Measuring magnesium, calcium and potassium isotope ratios using ICP-QMS with an octopole collision cell in tracer studies of nutrient uptake and translocation in plants

The ability of a quadrupole-based ICP-MS with an octopole collision cell to obtain precise and accurate measurements of isotope ratios of magnesium, calcium and potassium was evaluated. Hydrogen and helium were used as collision/reaction gases for ICP-MS isotope ratio measurements of calcium and potassium in order to avoid isobaric interference with the analyte ions from (mainly) argon ions Ar-40(+) and argon hydride ions (ArH+)-Ar-40-H-1. Mass discrimination factors determined for the isotope ratios Mg-25/Mg-24, Ca-40/Ca-44 and K-39/K-41 under optimized experimental conditions varied between 0.044 and 0.075. The measurement precisions for Mg-25/Mg-24, Ca-40/Ca-44 and K-39/K-41 were found to be 0.09%, 0.43% and 1.4%, respectively. This analytical method that uses ICP-QMS with a collision cell to obtain isotope ratio measurements of magnesium, calcium and potassium was used in routine mode to characterize biological samples (nutrient solution and small amounts of digested plant samples). The mass spectrometric technique was employed to study the dynamics of nutrient uptake and translocation in barley plants at different root temperatures (10 degrees C and 20 degrees C) using enriched stable isotopes (Mg-25, Ca-44 and K-41) as tracers. For instance, the mass spectrometric results of tracer experiments demonstrated enhanced Mg-25 and Ca-44 uptake and translocation into shoots at a root temperature of 20 degrees C 24 h after isotope spiking. In contrast, results obtained from K-41 tracer experiments showed the highest K-41 contents in plants spiked at a root temperature of 10 degrees C.