A novel method for extracting potassium (K) from K-poor and sodium-rich samples for high-precision stable K isotope analysis†

Reconstruction of stable potassium isotope composition (⁴¹K/³⁹K) in ancient seawater has significant implications for our understanding of the fundamental controls on the long-term stability of Earth's climate and ocean chemistry, but the relevant research is hampered by scarce preservation of seawater K in geological records. Marine halite (NaCl) samples are potentially valuable archives of seawater K isotope signatures because they often contain abundant fluid inclusions directly sourced from evaporated ancient seawater. However, the extremely high Na matrix relative to low K concentrations in halite presents a formidable challenge for K isotope analysis. This study reports a novel and efficient K extraction method that can separate K from a high Na matrix by precipitating K as a sparsely soluble salt using sodium tetraphenylborate (TPB). Development of this method was based on an in-depth understanding of the TPB precipitation reaction and solubility equilibrium of relevant TPB salts. Key parameters, including reaction time, precipitate recovery and rinse, and further K purification for isotope analysis, have been optimized through extensive laboratory experiments. It is demonstrated that this new method can extract small quantities of K down to 20 μg from a large quantity of the NaCl matrix up to 1 gram. In addition, preliminary results show that this new extraction method can be also applied to prepare carbonate samples, which represent another important type of sample pertinent to K isotope research, notoriously difficult to analyze due to their low K concentrations and high Ca matrices. For the first time, this study demonstrates the utility of TPB chemistry in preparing challenging K-poor, matrix-rich samples for high-precision stable K isotope analysis.