enzymic hydrolysis of proteins, the rapid uptake of these compounds by bacteria would lead us to expect that free amino acids would normally constitute a minor part of the dissolved organic pool. This is precisely what we do find the concentration of free amino acids seldom exceeds 150 xg/l in the open ocean. It would be expected that the concentration of combined amino acids would be many times as great. There have been relatively few measurements of proteins and peptides, and most of the measurements were obtained by measuring the free amino acids before and after a hydrolysis step. Representative methods of this type have been described [245-259]. Since these methods are basically free amino acid methods, they will be discussed next in conjunction with those methods. 

The simultaneous determination of dissolved organic carbon and phosphorus is feasible [39]. Phosphoglyceric acid (1 g equivalent P) was added to membrane-filtered, pre-irradiated seawater, and the phosphate was measured before and after ultraviolet irradiation using an autoanalyser. The recovery of organic phosphorus was 100%. 

Because they are so intimately related, for the purposes of this discussion, the categories of total and dissolved organic carbon are combined in this section. Also it is doubtful in any case that anyone ever measures a true total organic carbon in seawater. The total organic carbon should include... 

Armstrong and Boalch [60] have examined the ultraviolet absorption of seawater, particularly in the wavelengths between 250 and 300 nm, where the absorption is considered to result from the presence of aromatic compounds. Light absorption is a particularly useful measure, if it can be made to work, since it is not too difficult to construct an in situ colorimeter which can produce continuous profiles of dissolved organic carbon with distance or depth [71]. 

An example of an automated colorimetric method, oxidation by ultraviolet irradiation for the determination of dissolved organic carbon in the presence of potassium persulfate [ 87], is that of Schreurs [86 ]. The method uses a Technicon analyser to measure dissolved organic carbon, is fast and precise, and may be used to measure dissolved organic carbon in both seawater and fresh water over the range 0.1 -10 mg C per litre. 

Gershey et al. [58] concluded that a continuous and automated photooxidation procedure of the type described by Collins and Williams [30], with the reported modifications, will probably satisfy most of the needs of the oceanographer concerning measurement of dissolved organic carbon. The convenience and rapidity of the method opens up a new area of research the study of the small-scale temporal and spatial variations of the dissolved organic carbon content of the oceans. 

Miscellaneous Methods for Measurement of Dissolved Organic Carbon... 

Adsorption isotherm of (Aldrich) humic acid (HM) on 8-AI203 as a function of pH. Extent of adsorption was determined both by measurements of light absorption at 254 and 436 nm, respectively and by measurements of dissolved organic carbon (DOC) of the residual HM in solution (original concentration = 25 mg per liter). 

The ability to make analytical measurements depends intimately on the availability of well-defined standards and calibrants. Many measurements of analytes in seawater (such as dissolved organic carbon and dissolved organic nitrogen) cannot be compared among laboratories because of the lack of appropriate reference materials and blanks for instrument calibration and testing. Intercomparison exercises are critical (NRC, 1993, p. 75). 

Table 2 shows the mean values of the main chemical parameters measured between 2000 and 2004. Dependent on the lake and the parameter the sample number varies between 7 and 10 (data not shown). The chemical water composition is typical for carbonate-poor mountain regions low conductivity, alkalinity and pH and small nutrient and DOC (dissolved organic carbon) concentrations. Data of DOC are not shown, they vary between 0.14 and 0.70 mg C Ortho-phosphate is always smaller than the quantification limit of 4.3 P pg and ammonia and nitrite are negligibly small compared to nitrate (data not shown). 

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