Effect of extraction pH on metal speciation in plant root extracts

The species patterns of nutrient and trace metals (K, Ca, Mg, Mn, Fe, Zn) obtained by extraction of plant roots have been determined as a function of extraction pH in the range 4-9. The extractable metal concentrations were subdivided into low-molecular-weight (<10 kDa) and high-molecular-weight (>10 kDa) metal species by TXRF analysis. Except for pH 9, the low-molecular-weight fraction is predominant. This fraction was further separated by HPLC with AAS detection. It was demonstrated that the nature of the metal species changes dramatically when the extraction pH is changed, especially in the range 9-7. Information about the chemical nature of chromatographically separated metal species is obtained by use of different electrochemical detectors (amperometric detection at a glassy carbon or copper electrode and pulsed amperometric detection) and diode-array UV detection.