Ion chromatography performances evaluated from the third AQUACON freshwater analysis interlaboratory exercise

This paper describes a collaborative interlaboratory exercise for freshwater analysis held in the framework of the Analytical Quality Control and Assessment Studies in the Mediterranean Basin Project (AQUACON). Two sample types were prepared and distributed: the first for the analysis of pH, conductivity and major ions (calcium, magnesium, sodium, potassium, chloride, sulphate, bicarbonate) and the second for algal nutrients (ammonium, nitrate, phosphate, total phosphorus and total nitrogen, silicate). Two solutions with different concentration levels were prepared for each of the samples; homogeneity throughout bottles and stability of samples in time were tested from the two organising laboratories. Results were obtained from 155 laboratories from 30 countries around the world. A detailed examination of results obtained by ion chromatography showed that an important source of error was the neglect of the non-linearity of calibration curves. Repeatability and reproducibility tests were performed in a selected number of experienced laboratories in accordance with the ISO standard 5725 for anion and cation ion chromatography determination. The best obtainable repeatability and reproducibility limits of IC methods of water analysis are about 5 and 10%, respectively.