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.