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Quimica Analitica - Quimica UMA

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WATER ANALYSIS / Seawater ­ Dissolved Organic Carbon 277 For more detailed study of dissolved oil components GC and/or GC­MS techniques are generally used. The oil is solvent extracted from the sample and after a cleanup procedure may be directly injected into the GC. Interpretation of such data is generally difficult because of the degraded nature of the oil, the compound-selective nature of the dissolution process, and its mixture with other dissolved components. In practice, it is more common to analyze bulk oil samples from a slick rather than to examine the dissolved fraction. Only if there were a need to assess the extent to which the more toxic, watersoluble components were spreading within the water would such a detailed analysis be contemplated. Organic Contaminants Other than Oil Emerging Techniques The primary ideal of the marine chemical analyst is to have access to equipment capable of providing realtime, in situ data for every chemical of interest, thus permitting the chemical data produced to provide the best possible insight into the myriad processes going on within the oceans. Needless to say this ambition is a very long way from being realized. However, there are one or two analytical areas where there are signs that progress could be made in this direction. The development of small, relatively rugged GC­MS systems for ship or in situ use represents the way in which many analyses of dissolved organic compounds in seawater will be performed in the future. See also: Water Analysis: Seawater ­ Inorganic Compounds; Organic Compounds; Biochemical Oxygen Demand; Oil Pollution. It can be assumed that virtually all reasonably persistent organic compounds used in human society will eventually find their way into the oceans through a combination of atmospheric transport and terrestrial runoff. The variety of compounds involved presents significant challenges to the analyst. The techniques for the determination of dissolved organic contaminants in seawater are generally very similar to those utilized for natural compounds. Adsorption onto Amberlite XAD-2 or polyurethane foam or, alternatively, solvent extraction into for example hexane, are common procedures. Cleanup using column chromatography, concentrated H2SO4 (for chlorinated pesticides), or occasionally thin-layer chromatography are then followed by GC using flame ionization detection (FID), electron capture detection (ECD), or mass spectrometric detection. Less commonly LC is used with either UV absorption (including diode array systems) or fluorescence detection. LC­MS is also now finding applications in marine systems though largely in sedimentary biogeochemistry. Further Reading Barcelo D and Hennion MC (1997) Sampling of polar pesticides from water matrices. Analytica Chimica Acta 338(1­2): 3­18. Grasshoff K, Ehrardt M, and Kremling K (1999) Methods of Seawater Analysis, 3rd edn. New York: Wiley-VCH. Sharp JH, Carlson CA, Peltzer ET, et al. (2002) Final dissolved organic carbon broad community intercalibration and preliminary use of DOC reference materials. Marine Chemistry 78(4): 171­184. Spyres G, Nimmo M, Worsfold PJ, Achterberg EP, and Miller AEJ (2000) Determination of dissolved organic carbon in seawater using high temperature catalytic oxidation techniques. Trac-Trends in Analytical Chemistry 19: 498­506. Sugimura Y and Suzuki Y (1988) A high-temperature catalytic-oxidation method for the determination of nonvolatile dissolved organic-carbon in seawater by direct injection of a liquid sample. Marine Chemistry 24(2): 105­131. Seawater ­ Dissolved Organic Carbon G Spyres, City University of New York, Flushing, NY, USA M Nimmo, University of Plymouth, Plymouth, UK & 2005, Elsevier Ltd. All Rights Reserved. Introduction The anthropogenically induced increase of greenhouse gases (e.g., CO2) in the atmosphere has triggered what is now widely recognized as the `global warming' effect. Due to the potential adverse climatic conditions associated with global warming, interdisciplinary, international scientific environmental research programs (e.g., International Geosphere Biosphere Programme) in the past decade have increasingly focused on the global carbon cycle and the quantification of fluxes of atmospheric CO2 at marine and terrestrial system interfaces. In part, these studies are aimed at resolving the discrepancies in the 278 WATER ANALYSIS / Seawater ­ Dissolved Organic Carbon estimates of carbon reservoirs and fluxes between reservoirs. Of particular importance is the dissolved organic carbon (DOC) pool in the oceans, being equivalent to that of atmospheric CO2 (700 pg C). DOC is an important source of carbon for community respiration. Indeed, a net oxidation of only 1% of seawater DOC within a year would be sufficient to generate a CO2 flux larger than that produced by fossil fuel burning. Hence, its accurate quantification in ocean systems, with both spatial and temporal