Characterization of Inert Copper Species Using Solvent-Extraction ICP-MS

A fraction of oceanic copper is thought to be bound so tightly that it renders the complex chemically non-labile. This fraction is has been referred to as “inert copper”, and may impact copper residence time and distribution. Traditional techniques for copper speciation measurements using the Competitive Ligand Exchange – Adsorptive Cathodic Stripping Voltammeter (CLE-ACSV) are useful to determine free cupric ion and labile complexes, but are less useful to characterize inert complexes; CLE-ACSV is prone to artifacts like wall loss and error propagation at long equilibration times or high competing ligand concentrations.

Copper speciation measurements were taken on samples from GEOTRACES GP15, using a novel method which utilizes liquid to liquid extraction with toluene as the solvent. The method, originally developed for the Graphite Furnace - Flameless Atomic Absorbance Spectrophotometer, was miniaturized for application on the Inductively Coupled – Mass Spectrometer (ICP-MS). A strong chelator, 8-hydroxyquinoline (oxine), was used at high concentration to form a solvent-extractable copper bis-complex. The non-extractable fraction was considered to be inert. Diethylamine Triamine Pentetic Acid (DTPA) at millimolar concentrations was used to titrate against added oxine in UV-irradiated chelator-free water to calibrate the system at equilibrium and to set lower limits on the binding strength of the inert copper complexes.