Measurement of labile Cu in soil using stable isotope dilution and isotope ratio analysis by ICP-MS

Isotope dilution is a useful technique to measure the labile metal pool, which is the amount of metal in soil in rapid equilibrium (< 7 days) with the soil solution. This is normally performed by equilibrating soil with a metal isotope, and sampling the labile metal pool by using an extraction ( E value), or by growing plants ( L value). For Cu, this procedure is problematic for E values, and impossible for L values, due to the short half-life of the Cu-64 radioisotope (12.4 h), which makes access and handling very difficult. We therefore developed a technique using enriched Cu-65 stable isotope and measurement of Cu-63/Cu-65 ratios by quadrupole inductively coupled plasma mass spectrometry (ICP-MS) to measure labile pools of Cu in soils using E value techniques. Mass spectral interferences in detection of Cu-63/Cu-65 ratios in soil extracts were found to be minimal. Isotope ratios determined by quadrupole ICP-MS compared well to those determined by high-resolution ( magnetic sector) ICP-MS. E values determined using the stable isotope technique compared well to those determined using the radioisotope for both uncontaminated and Cu-contaminated soils.