BACKGROUNDOne Rb-Sr certified reference materials (GBW04411) and one Sm-Nd certified reference materials (GBW04419) for geological age, which are potash feldspar and basalt respectively, were produced and certified about 30 years ago. They have only one certified value and their matrices differ from many other geological samples. To better assess the quality of chromatographic separation, measurement procedures and mass spectrometry performance when analyzing Rb-Sr and Sm-Nd, three CRMs which are peridotite, eclogite and granite for Rb-Sr and Sm-Nd analysis were produced, and labelled GBW04139, GBW04140 and GBW04141 respectively.

OBJECTIVESTo prepare certified reference materials for analysis of Rb-Sr and Sm-Nd isotopes of different types of rocks.

METHODSPeridotite, eclogite and granite from typical areas in China were used as candidates. Referring to 'Determinations for isotopes of lead, strontium and neodymium in rock samples (GB/T 17672-1999)', 'Determination of Rb-Sr isotopic geological age and Sr isotope ratio in rocks and minerals (DZ/T 0184.4-1997)', 'Determination of Sm-Nd isotopic geological age and Nd isotope ratio in rocks and minerals (DZ/T 0184.6-1997)', Rb, Sr, Sm and Nd were purified by chromatographic separation, and their concentrations were analyzed by ID-TIMS, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios were determined by TIMS and MC-ICP-MS, respectively.

RESULTSFor homogeneity testing of the three CRMs, fifteen bottles of each were randomly selected (from the 200 bottles prepared) for analysis. F-testing was used to study homogeneity. The result was insignificant (1 < F < F_critical (v_among, v_within), demonstrating that the three CRMs had very good homogeneity. On the basis of the homogeneity study, the minimum sample required to ensure homogeneity was 1g for Peridotite, and 0.3g for Eclogite and Granite. The long-term stability of the samples was evaluated five times over 26 months. A linear model was used as a basic model for evaluating stability of Rb, Sr, Sm, Nd concentration and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd. Given that|b₁| < t_{0.95, 3}×s(b₁), the slope was insignificant and no instability was observed. The certified value was calculated from the unweighted means of the results submitted by the participating laboratories, including Institute of Geology, Chinese Academy of Geological Sciences; Institute of Geology and Geophysics, Chinese Academy of Sciences; Tianjin Center, China Geological Survey; School of Earth and Space Sciences, University of Science and Technology of China; Wuhan Center, China Geological Survey; Beijing Research Institute of Uranium of Geology; School of Earth Sciences and Engineering, Nanjing University; First Institute of Oceanography, State Oceanic Administration of China; and National Research Center for Geoanalysis. Uncertainties associated with batch characterisation (u_char), possible between-bottle variations (u_bb or u_bb') and those derived from effects related to long-term storage (u_s) can be expressed as standard uncertainties and combined as follows: U_\rmCRM=k \times \sqrt u_\rmchar^2 + u_\rmbb^2 + u_\rms^2 A coverage factor of k=2 was used. Certified values and uncertainties of GBW04139, GBW04140 and GBW04141 were shown in the following table. The accuracy of isotope ratio reached or was better than that of similar standard materials.

CRMs and their codes	Certified value±UCRM
	Rb(μg/g)	Sr(μg/g)	87Sr/86Sr	Sm(μg/g)	Nd(μg/g)	143Nd/144Nd
Peridotite certified reference material for Rb, Sr, Sm, Nd concentration and 87Sr/86Sr, 143Nd/144Nd (GBW04139)	0.17±0.02	12.7±0.5	0.704465±0.000067	0.10±0.02	0.36±0.03	0.512647±0.000081
Eclogite certified reference material for Rb, Sr, Sm, Nd concentration and 87Sr/86Sr, 143Nd/144Nd (GBW04140)	4.1±0.2	563±33	0.704915±0.000052	3.3±0.3	12.6±0.7	0.512264±0.000026
Granite certified reference material for Rb, Sr, Sm, Nd concentration and 87Sr/86Sr, 143Nd/144Nd (GBW04141)	64±3	382±13	0.713109±0.000071	2.4±0.2	15.1±0.7	0.511142±0.000023

CONCLUSIONSAll the rocks used for CRMs were collected from typical regions of Peridotite, Eclogite and Granite, the matrix were consistent with the geological samples. The three CRMs can meet the requirements of Rb-Sr and Sm-Nd analysis for rock samples.