Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization—Headspace SPME GC–MS

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.