Elemental concentrations (Ca, Cs, K, Mg, Sr) in a range of crops and associated soils from the UK and Spain

Prior to analysis crop samples were freeze-dried, re-weighed and then finely ground (<2mm, using either an agate mill or a pestle and mortar); soil samples were air (~20 oC) or oven dried (~60 oC) prior to being finely ground (<2mm, using an agate mill).

In preparation for ICMPS/ICPOES analysis the finely ground soil and crop samples were re-weighed and digested in aqua regia reagent (3:1 mixture of concentrated HNO3 and HCl ‘Aristar’ grade acids) at 175°C for 12 minutes using a microwave digestion system (CEM, MarsXpress). A second sub-sample of the soil samples was also extracted using 0.1M BaCl2 followed by centrifugation (see Lofts et al. 2001. https://doi.org/10.1046/j.1365-2389.2001.00358.x). Filtered (0.45 µm pore size) and acidified (1% v/v HNO3, ultra-pure) pore waters and irrigation waters were analysed directly. All samples were then diluted 100-fold prior to analyses. Analysis of the digests, extracts and pore waters was conducted by ICPMS for Cs and Sr (Perkin Elmer, NeXION 300D) or ICPOES for Ca, K and Mg (Perkin Elmer, Optima 7300 DV).

ICPMS and ICPOES quantitative measurements were made using external calibration after spiking the digests with 10 µg l-1 Ga, In and Re as internal standards to compensate for instrumental drift and matrix effects. ICPMS and ICPOES instrument limits of detection (LOD) were calculated by using the mean and three standard deviation measurements for the digestion reagent blank. Total method LODs for each element were then calculated to take account the sample mass and the dilution arising from the digestion procedure.

A Shimadzu TOC-L analyser, equipped with a TNM-L module, was used to measure NPOC. Before analysis, samples were acidified with 1M HCl and then purged with Zero grade air for six minutes to remove any inorganic carbon. The sample was then injected into the analyser and the remaining carbon, as NPOC, measured by combustion at 720°C, with a catalyst, which converted all carbon to carbon dioxide. The carbon dioxide was measured by an infra-red detector.

Data were recorded into Microsoft Excel workbooks from laboratory notebooks and instrument reports. A proportion of the data (approximately 20%) has been validated by somebody other than the person making the initial entry.