A novel high-temperature combustion based system for stable isotope analysis of dissolved organic carbon in aqueous samples. II: optimization and assessment of analytical performance.

Abstract

Dissolved organic carbon (DOC) plays an important role in carbon cycling, making precise and routine measurement of δ(13)C values and DOC concentration highly desirable. A new promising system has been developed for this purpose. However, broad-scale application of this new technique requires an in-depth assessment of analytical performance, and this is described here. A high-temperature combustion Total Organic Carbon analyzer was interfaced with continuous flow isotope ratio mass spectrometry (TOC/IRMS) for the simultaneous analysis of the bulk DOC concentration and δ(13)C signature. The analytical performance (precision, memory effects, linearity, volume/concentration effects, accuracy) was thoroughly evaluated, including realistic and challenging conditions such as low DOC concentrations and natural DOC. High precision (standard deviation, SD predominantly ≤ 0.15 ‰) and accuracy (R(2) = 0.9997) were achieved for the δ(13)C analysis of a broad diversity of DOC solutions. Simultaneously, good results were obtained for the measurement of DOC concentration. Assessment of natural abundance and slightly (13)C-enriched DOC, a wide range of concentrations (~0.2-150 mgC/L) and injection volumes (0.05-3 mL), demonstrated minor/negligible memory effects, good linearity and flexible usage. Finally, TOC/IRMS was successfully applied to determine low DOC concentrations (<2 mgC/L) and DOC from diverse terrestrial, freshwater and marine environments (SD ≤ 0.23 ‰). TOC/IRMS enables fast and reliable measurement of DOC concentrations and δ(13)C values in aqueous samples, without pre-concentration and freeze-drying. Further investigations should focus on complex, saline matrices and very low DOC concentrations, to achieve a potential lower limit of 0.2 mgC/L. Thus, TOC/IRMS will give DOC research in terrestrial and aquatic environments a huge impulse with high-resolution, routine δ(13)C analysis.