A framework for efficient chemical separation of tungsten and other trace elements using organic acids for high-precision mass spectrometry

High-precision measurements of isotopic compositions by mass spectrometry require sample solutions that are highly pure with respect to the element of interest. In order to achieve such high elemental purity, often for elements that are present in trace to ultratrace amounts in basaltic samples, chemical separation protocols are employed to separate major (e.g., Fe, Mg, Al, Ca, Ti) and trace elements while retaining the selected analyte. These protocols traditionally employ acidic solutions that typically include a suite of strong acids and hydrofluoric acid. Some other common-practice protocols utilize organic acids. Organic acids are particularly useful for separating elements with similar chemical behaviors on standard cation and anion resins. For example, separation of Ti from W in large (>1 gram) basaltic samples has traditionally employed acetic acid, which preferentially forms complexes with W that are retained on anion resins. We improve the efficiency of this separation protocol by instead using citric acid, which has highly disparate complexation behavior for Ti and W. The protocol is capable of producing a high W yield (approaching 100%) and can reduce the Ti/W ratio of basaltic samples from 10⁵ to <1 following a standard cation separation protocol. We then examine the chemical behaviors that enable such efficient separation of trace elements using organic acids, including pH, redox state, ligand availability, and sample size. Given the large range of organic acids available, careful selection of acid-analyte pairs and eluant formulae may enable a variety of high-efficiency separation techniques with minimal adaptation of our framework method. We illustrate this principle by adapting our citric acid protocol for high-efficiency separation of Mo and Sn (both typically with 90-100% yields), and further explore using salicylic acid for high-efficiency separation of other ultratrace metallic elements. These applications of our framework method reveal that organic acids are a currently underutilized tool for chemical separation protocols, particularly those protocols designed for use with large sample masses. Further, since many organic acids are inert and pose relatively little health risk compared to strong acids, our framework may be particularly for mitigating laboratory safety concerns.