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Another method employs wet oxidation and UV irradiation of the sample, which is contained in a recirculating stream of demineralized water. A conductivity cell located in this stream measures the increase in conductivity due to the CO2 resulting from the TOC. A larger sample of water is acidified and sparged with the carrier air. As CO2 is driven from the sample due to the TIC, it is dissolved in

FIG. 7.8.17 FID analyzer with wet oxidation.

18.3 mO-cm demineralized water. The resulting increase in conductance becomes the new baseline for the next step, which entails oxidation of the TOC remaining in the water.

This oxidation is accomplished by UV radiation which oxidizes the TOC to CO2. The added CO2 raises conduc-

FIG. 7.8.18 Differential-conductivity-type online organic carbon analyzer capable of detecting ppb levels of TOC in high-purity water. (Reprinted, with permission, from Ionics Inc.)

tivity to a logarithmically higher level in proportion to the TOC present. The water is then automatically demineral-ized as it passes through an ion exchange resin bed to prepare it for the next analysis.

This method is best suited for the measurement of low-TOC levels in solids-free samples, such as in drinking water. This method claims sensitivities in the ppb range.

Figure 7.8.18 shows a high-sensitivity, online TOC analyzer used in boiler feedwater; condensate return; or semiconductor, nuclear, or pharmaceutical plant water supply applications where ultrapure water is required. This analyzer takes a 25 cc/min continuous sample from the process water, mixes it with oxygen, and irradiates it with UV light in the reaction chamber. In the presence of oxygen and catalyzed by the UV radiation, the carbon molecules are oxidized into carbon dioxide. As the carbon dioxide is dissolved in the water, its conductivity increases. The analyzer interprets the difference between the conductivities of inlet and outlet water as a measure of the TOC.

This analyzer is continuous, fast (90 sec response time), and sensitive. Its span can be as narrow as 0 to 100 ppb. The sample can contain solids, but their particle size must be under 200 /.

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