Total organic iodine measurement: a new approach with UPLC/ESI-MS for off-line iodide separation/detection

Total organic iodine (TOI) is a collective parameter and a toxicity indicator for all the iodinated disinfection byproducts in a water sample. The currently used TOI measurement method involves adsorption of organic iodine onto activated carbon, pyrolysis of adsorbed organic iodine to hydrogen iodide, absorption of hydrogen iodide into a solution, and off-line separation/detection of iodide in the absorption solution using ion chromatography coupled with conductivity detection. In this study, a new approach with ultra performance liquid chromatography/electrospray ionization-mass spectrometry (UPLC/ESI-MS) detection by setting selected ion recording of m/z 127 in the negative ion mode (instead of ion chromatography-conductivity detection) was developed for off-line iodide separation/detection in the TOI measurement. An obstacle with the new approach was found to be a series of adducts (with the same m/z value as iodide) formed in the mobile phase and at the ESI-MS sample cone, which were effectively eliminated by acidifying the absorption solution with formic acid and optimizing the instrumental parameters. By comparing the calibration curves of seven aliphatic and aromatic iodine-containing standard compounds, iodoacetic acid was determined to be an appropriate calibration standard for the TOI measurement. This new approach was found to be more sensitive, accurate and rapid. The quantitation limit of TOI was 5 μg/L as I for a 40 mL water sample or 2.5 μg/L as I for an 80 mL water sample. TOI recoveries were 94-103% for seven iodine-containing compounds and 91-108% for two wastewater and six tap water samples. The run duration for analyzing iodide in the absorption solution was only 4 min. With the new approach, TOI concentrations in various types of water samples were successfully analyzed and compared. This approach provides a sensitive tool for investigating the formation and control of iodinated DBPs in disinfected drinking water and wastewater.