Research on the Precision Evaluation Standard for Determination of Zn in Soils

BACKGROUND The current technical specification, or standard method, to obtain the precision control level of a certain test item is to send the standard sample and a certain number of actual samples, with a known concentration level, to specific laboratories for comparison testing. OBJECTIVES To provide a more effective, representative and universal precision control evaluation standard. METHODS 871 actual soil samples with different concentration gradients, types (34 kinds) and geographical representation from 31 provinces (autonomous regions and municipalities) were selected and distributed blind to 76 different laboratories. Each sample was subjected to indoor and inter-laboratory parallel comparison tests by 2 to 4 different laboratories. Two standard methods, flame atomic absorption spectrometry (AAS) and wavelength dispersive X-ray fluorescence spectrometry (WD-XRF), which are commonly used in soil ecological environment monitoring in China, were selected. RESULTS When the concentration of Zn is less than 50mg/kg, it is proposed to control RD ≤ 15% in laboratory and RD' ≤ 20% inter laboratories for the open code test samples and standard samples, while RD ≤ 20% in laboratory and RD' ≤ 30% inter laboratories for the blind test samples. In the case of the concentration of Zn is 50-90mg/kg, it is proposed to control RD ≤ 10% in laboratory and RD' ≤ 20% inter laboratories for the open code test samples and standard samples, while RD ≤ 15% in laboratory and RD' ≤ 25% inter laboratories for the blind test samples. When the concentration of Zn is higher than 90mg/kg, it is proposed to control RD ≤ 10% in laboratory and RD' ≤ 15% inter laboratories for the open code test samples and standard samples, while RD ≤ 10% in laboratory and RD' ≤ 20% inter laboratories for the blind test samples. CONCLUSIONS There is no significant difference in the precision control results under AAS and XRF analysis methods. Different soil types may affect the precision control level. The main reason is that different matrix compositions of different soil types lead to different levels of digestion or compaction. Therefore, in actual monitoring work, it is also necessary to consider the difference in requirements of analysis and test conditions for different types of soil and the comparability of test results.