**2.3. Sample preparation**

Plant samples were washed with tap water and then with distilled water to remove debris and surface contamination. Samples were then bulked and air dried to remove excess moisture. Similarly, samples of the soils collected were bulked into composite samples and air dried for three days (72 hours).

Dried plant and soil samples were ground using a porcelain mortar and pestle and sieved to attain a uniform particle size. Each sample was put in a small transparent polythene bag and labeled.

## **2.4. Metal analysis**

Analysis of the elemental content of the samples was done using the Energy dispersive X-ray fluorescence Spectroscopy (EDXRF) method [21].

The samples were ground manually to powder with an agate mortar and pestle to grain size of less than 125µm. Pellets of 19mm diameter were prepared from 0.3-0.5kg powder mixed with three drops of organic liquid binder and pressed at 10 tons of pressure in a hydraulic press.

Measurements were performed using an annular 25mCi 109Cd as the excitation source, that emits Ag – K rays (22.1 KeV) in which case all elements with lower characteristic excitation energies are accessible for detection in the samples. The system consists furthermore of Si (Li) detector, with a resolution of 170eV for the 5.90KeV line, coupled to a computer controlled ADC – card.

Quantitative analysis of the sample was done using the Emission Transmission (E-T) method and that involves the use of pure target material (Mo) to measure the absorption factors in the sample.

The Mo target served as a source of monochromatic X-rays, which are excited through the sample by primary radiation and then penetrate the samples on the way to the detector. In this way, the absorption factor is experimentally determined which the program uses in the quantification of concentration of the elements. In addition, the contribution to the Mo-K peak intensity by the Zr-K is subtracted for each sample.

Sensitivity calibration of the system was performed using thick pure metal foils (Ti, Fe, Co, N, Zn, Nb, Zr, Mo, Sn, Ta and Pb) and stable chemical compounds (K2CO3), CaCO3, Ce2O3), WO3, ThO2, U3O3). The spectra for the samples were collected for 3000s with the 109Cd source and the spectra were then evaluated using the AXIL-QXAS program [22]. 109Cd source was used for the analysis of K, Th, Y, Zr, Nb and Mo.

The accuracy and precision of the measurements was confirmed though an analysis of IAE – V10 (hay powder) and IAEA – 259 (cabbage) certified reference material, distributed by International Atomic Energy Agency (IAEA).
