Influence of Milling or Cutting Procedures on Trace Element Contents of Plant Samples

Abstract

Several milling and cutting procedures have been tested for their suitability in trace element analysis of non - contaminated plant samples. Barley and wheat were selected because of their low trace element contents, and lucerne in order to test cutting with rotating steel knives. Grains and lucerne plants were distributed as a ring test, in order to yield processed and homogenized samples ready for decomposition and determination. The samples were collected and analyzed for their trace element contents in one lab, in order to avoid differences due to systematic errors of analysis. The trace elements of interest were chosen to detect abrasion of the mills (Fe, Cr, Ni, Mo, Co), contamination from dust (Pb, Cd), and cross contamination from animal feed premixes (Cu, Mn, Zn), which are often processed in the same labs as well. Whereas the results for Mn, Mo, and Cd approached Gaussian frequency distribution, the results for Cr, Ni, and Fe were clearly asymmetrically distributed, presumably due to many contaminated samples. In order to recognize outliers for asymmetric frequency distributions, construction of a symmetrical frequency distribution around the median is proposed, to which Gaussian statistics was applicable. The precision of the subsequent analytical procedure was evaluated with the same, barley sample and among the same labs in a ring test one year afterwards. As a result, the precision of the median- corrected frequency distribution of milling/cutting and of the analysis were within then same range, whereas the scatter of data from elements prone to abrasion like Fe, Cr, Ni, was much larger from the original data set. As sources for contamination, bad maintenance of the milling room, as well as abrasion of rotor and sieve from continuously operating mills (stainless steel - titanium) could be recognized. There was no significant difference between continously and discontinously operating devices, nor between different modes of cleaning, like washing, discarding the first portion, or just blowing and brushing.