PURPOSE: To achieve a stable and continuous measurement by preventing a slightly soluble salt from being generated, and filter from being clogged by using an eluent containing pyromellitic acid.

CONSTITUTION: An ion chromatography system which is left to stand outdoors can be easily contaminated as compared with one in a laboratory, an eluent component and a cation such as calcium may react with each other to easily generate a slightly soluble salt, column and filter are clogged, and the pressure of the column, etc., increases, thus making it difficult to perform a stable and continuous measurement for a long time. Therefore, by using each eluent containing tartaric acid and pyromellitic acid, the separation performance in the case of pyromellitic acid is nearly equal to or more than that of tartaric acid. Also, in the case of pyromellitic acid, no column pressure change is generated, even when rainwater is measured repeatedly 300 times. A retention time tends to become shorter along with the deterioration of the column. However, in the case of the pyromellitic acid, a retention time is stable for a time exceeding 3 months and the deterioration of the column can be suppressed by the use of the pyromellitic acid eluent.