Do you know about - Total Organic Carbon Analysis
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We had a good read. For the benefit of yourself. Be sure to read to the end. I want you to get good knowledge from Ear Institute .Total Organic Carbon (TOC) is a rapid method that analyzes for organic carbon and expresses the result as the amount of carbon found. It is a non-specific method unable to distinguish between various organic species and only indicating that organic carbon compounds are present. Organic carbon analyzers operate by the determination of the amount of total carbon present in a sample aliquot. Total carbon consists of inorganic and organic carbon. The inorganic carbon, present as carbonate or bicarbonate ions, must be removed or quantified prior to the analysis of organic carbon. Once the inorganic carbon is removed, subsequent analysis of the sample aliquot assumes that all carbon remaining is organic.
Discussion
Methodology used to remove inorganic carbon relies on acidification that converts all bicarbonate and carbonate ions to carbon dioxide that is then purged out of the sample using an inert gas. If quantification of inorganic carbon is desired it is purged into a detector, otherwise, it is vented to atmosphere. Once the inorganic carbon is removed the remaining organic carbon is oxidized to carbon dioxide that is purged by the inert gas into the detector.
Carbon Measurement Techniques
In about 1630 a Flemish scientist, Jan Baptista van Helmont identified a gas given off by the burning of wood as carbon dioxide. He also noted that air is a combination of gases. In 1756, Joseph Black, demonstrated that carbon dioxide occurred in natural air and could be created from other compounds. In his research on magnesium carbonates Black invented the analytical balance and used it to measure carbon dioxide by Loss on Ignition (LOI). The LOI test, where samples are heated and reduction in mass is measured, is the first quantitative test for carbon.
Organic matter in soil has been traditionally measured by LOI or chemical oxidation using dichromate solution. The dichromate, present as hexavalent chromium, reacts with reducing organic carbon in strong acid solution to form trivalent chromium. Titration of the unused hexavalent chromium with ferrous iron yields a method capable of estimating the organic carbon present in a sample.
A steel or coal sample can be placed in a furnace, or heated tube, and in the presence of oxygen the carbon converts to carbon dioxide. The carbon dioxide can be collected and measured, or it can be determined by a carbon dioxide specific detector. This steel analysis apparatus provides a basis for the modern TOC analyzer. In 1924, T. D. Yenson of the Westinghouse Electric and Manufacturing Company patented a "measuring device" that placed steel samples in a horizontal 1000C furnace that combusted carbon in an oxygen carrier gas and collected the CO2 cryogenically. In 1948, American Cyanamid patented an IR gas analyzer, and in 1967 James Teal at Dow Chemical Company patented (applied for in 1962) a "Method and Apparatus for Determination of Total Carbon Content in Aqueous Systems". This apparatus is a combustion system similar to Yenson's device that injects aqueous samples directly into a stream of oxygen flowing through a 700 - 900C furnace measuring the CO2 generated by IR detection. The patent states that previously accepted methods for the determination of carbon in water were based on chemical oxidation methods at moderate temperatures. As far as I can tell, James Teal's device is the first combustion TOC analyzer for water and the previous methods he is referring to is the Chemical Oxygen Demand (COD). Teal's method reported an analytical range of 2 - 500 ppm Carbon and 98% or better combustion efficiency of all organic compounds tested.
Frustrated with an inability to achieve lower levels of detection on seawater when using existing TOC combustion analyzers (recall that Teal's analyzer has a lower limit of 2 mg/l), Menzel and Vaccarro (Menzel and Vacarro, The measurement of dissolved organic and particulate carbon in seawater., Limnol., Oceanography., 9: pp 138 - 142, 1964) devised an ampule based wet chemical oxidation technique based on earlier work by R.F. Wilson. (Wilson, Measurement of Organic Carbon in Seawater, Limnol. Oceanography, 6; 259 - 261, 1961). Wilson digested seawater samples using sodium persulfate at 100C. Menzel and Vacarro's ampule method allowed the processing of large numbers of samples at the same time. In 1965 Alan Fredericks and Donald W. Hood developed a TOC method based on Menzel and Vacarro's ampule method that determined TOC is seawater by gas chromatography. This gas chromatographic method was later adapted to use an IR detector, and a newly formed company, Oceanographic Institute Corporation (OIC) commercialized the instrument. This new TOC analyzer digested samples using persulfate chemical oxidation by autoclaving samples enclosed in ampules. An autosampler busted the ampule and swept the CO2 gas into an IR detector. This instrument was capable of analyzing carbon in seawater to as low as 0.2 mg/L. The ampules had a significant advantage in that samples could be collected and sealed at sea pending subsequent digestion and analysis on land.
Ehrhard (Deep Sea Research and Oceanography Abstracts, Vol. 16, 4, 1969, pp 393 - 394) developed a DOC method using a Technicon autoanalyzer. This method combined continuous flow, UV irradiation, and persulfate oxidation and collected the CO2 generated into a dilute sodium hydroxide solution measuring carbon by conductivity. Cauwet (Marine Chemistry Vol. 14, 4, 1984 pp 297 - 306) improved on Erhard's original procedure by optimizing pH, persulfate concentration, UV, and utilization of IR detection.
In 1988, Sugimara and Suzuki (Marine Chemistry 24, pp 105 - 131) reported a high temperature catalytic oxidation (HTCO) method for the analysis of seawater by direct injection of 200 microliters of sample into a 680C furnace containing a platinum catalyst. The method was rapid, precise, and allowed shipboard analysis. Moreover, the method reported higher TOC levels in seawater than previous methods, namely Menzel and Vacarro's, spurring a debate on whether there is undetected carbon by chemical oxidation, or whether the HTCO method produces erroneously high results. After much research it was determined that there was a little bit of truth in both arguments. Initial results generated by HTCO methods did not properly compensate for high blanks caused by carbon build up within the combustion tube, however, even when compensated for blank values the HTCO results were still slightly higher. After much research, it has been fairly well established that the HTCO methods oxidize bacteria, vegetation, and certain large molecular weight molecules with greater efficiency. The higher oxidation efficiency of HTCO methods loses its value at lower concentrations since the HTCO methods are limited in sample volume compared to chemical oxidation.
ConclusionTotal Organic Carbon (TOC) analysis is an attempt to measure carbon contained in organic molecules and report results as a single value. The value obtained is dependent upon the oxidation technique and no single oxidation technique is adequate for every purpose. While high temperature catalytic oxidation (HTCO) seems better, the smaller sample volumes introduce sampling error. The slightly lower oxidation efficiency of chemical methods is offset by the ability to digest fairly large sample volumes. Thus, when HTCO and chemical oxidation results are compared, even though HTCO results trend higher than chemical oxidation the results always seem to lie within each others experimental error. The optimum choice of analyzer should always be made based upon intended application and required sensitivity levels. For lower detections a method utilizing larger sample volumes (chemical oxidation) should be chosen. For carbon levels above 1 - 2 ppm, the choice of oxidation technique is not so clear cut.
Hard to oxidize compounds, often mentioned but rarely defined, include cellulose, alkaloids, large chain surfactants, and bacteria. If these compounds are known to exist in concentrations greater than 1 - 2 ppm then HTCO is the obvious choice of analyzer. If, however, they exist at lower concentrations then partial recovery is better than no detection at all and a chemical oxidation method should be used. Other factors such as catalyst fouling, blanks, and so forth also need to be considered. These factors will be discussed at a later date.
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