variability, is one essential requirement in establishing a reliable global C budget. Furthermore, DOC is an essential chemical driver of aquatic biogeochemical cycles. It influences the distribution and transport of key chemical constituents such as inorganic nutrients and trace metals, and can facilitate the vertical export of aquatic colloids and associated material via aggregation processes. The reliable detection of DOC is thus crucial in our understanding of aquatic biogeochemical cycles. Historical Overview of Applied Analytical Techniques Analytical techniques, which have been employed for the measurement of DOC in seawater, are (1) wet chemical oxidation (WCO), (2) ultraviolet (UV) oxidation, (3) dry combustion, and (4) high-temperature catalytic oxidation (HTCO). Prior to the 1980s, WCO was the most adopted method; its application to DOC seawater analysis was first reported in the 1930s. Oxidants used have included peroxydisulfate, dichromate in sulfuric acidic, and potassium peroxide. Prior to oxidation of the sample DOC, inorganic carbon was removed by purging of the acidified sample (pHo2). The CO2 produced from the oxidation process is measured by one of the number methods from nondispersive infrared gas analysis (IRGA), colorimetrically, and conductometrically by conversion to CH4. However, the limitation of WCO was the progressive weakening of the oxidant (particularly in the presence of Cl À ), during sample treatment, which may have led to incomplete oxidation. Furthermore, different reagents would have had different oxidation strengths, potentially leading to different oxidation efficiencies. UV oxidation methods were proposed during the same period as WCO, although they were not adopted until the 1970s. As with WCO, inorganic carbon is removed from the seawater prior to oxidation. Typically, water samples were exposed to a UV radiation source light via a quartz capillary coiled around the light source. Some UV radiation procedures included the addition of a chemical oxidant. The advantage of the UV oxidation approach compared to that of WCO was that oxidants were generated continuously. However, the main disadvantage with UV oxidation was the deterioration of the UV light source with age. This lowered the efficiency of the oxidation process and potentially led to noncomparability of different research group datasets. Dry oxidation methods (applied in the 1960s and 1970s) involved initial acidification and dehydration of the sample, followed by high-temperature combustion of the resultant `sea salt'. Sample dehydration was achieved by heating to dryness at 601C or by freeze-drying. The main disadvantage of these methods was the high degree of sample manipulation required during the dehydration step, enhancing the risk of sample contamination. The method is also labor intensive and cannot be automated preventing real-time analysis. Since the late 1980s, HTCO methods, which typically incorporate an Al/Pt catalysis combustion column, have been adopted. The popularity of this technique was, in part, due to the advances in the sensitivity and stability of the IRGA and its real-time direct injection capabilities, requiring lower sample volumes (50­200 ml) compared to those for WCO (5­100 ml). HTCO is rapid and precise (71­2% RSD) and can yield equivalent or greater amounts of DOC than WCO methods. In addition, it is effectively adopted for routine analyses and is stable for shipboard determinations. This was first proposed in a paper by Sugimura and Suzuki, whose HTCO method detected 1.5­5 times higher concentrations of DOC (180­280 mmol l À 1 C) in surface oceanic waters than had been previously reported by WCO methods. Underestimation of the DOC concentrations in seawater measured by WCO would have clearly meant an underestimation of the DOC pool in the oceans. The findings of Sugimura and Suzuki, however, were not always reproduced by other researchers, and were subsequently retracted as a result of inaccuracies due to insufficient consideration of the influence of gases other than CO2 and water/system blanks not subtracted from the initial data. Nevertheless, this work mobilized the scientific community into reevaluating the accuracy of conventional WCO analytical techniques, and paved the way to the subsequent adoption of HTCO as the preferred analytical method for the determination of seawater DOC. Subsequent to Sugimura and Suzuki, research findings, both supportive and critical of the HTCO method, appeared but there were inconsistencies with the published data. This led to a US National Science Foundation/National Oceanic and Atmospheric Administration/Department of Energy sponsored workshop on the `Measurement of Dissolved WATER ANALYSIS / Seawater ­ Dissolved Organic Carbon 279 Organic Carbon and Nitrogen in Natural Waters' (Seattle 1991) in order to critically evaluate the methodological procedures that essentially bear upon generating consistent results In this workshop, natural seawater samples were distribu

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