In the analysis of the concentration of trace elements in biological material there are many possible preanalytical sources of error. Precision and accuracy of trace element analyses are not so much determined by the analytical procedure--atomic absorption spectrometry--as by individual biological factors, contamination and loss of elements during sampling and sample preparation, influences resulting from the calibration procedure, or from the analytical equipment itself. Genetical factors like sex or race, ecological factors like age or pregnancy and short term factors like food intake, parenteral feeding or drug therapy can influence the final results. Different blood sampling methods with different duration and strength of compression may lead to hemolysis resulting in falsely elevated values of trace elements. Needles, syringes and tubes contribute to contamination by leaching trace elements from their walls or lead to loss of material by adsorption to the surface of the walls. As there is still no primary standard in order to guarantee precision and accuracy the calibration procedures have to be compared individually depending on the different matrices of the sample.