Precise and Accurate Compound Specific Carbon and Nitrogen Isotope Analysis of Atrazine: Critical Role of Combustion Oven Conditions

Compound-specific stable isotope analysis by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is increasingly used to assess origin and fate of organic substances in the environment Although analysis without isotopic discrimination is essential, it cannot be taken for granted for new target compounds. We developed and validated carbon isotope analysis of atrazine, a herbicide widely used in agriculture. Combustion was tested with reactors containing (i) CuO/NiO/Pt operating at 940 degrees C; (ii) CuO operating at 800 degrees C; (iii) Ni/NiO operating at 1150 degrees C and being reoxidized for 2 min during each gas chromatographic run. Accurate and precise carbon isotope measurements were only obtained with Ni/NiO reactors giving a mean deviation Delta delta C-13 from dual inlet measurements of -0.1-0.2 parts per thousand and a standard deviation (SD) of +/- 0.4 parts per thousand. CuO at 800 degrees C gave precise, but inaccurate values (Delta delta C-13 = -1.3 parts per thousand, SD +/- 0.4 parts per thousand), whereas CuO/NiO/Pt reactors at 940 degrees C gave inaccurate and imprecise data. Accurate (Delta delta N-15 = 0.2 parts per thousand) and precise (SD +/- 0.3 parts per thousand) nitrogen isotope analysis was accomplished with a Ni/NiO-reactor previously used for carbon isotope analysis. The applicability of the method was demonstrated for alkaline hydrolysis of atrazine at 20 degrees C and pH 12 (nucleophilic aromatic substitution) giving epsilon(carbon) = -5.6 parts per thousand +/- 0.1 parts per thousand (SD) and epsilon(nitrogen) = -1.2 parts per thousand +/- 0.1 parts per thousand (SD).