MEASURING ORGANIC CARBON continued
TOC analysis attempts to
measure carbon contained in
organic molecules and report results as a single value.
Figure 2 – Shimadzu TOC-L HTCO TOC analyzer.
In 1965, Alan Fredericks and Donald W. Hood developed a TOC method based on Menzel and Vacarro’s ampule method that determined TOC in seawater using 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 in- strument. This new TOC analyzer digested samples using persulfate chemical oxidation by autoclaving samples enclosed in ampules. An autosampler broke 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 sub- sequent digestion and analysis on land.
Ehrhard3 developed a TOC method using a
Technicon AutoAnalyzer. This method com- bined continuous flow, ultraviolet irradiation and persulfate oxidation and collected the CO2
solution measuring carbon by a change in conductivity. Cauwet4
In 1988, Sugimara and Suzuki5
original procedure by optimizing pH, persulfate concentration, UV and IR detection.
reported on a
high-temperature catalytic oxidation (HTCO) method for the analysis of seawater by direct injection of 200 μL of sample into a 680 ºC
generated into a dilute sodium hydroxide improved on Ehrhard’s
furnace containing a platinum catalyst of the TOC-L HTCO TOC analyzer (Shimadzu Scientific Instruments, Columbia, Md.) (see Figure 2). The method was rapid and precise and allowed shipboard analysis. Moreover, it demonstrated higher TOC levels in seawater than previous methods, namely Menzel and Vacarro’s, spur- ring 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 both arguments were valid. Initial results
AMERICAN LABORATORY 14 MAY 2016
generated by HTCO did not properly compen- sate for high blanks caused by carbon buildup within the combustion tube; however, even when compensating for blank values, the HTCO results were still slightly higher. Humic acids, bacteria, vegetation and certain high- molecular-weight molecules are oxidized with greater efficiency using HTCO. The higher oxidation efficiency of HTCO loses its value at lower concentrations (below about 1.0 mg/L of carbon) since HTCO is limited in sample volume compared to chemical oxidation (Standard
Figure 3 – Comparison of chemical oxidation and HTCO in drinking water samples.
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