Fig. 2: Typical ultraviolet (UV) absorbance and element chromatograms obtained with the developed SEC-UV-ICP-MS/MS method for Kohala soil extracts together with their deconvolution.

Panel a shows the selenium (Se), iron (Fe), carbon (C), UV, sulfur (S), copper (Cu), and arsenic (As) chromatograms obtained for the NaOH extract of the topsoil (0–10 cm) from Kohala site S4 (intermediate soil organic carbon content), while panel b shows the same type of data for the ultrapure water extract of this soil. The optimized size exclusion chromatographic (SEC) separation involves two columns (Shodex OH-Pak 803-SB and 802.5-SB) connected in series and operated with a mobile phase containing 5 mmol L⁻¹ ammonium nitrate at pH 7 and 9.5 for ultrapure water and NaOH extracts, respectively. For Se, the mass flow chromatogram (unit, µg L⁻¹) resulting from the on-line isotope dilution is shown while the intensity chromatograms (unit, counts s⁻¹) are shown for the other elements. The background colors indicate the boundaries of the identified SEC fractions, except the free oxyanions (i.e., HSeO₃⁻/SeO₃²⁻, SO₄²⁻ and H₂AsO₄⁻/HAsO₄²⁻) that are indicated with arrows. The vertical dashed lines show the apex of the identified fraction peak(s). Note that the fraction F2 corresponding to the first C peak is noted F2a for NaOH extracts and F2b for ultrapure water extracts because for F2a the peak of C coincides with an UV peak, while for F2b there is no UV peak associated with the C peak, therefore representing non-aromatic, aliphatic organic matter.