Discrepancies and artifacts during preservation and analysis of the disinfection by-product 2,6-dichloro-1,4-benzoquinone

Abstract

Recent data showing fast degradation of the emerging disinfection by-product 2,6-dichloro-1,4-benzoquinone (DCBQ) in the presence of free chlorine seem incompatible with the high concentrations reported in drinking water distribution systems. The current study was conducted to reconcile this apparent incompatibility. Laboratory tests showed that the published protocol for DCBQ preservation, addition of formic acid without conventional reducing agents, was problematic. Formic acid does not rapidly reduce free chlorine, allowing chlorine residuals to persist during sample workup and analysis. Acidic conditions from formic acid addition along with a persistent free chlorine residual catalyzed additional DCBQ formation when organic precursors were present. This led to large positive analytical bias during formation potential testing using raw water and model precursors. DCBQ levels previously reported using the formic acid preservation method are likely to show a strong positive bias. For future testing, we recommend the use of glycine or arsenite followed by formic acid.