by William Lipps AL


Measuring Organic Carbon: A Brief History and Discussion of Methodologies


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 nonspecific method unable to distinguish between various organic species and indicates only that organic carbon com- pounds are present. TOC analyzers operate by determining the amount of total carbon in a sample aliquot. Total carbon consists of inorganic and organic carbon. 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 bicar- bonate and carbonate ions to carbon dioxide that is purged out of the sample using an inert gas. If quantification of inorganic carbon is de- sired, it is purged into a detector; otherwise it is vented to atmosphere. Once 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 the 1630s, Flemish scientist Jan Baptist van


Helmont identified the gas emitted by the burning of wood as carbon dioxide. In 1756, Joseph Black demonstrated that carbon diox- ide occurred in natural air and could be created from other compounds. While doing research on magnesium carbonates, Black invented the analytical balance and used it to measure carbon dioxide by loss on ignition (LOI). The LOI test, in which samples are heated and reduction


in mass is measured, is the first quantitative test for carbon.


Organic matter in soil has traditionally been 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 car- bon dioxide can be collected and measured or it can be determined by a carbon dioxide-specific detector. The initial steel analysis apparatus provides a basis for the modern TOC analyzer. In 1924, T.D. Yensen of the Westinghouse Electric and Manufacturing Company patented a “measuring device” that placed steel samples in a horizontal 1000 ºC furnace that combusted carbon in an oxygen carrier gas and collected the CO2


cryogenically. In 1948, American


Cyanamid patented an infrared (IR) gas ana- lyzer, and in 1967 James Teal at Dow Chemical Company patented a “Method and Apparatus for Determination of Total Carbon Content in Aqueous Systems.” This apparatus is a combus- tion system that is similar to Yensen’s device and manually injects aqueous samples directly, using a syringe, into a stream of oxygen flowing through a 700–900 ºC furnace measuring the CO2


generated with infrared absorbance. The


patent states that previously accepted meth- ods for the determination of carbon in water were based on chemical oxidation methods at moderate temperatures. Teal’s device appears to be the first combustion TOC analyzer for


AMERICAN LABORATORY 12 MAY 2016


water, and the previous method he is referring to is likely the chemical oxygen demand (COD) test. The method reported an analytical range of 2–500 ppm carbon and 98% or better combus- tion efficiency of all organic compounds tested.


Frustrated with an inability to achieve lower levels of detection on seawater using existing TOC combustion analyzers (recall that Teal’s analyzer has a lower limit of 2 mg/L), Menzel and Vaccarro1


Wilson digested seawater devised an ampule-based wet


chemical oxidation technique based on earlier work by R.F. Wilson.2


samples using sodium persulfate at 100 ºC. Menzel and Vacarro’s ampule method allowed the simultaneous processing of large numbers of samples. Figure 1 shows ampules similar to those used by Menzel and Vacarro.


Figure 1 – Ampules similar to those used by Menzel and Vacarro.


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