Doubts about VOC and TOC detection in water
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This post was last edited by qugd on 2020-3-20 at 10:59. Recently, I have been working on several projects, discussing with designers and users the issue of online measurement of volatile organic compounds in water, primarily light hydrocarbons (alkanes and alkenes with 3 to 5 carbon atoms). Since it is necessary to determine the concentration of these organic compounds in water, there are various testing methods and instruments available: 1. Use a method similar to that used for analyzing VOCs in water in standard procedures, that is, conduct analysis using chromatography by sweeping out the VOCs from the water sample with an inert gas. 2. Use the same inert gas as that employed in chromatographic analysis to sweep through the water sample, and then detect the VOCs that are carried away using an infrared gas analyzer. 3. Utilize a TOC analyzer; by combining different programs, it is possible to detect volatile organic carbon – that is, first measure the total TOC, and then measure the TOC after the volatile organic carbon has been removed (this value represents non-volatile organic carbon). The difference between these two values gives the amount of VOCs. This instrument uses an open, atmospheric-pressure sampling method; I suspect that during the measurement of total TOC, some of the volatile organic carbon may escape from the sample and be lost. Of these three different analysis methods, which one is closer to reality, or in other words, more reliable? I hope fellow netizens can offer their insights and discuss this.
I’m not familiar with the details; I’ll ask some experts in the field
We all do it: for VOCs, volatile organic compounds, we use headspace or sweep capture for pre-treatment, and then chromatographic analysis. As for accuracy, since the external standard method is used for calibration with standard samples, the accuracy is very high for individual characteristic parameters. Infrared is not used for VOC detection, so no comment will be made. TC=TOC+NPOC; the operating conditions and requirements for different instruments vary, which leads to differences in result accuracy, with some differences being quite significant. Some have high requirements for salt content, while in others complete oxidation of carbon does not occur; therefore, it is not possible to give a uniform assessment of their accuracy. Any analytical method has its limitations and applicability. One can only explore, accumulate experience, and conduct verification based on the actual circumstances.