Research Progress of Geological Reference Materials in China

Geological reference material is a primary standard to ensure the accuracy of geological sample analysis results. After more than 40 years of development, China has developed and certified thousands of geological reference materials covering different media such as rock, ore, soil, sediment, organism, and water. China’s metrological management department divides the certified reference material into two classes according to the classification class, the first-class reference material (GBW) and the second-class reference material [GBW(E)]. According to their attributes and application fields, they are divided into 13 categories^[6-7], and the geological reference materials belong to the seventh category [GBW07 and GBW(E)07]. It can be seen from the National Sharing Platform for Reference Materials (https://www.ncrm.org.cn) that by the end of the 13th Five-Year Plan, the total number of geological reference materials approved by the State Administration for Market Regulation is 1013, including 718 first-class certified reference materials, and the amount of first-class certified reference materials ranks first in the 13 categories (Fig.1). Geological materials are the most important and basic raw materials in the development of human society, with various types, complex components, and large differences in component content. It is because of the complexity of geological materials and the demand for reliable quantitative analysis that thousands of geological reference materials have been developed. Some reference materials developed by some industrial geological agencies are classified into other categories in classification management, such as isotope reference materials belonging to the fourth category, biological composition reference materials belonging to the tenth category, coal reference materials belonging to the eleventh category, these reference materials are widely used in related geological work, and in this paper they will be classified as geological reference materials. To systematically grasp the development status of geological reference materials in China, the first-class geological certified reference materials are comprehensively sorted, and are divided into six categories according to the type of matrix, property value and application scope: basic, ore, marine, environment, energy and special purpose (Table 1), and the development situation of each type, matrix characteristics and fixed value index respectively are summarized. On the basis of summarizing the results, on one hand, the main problems existing in the application of geological reference materials and the causes of these problems are analyzed. On the other hand, the detailed technical problems that may affect the uncertainty in the process of homogeneity, stability and characterization are discussed, and the views and suggestions are expounded.　　In this paper, the first-class geological certified reference materials are divided into the following six categories to summarize the development of each type: 　　(1) 260 basic geological reference materials have been developed, the type of matrix including rock, soil and river sediments. The characteristic components of the certified value are generally 70 elements and compounds, which are mainly used as monitoring standards for sample analysis in basic geological surveys and research.　　(2) 272 ore geological reference materials have been developed, including precious metal ore, metal ore, single mineral, and non-metallic ore. The specific mineral types are shown in Table 1. There are generally 20 property values, which are the main technical indicators in mineral exploration. They are mainly used as a monitoring standard for geological exploration, mineral processing and smelting, comprehensive utilization, commodity inspection and trade amongst other fields.　　(3) 16 marine geological reference materials have been developed, including marine sediments and marine minerals. The specific samples include 10 marine sediment reference materials namely, 1 near sea, 1 Yellow Sea, 2 South China Sea, 1 East China Sea, 3 deep Pacific Ocean, 1 Antarctic Ocean and 1 Arctic Ocean. Six marine mineral reference materials are three polymetallic nodules and three ocean cobalt-enriched crusts. The characteristic composition of the certification value is the same as that of the basic geological reference material, but slightly less, generally 51-71 kinds. This is a necessary chemical composition measurement standard for marine sediment measurement and marine mineral resource survey and research.　　(4) 101 standard materials for environmental geology have been developed, including the available state of soil elements, the form of soil and sediment elements, the valence state of groundwater elements, the extractable state of soil elements, biological inorganic elements, organic pollutants in soil and sediment and other reference materials. The characteristic components of the matrix and certified values are the total amount of inorganic elements in biological samples, the available states, and forms of elements in soil and sediment, the valence states of elements in water and organochlorine pesticides, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons in soil and sediment. This kind of reference material provides analytical quality monitoring standards for land quality geochemical surveys and evaluation, regional ecological geochemical evaluations, multi-objective geochemical surveys and detailed surveys of soil pollution.　　(5) 23 energy geological reference materials have been developed. The property values of oil-generating rock reference materials are organic carbon, pyrolysis hydrocarbon S₂, pyrolysis peak temperature T_max, and chloroform asphalt “A”. The property values of coal reference material are calorific value, ash, volatile matter, total sulfur, carbon, hydrogen, nitrogen, true relative density and phosphorus, chlorine, fluorine, and arsenic. The property values of the reference materials of highly evolved hydrocarbon source rock are pyrolysis hydrocarbon S₂, pyrolysis peak temperature T_max, and total organic carbon TOC. This is the monitoring standard for the exploration, evaluation and rational utilization of energy and mineral resources.　　(6) 129 reference materials for special purposes have been developed, including 20 synthetic reference materials for emission spectroscopy and 4 synthetic reference materials for X-ray fluorescence spectroscopy analysis. The property values are 28-29 inorganic chemical elements. There are 38 isotope reference materials, and the property values of those include isotope abundance and abundance ratio. There are 35 electron probe reference materials, and the property values are generally two principal components of their minerals. There are 15 reference materials for phase analysis, and the property values are the corresponding main phase. There are 5 soil limit water content reference materials, and the property values are 10mm liquid limit, plastic limit, and plastic index. There are 12 soil pH reference materials, and the property values are pH. It can be seen from the classification and summary that the geological reference material matrix in China is rich in types and quantities, which establishes an influential quality monitoring system for geological sample analysis, ensures the reliability and consistency of geological sample test data, and significantly improves the comparability and scientific value of relevant data.　　