Characterization of C3F7CN gas mixtures and their trace decomposition products detection using GC and photoionization mass spectrometry for electrical insulation applications

One of the most pressing environmental challenges in the energy sector is the replacement of SF₆, a potent greenhouse gas. Heptafluoro-isobutyronitrile (C 3 F 7 CN) based gas mixtures are being used to provide electrical insulation for a new generation of more environmentally friendly high-voltage gas insulated equipment. Equipment on the electricity network is expected to function >45 years and an emerging operational challenge is the build-up of decomposition products (some of which are toxic) over time, which reduces the electrical performance of the insulating gas and results in an increased risk of failure to the in-service equipment. An analytical methodology has been developed that enables the unambiguous ppm level identification and quantification of the gas decomposition products in C 3 F 7 CN gas mixtures, namely, CO, CF 4 , C 3 F 3 N and others, using GC–MS equipped with electron ionization (EI) and chemical ionization (CI) ion sources and electrostatic trap mass spectrometry with photoionization and atmospheric pressure chemical ionization (APCI). This paper reports for the first time the relative experimental ionization efficiencies of three main chemical classes of products in C 3 F 7 CN due to electrical discharges: fluorocarbons, hydrogenated fluorocarbons, and nitriles, with the key products include CF₄, C₂F₆, C₃F₆, C₄F₁₀, C₃HF₈, C₃H₂F₄ and C₃HF₇ relative to C₃F₇CN. A robust analytical methodology has been developed to measure the electron ionization efficiencies of decomposition products with their chemical composition verified using both GC–MS and photoionization mass spectrometry. This method significantly simplifies the concentration measurements of fluorocarbons, hydrogenated fluorocarbons and nitriles classes of compounds specific to C 3 F 7 CN gas mixtures used in electrical insulation applications, for which analytical standards are either not readily available or difficult to use due to acute toxicity. Accurate decomposition indicators can be provided to ensure equipment reliability for the power industry. • Gas contamination can impact the electrical performance of C 3 F 7 CN gas mixtures, thereby affecting the long-term reliability of gas-insulated equipment. • Characterization of abundant and trace components of insulating gases by GC–MS and photoionization mass spectrometry. • Development and validation of analytical methods for identification and concentration measurements of the decomposition products of C 3 F 7 CN and CO 2 gas mixtures at ppm level. • Experimental measurements of the relative electron ionization efficiencies for the decomposition products of C 3 F 7 CN and CO 2 gas mixtures.