Nitrite Interference in the Analysis of Nitrophenols In Wastewater Effluents

Elevated concentrations of 2-nitrophenol (ONP) and 4-nitrophenol (PNP) were measured in a municipal treatment plant's effluent when nitrite ion (NO2−1) was present. ONP and PNP concentrations were determined using USEPA's method 3520 continuous liquid-liquid extraction in conjunction with gas chromatography/mass spectrometry (GC/MS) analysis by USEPA method 8270. It was also observed that surrogates, compounds added during testing to check the extraction efficiency, were at unusually low levels when nitrite was present. It was determined that nitrite reacts with the surrogates producing low surrogate recoveries and also that nitrite reacts with phenol present in the sample to produce false positive effluent concentrations of ONP and PNP. It was determined in an inter-laboratory study that nitrite interference occurs with the separatory funnel extraction method 3510 also. Modification of method 3520's extraction pH to 4.5 corrected this interference problem; however, certain of the target analytes for method 8270 cannot be recovered in acceptable concentrations at this modified extraction pH. Therefore, the modified method cannot be used for all phenolic compounds typically analyzed by method 8270.