On-line measurements of metal emissions in energy conversion systems at very low concentrations are difficult to perform using existing techniques. Metals are of high importance due to their detrimental impact on human health, the environment and various industrial processes and/or equipment. Herewith and for the first time, we report the real-time detection and characterization of metals and trace elements in a Swiss biogas production plant using a novel technology based on an extractive electrospray ionization (EESI) source coupled to a high-resolution time-of-flight mass spectrometer (TOF-MS). The deployment examines the ability of the EESI-TOF to resolve highly transient signals, while providing a relatively straightforward and well-characterized platform for in-field diagnostic measurements. The EESI-TOF-MS was operated in the negative ion production and detection mode, and qualitative and quantitative results for a range of trace metals (e.g. Fe, Cu, Zn, Cd, Pb, etc.) were obtained. On-site results showed fast responses (1 Hz) and detection limits below 3 ng/m3. In addition, the EESI-TOF-MS was used for the off-line analysis of condensable trace metals of biogas samples collected using a continuous liquid-quench sampling system in 2-propanol. The results of the offline and online methods were compared and show good agreement.