Use and analysis by thermal ionisation mass spectrometry of 26Mg and 41K to assess mineral uptake in Scots pine (Pinus sylvestris L.)

Abstract

Use of ²⁶Mg and ⁴¹K as tracers allows the quantification of the uptake and internal cycling of Mg and K in plants. Application of thermal ionisation mass spectrometry (TIMS) used for a plant experiment is discussed here. Firstly, the sensitivity of the ²⁶Mg/²⁴Mg ratio to the amount of Mg loaded onto the mass spectrometer filament was assessed. Using NIST SRM-980 and amounts of Mg from 0.2 to 1.2 μg, no significant difference in the ²⁶Mg/²⁴Mg value after correction for isotope fractionation was observed. Analysis of SRM-980 produced a corrected mean ²⁶Mg/²⁴Mg value of 0.13960 ± 0.00006 (n = 10) close to the certified range (0.13932 ± 0.00026). Control of fractionation during K analysis by TIMS is important for accurate isotope determinations. Fractionation profiles for NIST SRM-985 using filament loadings of 1 and 5 μg K were plotted and, with the higher loading, produced a more stable ³⁹K/⁴¹K value. Conversion of K from a chloride to an iodide had no significant effect on the measured ratio. The SRM-985 mean ³⁹K/⁴¹K value was 13.916 ± 0.034, higher than the certified range (13.8566 ± 0.0063). Analysis of natural ²⁶Mg and ⁴¹K levels in needles, stem wood, stem bark, fine roots and coarse roots from Scots pine allowed the precision of the analysis to be defined. This information, in conjunction with a simple model, was used to discuss the design of a tracer study in plants using ²⁶Mg and ⁴¹K. Predicted whole tree ³⁹K/⁴¹K and ²⁶Mg/²⁴Mg values from a 95 day experiment were then calculated and compared with the actual values measured using TIMS.