Isotope ratio measurements of magnesium and determination of magnesium concentration by reverse isotope dilution technique on small amounts of Mg-26-spiked nutrient solutions with inductively coupled plasma mass spectrometry

In the present work, an analytical method has been developed for the measur ement of magnesium isotope ratios and the determination of magnesium concen tration in Mg-26-spiked plant nutrient solutions by the reverse isotope dil ution technique. The experimental parameters of the mass spectrometric meas urements using quadrupole-based inductively coupled plasma mass spectrometr y (ICP-MS) were optimized with respect to the lowest relative standard devi ation (RSD) in the magnesium isotopic ratio (Mg-25/Mg-24 and Mg-26/Mg-24). The RSD of the measured isotopic ratios was determined in different concent rations of isotopic standard solution (NIST SRM (TMg)-T-980-Mg-26-enriched isotopic standard) of natural isotopic abundances (from 1 to 50 mug/L with 1.9% to 0.22% RSD, respectively). Long-term isotopic ratio measurements usi ng 50 mug/L isotopic standard solution (NIST SRM 980) yielded 0.14% RSD for Mg-25/Mg-24 and 0.23% RSD for Mg-26/Mg-24. The accuracy of measured isotop ic ratios was 0.1% to 0.3% [Mg-25/Mg-24 0.12684 +/- 0.00018 (measured) vers us 0.12663 +/- 0.00013 (certified) and Mg-26/Mg-24 0.13958 +/- 0.00036 (mea sured) versus 0.13932 +/- 0.00026 (certified)]. Synthetic isotopic mixtures of NIST SRM 980 and highly enriched Mg-26 solutions were prepared and the measured isotopic ratios were compared with the calculated ones. The RSD of the isotope ratios measurements ranged between 0.14% to 0.07% for the Mg-2 5/Mg-24 ratio and 0.2-0.04% for the Mg-26/Mg-24 ratio. The developed method has been successfully applied for the determination of the magnesium isoto pe ratios and the concentration measurements in Mg-26-enriched plant nutrie nt solution samples with reverse isotope dilution technique. The nutrient s olutions investigated contained high amounts of calcium and potassium (20-3 5 mug/ml and 9.7-30 mug/ml, respectively). Magnesium concentrations were de termined by inductively coupled plasma optical emission spectrometry (ICP-O ES) and ICP-MS with external calibration and the measured values are in agr eement with the results obtained by the reverse isotope dilution technique. (Int J Mass Spectrom 202 (2000) 231-240) (C) 2000 Elsevier Science B.V.