In the FID analyzer, a small acidified sample is transported in the presence of an oxidizer through a heated vaporization zone. Here, the IC, in the form of CO2 plus any VOC, is driven off. The residual sample is sent through a pyrol-ysis zone to convert the remaining TOC to CO2. The CO2 is subsequently converted to methane in a nickel-reduction step. An FID detector measures the resulting methane.
The VOC is separated from the CO2 in a bypass column, reduced to methane, and routed to the same FID for an additional VOC analysis to be added to the dissolved organic carbon value.
Figure 7.8.17 shows another method that uses the FID to analyze the VOC directly after the TIC (CO2) is removed. In this method, catalytic oxidation combustion is replaced by a wet oxidation method. This method adds persulfate to the sample and exposes the solution to UV radiation to enhance oxidation efficiency. The resulting CO2 is sparged and converted in the nickel-reduction methanator, and its concentration is measured in the FID analyzer. This wet oxidation technique is available as an online analyzer.
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