Ammonium 6:2 fluorotelomer sulfonate: crystal structure, thermal analysis and use as internal standard in PFAS analysis

• Straightforward preparation of crystalline NH 4 + 6:2 fluorotelomer sulfonate. • Unique hydrogen bonding network provides extraordinary physicochemical stability. • Affordable volumetric primary standard for PFAS trace analysis. • Ideal PFOS- and PFOA proxy for screening PFAS remediation sorbents. • Convenient starting material for related salts of 6:2 fluorotelomer sulfonic acid. Alkali salts of the common fluorosurfactant 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanesulfonic acid ( 6:2 FTSA ) exhibit a remarkably low solubility in water, which enables the precipitation of these salts from water. The same applies for the corresponding ammonium salt 6:2 FTS NH 4 which can be obtained in crystalline form by slowly adding aqueous ammonia to an aqueous solution of FTSA . To understand the cause of this behavior on a molecular basis, an X-ray single crystal structure determination of 6:2 FTS NH 4 was performed, allowing to further examine the molecular interactions in the solid state responsible for its low solubility. A hydrogen-bonded layer structure results from four distinct N‒H···O interactions, that involve all four NH groups, the given ammonium ion and sulfonate-O atoms belonging to four different anion units. Columns of parallel aligned fluoroalkyl chains dominate the packing of molecules in the crystal. The characterization was accompanied by thermal as well as spectroscopic analysis. The lack of hygroscopicity and its high thermal stability make 6:2 FTS NH 4 a suitable gravimetric standard substance for analytical applications. HPLC MS/MS measurements showed that 6:2 FTS NH 4 can adequately substitute commercial 13 C labelled compounds at least for routine analysis of common perfluorinated contaminants using standard internal calibration.