At present, there are still some problems in the application and development technology of a large number of geological reference materials. The problems are discussed and analyzed, hoping to draw attention to improving the value determination class of geological reference materials. In the aspect of application, the specific problems existing in different types of geological reference materials and their causes at the present stage are analyzed and development suggestions are proposed, according to the work needs of current mineral resources, international cooperation, marine strategy, ecological civilization and other fields. In the aspect of development technology, some detail technologies which may affect the value determination class in the process of homogeneity, stability and characterization are analyzed. In the aspect of development technology, some detail technologies which may affect the setting in the process of homogeneity, stability and value setting are analyzed. The test method of homogeneity and the evaluation method of stability uncertainty are discussed. Suggestions are made for the uncertainty evaluation method of unexamined elements for homogeneity test and control limit of relative extended uncertainty.　　1. Three problems existing in the current application of geological reference materials are analyzed: 　　(1) The quantity of some types of reference materials developed is small, the gradient distribution of characteristic values is insufficient, and the matrix type is simple. For example, the amount of marine geology, micro-area analysis, energy and mineral resource reference material development is small. The ore-forming elements of key metal ore reference materials are mainly low content, lacking rich ore grade or higher content grade. The matrix types of the samples of organic pollutant reference materials are only soil and sediment, and the property values are only organochlorine pesticides, polychlorinated biphenyls, polycyclic aromatic hydrocarbons and other compounds, and the matrix types and target compounds are few. The main reasons for these problems are difficulty in obtaining samples, little working demand, difficulty in ensuring the uniformity and stability of the sample preparation technology, large error of analysis technology unable to determine the value.　　(2) There are many components in the geological reference materials with mature development technology that have no certified values. The reasons of low element content and test interference are mainly analyzed.　　(3) At the current class of analytical technology, there is only one measurement method for several elements, and for some elements, with the widespread application of modern instrumental analysis technology, the optional measurement method is gradually becoming singular. 　　2. The detail technology of four aspects in the development of reference material is discussed: 　　(1) Uniformity testing method. XRF is a widely used measurement method for the uniformity test of geological standard materials due to its high testing precision. However, due to its large sample size, XRF cannot meet the requirements of minimum sample size. Since the release and implementation of China National Technical Specification for Measurement “General and Statistical Principles for Characterization of Reference Materials” (JJF1343) in 2012, the uniformity of reference materials was tested by ICP-MS and ICP-OES under 0.1g weighing sample^[24-30]. The measurement precision of these two methods is not as good overall as that of XRF, and the accuracy of individual elements is poor, which may cause the numerical value to be too large when calculating the uncertainty introduced by uniformity, thus affecting the reasonable evaluation of the uncertainty of property value. For this reason, some scholars^[34-35] improved the XRF sample preparation device, studied the accuracy and precision of the method under 0.1g weighing sample, and verified the application effect for uniformity testing. However, only 10 major components in the reference materials of soil and stream sediments were tested under 0.1g weighing sample, while the reference materials of other matrix types and other trace elements, especially heavy elements, have not been studied and tested. Therefore, the feasibility of the XRF method with a sample size of 0.1g still needs to be verified.　　(2) Uncertainty calculation method for elements without uniformity test. The composition of geological materials is complex, and its reference substances generally have multiple attribute values, and the content of each component is greatly different, so it is usually difficult to test the uniformity of all attribute values. According to the provisions of JJF1646—2017, the characteristic of representative and less homogeneous should be selected for uniformity evaluation. The uncertainty of the untested elements can be evaluated according to the concentration and geochemical properties with reference to the uncertainty introduced by the tested elements. However, the exact calculation method was not specified. An evaluation method that considers both element properties, content and test methods (Table 2) is presented here. The specific calculation method is to multiply the relative uncertainty (U_r) of the uniformity of the detected element by the standard value μ of the undetected element and take it as the uncertainty component u_bb of the undetected element.　　(3) Evaluation method of stability uncertainty. At present, trend analysis method is generally used to evaluate the stability uncertainty (u_s) of geological reference materials, and the calculation formula is u_s=s(b₁)·t. It can be seen from the formula that the longer the stability monitoring time, the greater the stability uncertainty introduced. The stability uncertainty data of several reference materials were compared and determined to be the main contribution to the total uncertainty, which is inconsistent with the long-term observation of the stability of the geological reference materials. Therefore, this calculation method is not suitable to evaluate the uncertainty introduced into the stability of geological reference material. Meng et al.^[42] suggested using the analysis of variance to calculate, and the comparative analysis showed that u_s was still the main contribution to the total uncertainty. Wang et al.^[43] used range method to calculate u_s, but the application of this method in geological reference materials has yet to be verified. In conclusion, the reasonable evaluation of stability uncertainty of matrix reference materials with more test error sources needs to be studied further.　　(4) Control limits for relative extended uncertainty. The determination of the property value of geological reference materials usually has certain control requirements for relative extended uncertainty (U_rel), but these control limits are rarely introduced in the public information of the reference material. It was not until the specification JJF1646 published in 2017 that U_rel was required in principle. U_rel is the embodiment of the quality of property value data. U_rel control limits in Table 3 are proposed on the basis of the principle requirements of JJF1646—2017 by referring to the control limits in the development of existing reference materials and the analysis of different types of geological reference materials. In the process of development, the developer also needs to comprehensively consider the matrix condition, setting index, element content of the sample and formulate reasonable U_rel control limits of uncertainty in combination with the current analysis technology class, to obtain accurate and reasonable property value and uncertainty.