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BACKGROUND Soil environmental reference materials are important technical tools for the quality control of soil environmental monitoring. However, there is no self-developed environmental matrix reference material with total organic carbon (TOC). In particular, a soil standard sample supporting the combustion oxidation-non-dispersive infrared method has not been developed yet. OBJECTIVES To develop a soil TOC reference material that can support standard methods for the determination of TOC in soil. METHODS Agricultural land soil with a high organic carbon content was selected as the raw material. The soil TOC reference materials were prepared after drying, grinding, mixing, bottling, and sterilization. Ten bottles of the reference materials were randomly sampled in layers, and their homogeneities were determined using combustion oxidation-non-dispersive infrared (NDIR) absorption method. The data were assessed using a single element analysis of variance. The reference material was stored at room temperature, and its stability was determined for 18 months. The test results were assessed using a linear fitting model. Eleven laboratories were organized as the cooperation to determine the TOC content of the prepared reference material using combustion oxidation-NDIR absorption method and potassium dichromate volumetric method. All measurement data were statistically analyzed to obtain the certified value and uncertainty of the reference material. RESULTS The soil TOC reference material exhibited good homogeneity and stability. After evaluation, the F value was determined to be less than the critical value of F0.05 (9, 10). Additionally, the relative uncertainty (ubb) of the uniformity between bottles was 1.5%, and the sample exhibited good uniformity. The stability uncertainty was 1.2%, which indicated good stability. The certified value of the soil TOC reference material was determined to be (25.2±1.4)mg/g, using the grand mean of the collaborative determination results from multiple laboratories. CONCLUSIONS A soil TOC reference material was developed using combustion oxidation-NDIR absorption method. This reference material can be used to support the standard determination methods of TOC in soil and meets the requirements of the studies on TOC content in soil. Moreover, it is comparable with similar foreign samples.

In China, the mining district and surrounding areas have been high risk areas for soil pollution by heavy metals. These are also the main areas for environmental monitoring of soil. Unfortunately, soil certified reference materials (CRMs) in which matrix and levels of heavy metals consistent with the actual soil in thse aeras are not available. In this study, based on the pollution characteristics of the heavily polluted soil area and high-risk area, and requirements of practical environmental monitoring, agricultural soil surrounding the mining district of red soil area in southern China was collected and prepared to soil certified reference materials according to GB/T 15000 series rules. The collected material was dried at low temperature, ground, sieved, homogenized and radiation-sterilized. Twenty bottles of samples were randomly chosen in order to test the homogeneity. Results show that standard uncertainty due to between-bottle homogeneity ranges from 0.27% to 2.1% (in Table 2), indicating a good homogeneity. The minimum sampling weight was 0.2 g. X-ray Fluorescence Spectrometry method was used for long-term stability study. Stability was assessed by trend analysis, under the preservation conditions of room temperature without direct sunlight. No instability was observed during 12-month monitoring periods, and standard uncertainty due to long-term stability ranged from 0.06% to 2.1% (in Table 3). Elemental contents were determined by 13 laboratories using two different methods. The elements include Ag, Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Se, Ti, Tl, V and Zn. Total uncertainty results from uncertainties in certificated values (uchar), between-bottle homogeneity (ubb) and long-term stability (ults). The expanded relative uncertainty of most elements is smaller than 7%, with the exception of Se. The reference material was approved as the national reference material (GSB 07-3272—2015). The presented CRMs were expected to be useful for method validation or quality control of heavy metal analysis in mining districts and surrounding areas by routine testing laboratories.

Matrix-type certified reference materials (CRMs) prepared from real environmental samples are used to ensure the reliability of environmental analyses and the comparability of results from different laboratories. Internationally, the National Institute of Standards and Technology (NIST) in the USA, the Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC) in the European Union, and the UK Laboratory of the Government Chemist (LGC) have developed sediment CRMs for Polycyclic Aromatic Hydrocarbons (PAHs). Although some matrix-type CRMs are available for the quantification of PAHs in sediment and soils, their PAH contamination is mostly at the mg/kg level. Thus, they are not suitable for the QA of analyses of PAHs at the μg/kg level. In China, by restricting preparative techniques and value-assignment techniques of natural matrix CRMs, no CRMs of PAHs in sediment have been developed. Therefore, based on the foreign research experience, CRMs of PAHs in river sediment have been developed for Chinese environmental monitoring and scientific research. A method for developing CRMs of PAHs in sediment is described in this paper. Sediment sample was collected from Songhua River, and air-dried, ground, homogenized and sterilized. A good homogeneity was obtained by the testing for 16 kinds of PAHs from 15 units selected by a random stratified scheme covering the whole batch, and 3 sub-samples were analyzed in each unit. At 30℃ without direct sunlight, the concentrations of 16 PAHs were not changed during monitoring period of 16 months. Based on the results from difference analytical techniques including Gas Chromatography/Mass Spectrometry and Reversed-phase Liquid Chromatography by 11 collaborating laboratories, reference values for 14 PAHs are given, and reference values for 2 PAHs are in the range of 6.9-158 μg/kg. Expanded uncertainty of each PAH is 1.5-25 μg/kg. Since the values of PAHs in the CRM are much lower than those in other CRMs and are comparable to the actual PAHs content in sediment or soil, the CRMs are suitable for PAH monitoring in sediment and soil samples and method validation.