Gas chromatography/isotope ratio mass spectrometry of recalcitrant target compounds: Performance of different combustion reactors and strategies for standardization

RATIONALE Compound-specific isotope analysis (CSIA) relies on continuous flow combustion of organic substances to CO₂ and N₂ in a miniature reactor to measure ¹³ C/¹² C and ¹⁵ N/¹⁴ N stable isotope ratios. Accurate analysis is well established for many volatile hydrocarbons. In contrast, compounds which contain hetero and halogen atoms are less volatile and may be more recalcitrant to combustion. METHODS This study tested carbon and nitrogen isotope analysis of atrazine, desethylatrazine (DEA), dichlobenil and 2,6-dichlorobenzamide (BAM) by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) with multiple reactor tubes of two different kinds (conventional CuO/NiO/Pt and a NiO tube/CuO-NiO reactor prototype). RESULTS The advantages of the NiO tube/CuO-NiO reactor were the absence of an additional reduction reactor, the possibility of routine reoxidation in nitrogen isotope analysis, and reliable atrazine and DEA measurements over several hundred injections. In contrast, BAM analysis showed good accuracy for carbon, but notable variations in the trueness of nitrogen isotope ratios. Accurate carbon and nitrogen analysis was nevertheless possible by bracketing samples with external compound-specific standards and subsequent offset correction. CONCLUSIONS We conclude that instrument data should never be taken at its 'face value', but must consistently be validated with compound-specific standards of the respective analytes.