This report is intended to be the 7th deliverable report of the project 21GRD09 MetroPOEM describing the production of UME MetroPOEM CRM, a seawater material certified for the isotope amount ratios of selected elements: B, Cd, Li, Ni, Pb and U. The certified reference material was produced in accordance with the requirements of ISO 17034:2016 standard [1]. The collection of starting material, 250 L of seawater, was conducted at the station UE67, which was located at the edge of German EEZ in the North Sea (55° 25’ N; 4° 5’ E). The sample was filtered before filling into pre-cleaned 25 L HDPE carboys and acidified using subboiled HNO3 for the stabilisation. The carboys were sealed and individually packed in clean PE foil, and stored at 4 °C prior to transport to TÜBİTAK UME by Hereon for further processing. After preliminary analysis, the material was spiked with the elements whose mass fractions were below the target levels to obtain the material having suitable concentrations to be measured in standard analytical laboratories. After filling into pre-cleaned 250 mL polypropylene bottles, the whole batch was sterilized with 25 kGy -radiation by 60Co source. Between-unit homogeneity and stability during dispatch and storage conditions were assessed in accordance with ISO 33405:2024 standard [2] by 21GRD09 MetroPOEM Project [3] partners in a collaborative manner. The characterisation was conducted by an interlaboratory study where, in addition to the project partners, several expert laboratories were also involved in isotope ratio measurements. Uncertainties of the characterisation measurements were estimated in compliance with the Guide to the Expression of Uncertainty in Measurement (GUM) [4]. Certified values include uncertainties arising from possible inhomogeneity, instability and characterisation measurements. The material is intended to be used for calibration or quality control purposes in laboratories performing isotope ratio measurements.