Akimasa MASUDARare earth element (REE) determinations with high accuracy and precision were performed on four geological standard rocks (JA-l, JB-l, -2 and -3) by inductively coupled plasma-mass spectrometry (ICP-MS). In order to achieve this, internal standardization, matrix matching or separation of REE from the matrix elements are recommended. In these analyses, REE data obtained by ICP-MS show excellent agreement (within 5%) with those obtained by isotope dilution method using thermal ionization mass spectrometer. Among these analytical methods of ICP-MS, the most reliable data can be obtained by analysis using a sample solution free from matrix elements. Mass spectrometry using inductively coupled plasma as an ion source (ICP-MS) is a new technique for elemental and isotopic analysis.'-5 Simultaneous multielement determination of trace elements can be performed by ICP-MS, giving simple spectra and excellent low limits of detection. These advantages are promoting wide application of ICP-MS to various analytical studies such as trace element determination in geochemical samples. [6][7][8][9][10][11] Rare earth element (REE) determinations in geochemical samples have also been carried out by ICP-MS. Date and Gray' reported the measurement of REE in three standard reference silicate rocks (SY-3, GSP-1 and NIM-G) with no pretreatment except removal of silica during acid digestion. They showed relatively good agreement between REE abundance data obtained with ICP-MS and previously published data, the differences being generally 10 -40%. Longerich et al." studied the effects of ICP-MS instrumental parameters on the determination of REE and they determined REE abundances in standard rocks BCR-1 and SY-2 without preconcentration. REE abundances reported by them are 10 -20% lower than the accepted values in most cases. Furthermore, Lichte et al.12 presented REE abundance data using ICP-MS with deviations of 2-50% from those obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) and neutron activation analysis (NAA). In their analysis by ICP-MS, samples were analyzed directly following an acid digestion and Cd was used as an internal standard element.Although these earlier papers have given an indication of the effectiveness of the ICP-MS technique for REE analysis, more precise data on REE are necessary for the strict interpretation of REE patterns in geochemical samples.Thus, in this paper, we perform REE determinations with ICP-MS by four different analytical approaches described later, aiming at establishment of a reliable analytical method.The REE data obtained by ICP-MS are compared with those obtained by a thermal-ionization mass spectrometric isotope dilution technique (TIMS-ID), which is currently considered to be a "bench mark" technique for REE analysis.