AWWA ACE94238

The determination of Trihalomethanes (THMs) in water is generally accomplished by purge and trap gas chromatography. The analysis of these compounds at low levels in water, Part Per Billion (PPB) and Sub PPB, is difficult because the common gas chromatographic detectors lack sensitivity to the full array of compounds. For example, the Flame Ionization Detector (FID), while a good general purpose detector, is not very sensitive to chloroform. The Photo Ionization Detector (PID) is not suitable since the ionization potential of some of these compounds typically exceeds 10.6 electron volts. The Electron Capture Detector (ECD), while sensitive to all halogenated compounds, has two limitations: greatly differing response factors from compound to compound; and poor linearity. The Electrolytic Conductivity Detector (ELCD), commonly used in series with the PID, is difficult to operate outside of the laboratory environment. An on-line purge and trap gas chromatographic system has been developed using the Argon Ionization Detection (AID). The AID is capable of ionizing chloro- and bromomethanes due to its high ionization energy of 11.7 electron volts. The system is fully automated and can detect low PPB levels and sub PPB levels of compounds such as chloroform and bromoform.