BACKGROUND With the comprehensive development of the agricultural ecological geological survey and the improvement of people's attention to the ecological environment in our country, the demand for biogeochemical reference materials in biological analysis food analysis and agricultural products analysis is increasing. The existing biogeochemical reference materials are in short supply, and there is an urgent need to develop new reference materials. With the full deployment and development of agro-ecological geological surveys, the demand for biogeochemical reference materials is increasing.

OBJECTIVES To reproduce tea composition reference materials (GBW10016a & GBW10052a) to take the place of old tea composition reference materials (GBW10016 & GBW10052).

METHODS According to the development needs of geological surveys, the focus of this paper is on the preparation of two tea reference materials, GBW10016a and GBW10052a. The progress includes sampling, processing and preparation, particle size test, uniformity test, stability test, content test, and calculation of identification value and uncertainty.In the sampling process, the sample collection locations are Wuyishan City, Fujian Province and Wuyuan County, Shangrao City, Jiangxi Province. The candidates are oolong tea and green tea. The tea samples both weigh 400kg.In the processing and preparation, the samples were cleaned, freeze-dried, heated and dried, ball milled, screened, sub-packed and inactivated. In the particle size test, the BT-9000s laser particle size distributor was used to test the particle size of the samples. The particle size of GBW10016a & GBW10052a centrally distributed in the range of 75μm to 200μm, and the cumulative distribution content reached 98% in the range of 175μm to 200μm.In the uniformity testing process, 15 bottles of samples were randomly selected for analysis of more than 30 elements. The analysis results show that the relative standard deviation of the elements is less than 7%, the measured values F in the variance test are less than the critical value F. It shows that the uniformity of the samples is acceptable.In the stability testing process, the short-term and long- term stability of the samples are good according to the analysis and testing in different time periods.In the content testing process, a total of 61 indicators were analyzed by accurate and reliable methods such as inductively coupled plasma-mass spectrometry/optical emission spectrometry (ICP-MS/OES) and atomic fluorescence spectrometry (AFS) in multiple laboratories.In the process of calculation of identification value and uncertainty, the Grubbs algorithm and the Dixon algorithm were carried out to check and exclude individual out-group data in the group at the first step. The Cochrane algorithm was used to carry out precision testing at the second step. The Shapiro-Wilk algorithm was used to complete the data normality test.The whole preparation process of the tea reference materials follow three specifications: Technical Norm of Primary Reference Materials (JJG 1006—1994), General and Statistical Principles for Characterization of Reference Materials (JJF 1343—2012) and The Production of Reference Materials for Geoanalysis(JJF 1646—2017).

RESULTS GBW10016a (GSB-7a) and GBW10052a (GSB-30a) tea reference materials are successfully reproduced, and have been approved as national first-class reference materials. The GBW10016a (GSB-7a) and GBW10052a (GSB-30a) are characterized by strong representativeness, multi-component planting, and accurate and reliable customization results. A total of 61 indicators are involved in this reproduction: Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Gd, Ge, Hf, Hg, Ho, I, K, La, Li, Lu, Mg, Mn, Mo, N, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, S, Sb, Sc, Se, Si, Sm, Sn, Sr, Tb, Th, Ti, Tl, Tm, U, V, Y, Yb, Zn. GBW10016a (GSB-7a) contains 54 elements with certified values and uncertainties, and 3 elements with reference values. GBW10052a (GSB-30a) contains 57 elements with certified values and uncertainties, and 3 elements with reference values. For GBW10016a (GSB-7a), Ag and Sn are added, and Ge and Tl are upgraded from the reference values of the original batch (GBW10016) to the certified values. For GBW10052a, Al, Ag, Cl, I, N, S, Sb, Sc, Si and Sn are upgraded from the reference values of the original batch (GBW10052) to the certified values. Due to the current level of analysis technology and objective conditions, the elements of Hf and Nb cannot be given reference values.

CONCLUSIONS The reference materials effectively support the needs of geochemical investigation and evaluation of the agricultural ecological environment, analysis of biological samples and food and agricultural products. With the development of ecological environmental research and food safety and hygiene inspection, people pay more and more attention to the characteristic elements, and the requirements are increasing. Problems still exists in the analysis of biological samples, such as discrete analysis results of the same method between laboratories and systematic errors among the main analysis methods of individual elements. There are technical difficulties in the analysis of some elements. Analysis and testing technology needs to be continuously improved to cope with the characteristics of a special biological sample matrix, low composition content and difficult testing.