Application of an Optimized Total N-Nitrosamine (TONO) Assay to Pools: Placing N-Nitrosodimethylamine (NDMA) Determinations into Perspective

Although N-nitrosodimethylamine (NDMA) has been the most prevalent N-nitrosamine detected in disinfected waters, it remains unclear whether NDMA is indeed the most significant N-nitrosamine or just one representative of a larger pool of N-nitrosamines. A widely used assay applied to quantify nitrite, S-nitrosothiols, and N-nitrosamines in biological samples involves their reduction to nitric oxide by acidic tri-iodide, followed by chemiluminescent detection of the evolved nitric oxide in the gas phase. We here describe an adaptation of this method for analyzing total N-nitrosamine (TONO) concentrations in disinfected pools. Optimal sensitivity for N-nitrosamines was obtained using a reduction solution containing 13.5 mL glacial acetic acid and 1 mL of an aqueous 540 g/L iodide and 114 g/L iodine solution held at 80 degrees C. The method detection limit for N-nitrosamines was 110 nM using 100 microL sample injections and NDMA as a standard. N-nitrosamines featuring a range of polarities were converted to nitric oxide with 75-103% efficiency compared to NDMA. Evaluation of potential interfering species indicated that only nitrite and S-nitrosothiols were a concern, but both interferences were effectively eliminated using group-specific sample pretreatments previously employed for biological samples. To evaluate the low TONO concentrations anticipated for pools, 1 L samples were extracted by continuous liquid-liquid extraction with ethyl acetate for 24 h, and concentrated to 1 mL. N-nitrosamine recovery during extraction ranged from 37-75%, and there was a potential for artifactual nitrosation of amines during solvent reflux in the presence of significant nitrite concentrations, but not at the low nitrite concentrations prevalent in most pools. Using the 1000-fold concentration factor and 56% average extraction efficiency, the method detection limit would be 62 pM (5 ng/L as NDMA). The TONO assay was applied to six pools and their common tap water source in conjunction with analysis for specific nitrosamines. Even accounting for the range of N-nitrosamine extraction recoveries, NDMA accounted for an average of only 13% (range 3-46%) of the total nitrosamine pool.