Micromolar fluoride contamination arising from glass NMR tubes and a simple solution for biomolecular applications

Abstract

Fluorine (19F) NMR is emerging as an invaluable analytical technique in chemistry, biochemistry, material science, and medicine, especially due to the inherent rarity of naturally occurring fluorine in biological, organic, and inorganic compounds. Thus, we were surprised to identify an unexpected peak in our 19F NMR spectra, corresponding to free fluoride, which appears to leach out from various types of new and unused glass NMR tubes over the course of several hours. We quantified this contaminant to be at micromolar concentrations for typical NMR sample volumes across multiple glass types and brands. We find that this artefact is undetectable for samples prepared in quartz NMR tubes within the timeframes of our experiments. We also observed that pre-soaking new glass NMR tubes combined with rinsing removes this contamination below micromolar levels. Given the increasing popularity of 19F NMR across a wide range of fields, the long collection times required for relaxation studies and samples of low concentrations, and the importance of avoiding contamination in all NMR experiments, we anticipate that our simple solution will be useful to biomolecular NMR spectroscopists.