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

Use of Mg-26 and K-41 as tracers allows the quantification of the uptake an d internal cycling of Mg and K in plants. Application of thermal ionisation mass spectrometry (TIMS) used for a plant experiment is discussed here. Fi rstly, the sensitivity of the Mg-26/Mg-24 ratio to the amount of Mg loaded onto the mass spectrometer filament was assessed. Using NIST SRM-980 and am ounts of Mg from 0.2 to 1.2 mu g, no significant difference in the Mg-26/Mg -24 value after correction for isotope fractionation was observed. Analysis of SRM-980 produced a corrected mean Mg-26/Mg-24 value of 0.13960 +/- 0.00 006 (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 load ings of 1 and 5 mu g K were plotted and, with the higher loading, produced a more stable K-39/K-41 value. Conversion of K from a chloride to an iodide had no significant effect on the measured ratio. The SRM-985 mean K-39/K-4 1 value was 13.916 +/- 0.034, higher than the certified range (13.8566 +/- 0.0063). Analysis of natural Mg-26 and K-41 levels in needles, stem wood, s tem bark, fine roots and coarse roots from Scots pine allowed the precision of the analysis to be defined. This information, in conjunction with a sim ple model, was used to discuss the design of a tracer study in plants using Mg-26 and K-41. Predicted whole tree K-39/K-41 and Mg-26/Mg-24 values from a 95 day experiment were then calculated and compared with the actual valu es measured using TIMS.