Measurements of marine nutrient concentrations are necessary to investigate global biogeochemical cycles, including in the context of climate change, as nutrients are closely coupled to the carbon cycle. To study these phenomena on a global scale, measurements must be internationally comparable between laboratories and calibrated to a metrologically traceable reference standard. In this chapter, we describe how well-trained chemical analysts following Good Laboratory Practice can meet these requirements. Inter-laboratory comparability is not possible without first achieving consistent and reproducible measurements within a single laboratory. First, the measurement reproducibility must be monitored and kept constant within each session of analysis. Second, this criterion should be maintained from one analysis session to the next, for example by using an internal laboratory reference standard and/or conducting duplicate measurements across different sessions. Third, an internationally accepted Certified Reference Material (CRM) should be used to assess the comparability of the data with other laboratories worldwide. Regular participation in laboratory inter-comparison exercises can also reveal systematic bias, thus helping to improve general data quality. Improving analytical quality involves many aspects, such as calibrating volumetric flasks and pipettes gravimetrically to reduce systematic errors, but also a number of seemingly small procedural improvements that can noticeably improve measurement precision and consistency. Each step in the overall procedure from sampling, calibration and analysis through to data reporting contributes to the precision and accuracy of the results.