Lemna minor (Aquatic plant, duck weed)
The plant selected for the candidate reference material, duckweed (Lemna minor) is small sized, floating species with small leaves (2-3 mm diameter) organised in disc-shaped form, with a single root of 1-3 cm length. There is no contact of the roots with the sediment, but there is the possibility of adsorption of organic and inorganic suspended matter on the surfaces of the roots and leafs. Lemna minor prefers stagnant fresh water areas. The duckweed was sampled by LSA in canals of the Beemster polder (The Netherlands). In order to prevent metal contamination, all materials in contact with the samples were metal free, i.e. sampling sieves, containers, sorting basins, crusher and storage bags were of polythene, sampling rods were made of glass-fibre-epoxy, and the collection sieves were of nylon. Sampling took place in September 1996. The plants were collected from the water surface using 5 mm mesh sieves connected to 4 m long rods. After leaking out of most of the adhering water the duckweed was collected in pre-cleaned plastic 120 L containers. The plant material was then transported to a laboratory container for cleaning and preparation. The handling consisted of the manual separation of the plant material from non-plant materials such as small branches and leaves, aquatic snails and silt (sediment particles, suspended organic matter, decomposed plant material). The collected plant volume in each of the 120 L containers was separated into sub-samples in containers with local surface water in order to allow a first sedimentation and separation of heavy particles. From these containers, sub-samples of about 2 L plant material were transferred into water basins of 2 m long, 25 cm wide and 8 cm deep. This small amount of plants easily spreads over the water surface and simplifies the manual picking out of all non desired material. At the same time, sedimentary particles are allowed to settle. The water of these basins was replaced frequently with local surface water in order to reject the suspended, colloidal and partly deposited fractions. The thus pre-cleaned plant material was dewatered by sieving (nylon sieves, 1 mm mesh) and again washed in local fresh water in sedimentation basins. These washings/sedimentation steps had to be repeated 3-5 times, depending on the suspended sediment loads found in the samples. The decision on termination of the washing process was made by visual inspection of the water. After the final washing step a sub-sample of about 10 kg was filled into a nylon sieve bag and predried by centrifugation. The resulting material was then transferred into a thermo-insulated 60 L polythene container and shock-frozen with liquid nitrogen. This step prevented possible degradation of the plant material. The volume of the frozen plants was reduced by manual crushing and the 14 product was finally filled into 6 L polythene bags. The total mass of the prepared material resulted in 680 kg wet-weight. The frozen plants were transported deep-frozen to the IRMM for further treatment and bottling of the materials. There the material was air dried in an oven at 60 °C for 60 h, using a temperature controlled programme. This was followed by jet-milling and ultra fine classification of the powders. The top particle size was set at <180 µm. After homogenisation in a Turbula mixer, the material was bottled in 100 ml well cleaned brown glass bottles with polythene inserts and plastic screw caps (about 10 g of powder). Before closing, the vials were filled with argon. The moisture content was determined at 1.02 ± 0.21 % (mass fraction, Karl Fischer, n=20). The particle size analysis showed a top particle size of 90 mm (Kramer et al., 1998).
keywords: environmental material , biological material , plant material, trace elements , heavy metals , rare earth elements (REEs)