Rationale Bulk stable isotope analysis (BSIA) of dissolved matter (e.g. dissolved organic carbon, total nitrogen bound (TN b ), etc.) is of particular importance since this pool is a prime conduit in the cycling of N and C. Studying the two elemental pools is of importance, as transformation and transport processes of N and C are inextricably linked in all biologically mediated systems. No system able to analyze natural abundance stable carbon and nitrogen isotope composition in aqueous samples (without offline sample preparation) and simultaneously has been reported so far. Extension of the high‐temperature combustion (HTC) system, to be capable of measuring TN b stable nitrogen isotope composition, is described in this study. Methods To extend the TOC analyzer to be capable of measuring TN b , modifications from the HTC high‐performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) interface were implemented and expanded. A reduction reactor for conversion of NO x into N 2 was implemented into the new developed system. The extension addresses mainly the development of the focusing unit for nitrogen and a degassing device for online separation of TN b from molecular nitrogen (N 2 ) prior to injection. Results The proof of principle of the system was demonstrated with different compound solutions. In this initial testing, the δ 15 N AIR‐N2 values of the tested compounds were determined with precision and trueness of typically ≤0.5‰. Good results (u ≤ 0.5‰) could be achieved down to a TN b concentration of 40 mgN/L and acceptable results (u ≤ 1.0‰) down to 5 mgN/L. In addition, the development resulted in the first system reported to be suitable for simultaneous and direct δ 13 C and δ 15 N BSIA of aqueous samples. Conclusions The development resulted in the first system shown to be suitable for both δ 13 C and δ 15 N direct BSIA in aqueous samples. This system could open up new possibilities in SIA‐based research fields. Copyright © 2016 John Wiley & Sons, Ltd.