Introduction
Total Organic Carbon (TOC) is a rapid system that analyzes for organic carbon & expresses the result as the volume of carbon identified. It’s a non-particular system unable to distinguish between different organic species & only indicating that natural carbon compounds are present. Natural carbon offsetting companies operate by the determination of the quantity of total carbon present in a sample aliquot. Total carbon consists of inorganic and natural carbon. The inorganic carbon, present as carbonate or bicarbonate ions, must be removed or quantified preceding 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 CO2 that’s then purged out of the sample employing 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 Approaches
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 mixture of gases. In 1756, Joseph Black, demonstrated that CO2 took place in natural air and could be created from other compounds. In his analysis 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 1st quantitative test for carbon.
Natural matter in soil has been traditionally measured by LOI or chemical oxidation utilizing dichromate remedy. The dichromate, present as hexavalent chromium, reacts with reducing natural carbon in strong acid answer to form trivalent chromium. Titration of the unused hexavalent chromium with ferrous iron yields a method capable of estimating the natural carbon present in a sample.
A steel or coal sample may be placed in a furnace, or heated tube, & in the presence of oxygen the carbon converts to CO2. The CO2 can be collected and measured, or it might be subject to a CO2 specific detector. This steel analysis apparatus gives 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 & 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 “Process and Apparatus for Determination of Total Carbon Content in Aqueous Programs”. This apparatus is a combustion procedure comparable to Yenson’s device that injects aqueous samples directly into a stream of oxygen flowing via a 700 – 900C furnace measuring the CO2 generated by IR detection. The patent states that previously accepted procedures for the determination of carbon in water were reliant on chemical oxidation procedures at moderate temperatures. As far as I can tell, James Teal’s device is the very first combustion TOC analyzer for water plus the earlier tactics he is referencing is the Chemical Oxygen Demand (COD). Teal’s process reported an analytical range of 2 – 500 ppm Carbon & 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 & 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 strategy dependant on earlier work by R.F. Wilson. (Wilson, Measurement of Natural Carbon in Seawater, Limnol. Oceanography, 6; 259 – 261, 1961). Wilson digested seawater samples making use of sodium persulfate at 100C. Menzel and Vacarro’s ampule process allowed the processing of large numbers of samples at the same time. In 1965 Alan Fredericks & Donald W. Hood developed a TOC method dependant upon Menzel & Vacarro’s ampule system that determined TOC is seawater by gas chromatography. This gas chromatographic strategy was later adapted to use an IR detector, and a newly formed company, Oceanographic Institute Business (OIC) commercialized the instrument. This new TOC analyzer digested samples utilizing persulfate chemical oxidation by autoclaving samples enclosed in ampules. An autosampler busted the ampule & swept the CO2 gas into an IR detector. This instrument was capable of analyzing carbon in seawater to as low as 0.two mg/L. The ampules had a fair advantage in that samples could be collected and sealed at sea pending subsequent digestion and analysis on land.
Ehrhard (Deep Sea Study and Oceanography Abstracts, Vol. 16, four, 1969, pp 393 – 394) developed a DOC procedure employing a Technicon autoanalyzer. This strategy combined continuous flow, UV irradiation, & persulfate oxidation and collected the CO2 generated into a dilute sodium hydroxide solution measuring carbon by conductivity. Cauwet (Marine Chemistry Vol. 14, four, 1984 pp 297 – 306) improved on Erhard’s original process by optimizing pH, persulfate concentration, UV, & utilization of IR detection.
In 1988, Sugimara & Suzuki (Marine Chemistry 24, pp 105 – 131) reported a high temperature catalytic oxidation (HTCO) approach for the analysis of seawater by direct injection of 200 microliters of sample into a 680C furnace containing a platinum catalyst. The procedure was rapid, precise, & allowed shipboard analysis. Furthermore, the method reported higher TOC levels in seawater than prior practices, namely Menzel and Vacarro’s, spurring a debate on whether there’s undetected carbon by chemical oxidation, or whether the HTCO approach yields erroneously high outcomes. After much research it was determined that there was a little bit of truth in both arguments. Original outcomes generated by HTCO tactics did not effectively compensate for high blanks brought on by carbon develop up within the combustion tube, yet, even when compensated for blank values the HTCO results were nonetheless somewhat further. After much investigation, it has been fairly well established that the HTCO techniques oxidize bacteria, vegetation, & certain sizeable molecular weight molecules with greater efficiency. The further oxidation efficiency of HTCO practices loses its value at lower concentrations since the HTCO approaches are confined in sample volume compared to chemical oxidation.
ConclusionTotal Natural Carbon (TOC) analysis is an try to measure carbon neutrality contained in natural molecules and report outcomes as a single value. The value obtained is dependent upon the oxidation technique and no single oxidation tactic is adequate for each purpose. While high temperature catalytic oxidation (HTCO) seems better, the smaller sample volumes introduce sampling error. The somewhat lower oxidation efficiency of chemical strategies is offset by the capacity to digest fairly big sample volumes. So, when HTCO and chemical oxidation results are compared, albeit HTCO outcomes trend higher than chemical oxidation the results generally seem to lie inside each others experimental error. The optimum choice of analyzer will need to always be made based upon intended application & needed sensitivity levels. For lower detections a system utilizing bigger sample volumes (chemical oxidation) should be chosen. For carbon levels above one – two ppm, the choice of oxidation strategy is not so easy.
Challenging to oxidize compounds, generally mentioned yet hardly ever defined, include cellulose, alkaloids, big chain surfactants, and bacteria. If these compounds are known to exist in concentrations higher than one – two ppm then HTCO is the clear choice of analyzer. If, though, they exist at lower concentrations then partial recovery is better than no detection at all plus a chemical oxidation system need to be used. Other aspects such as catalyst fouling, blanks, and so forth also have to be considered. These aspects will be mentioned at a later date.