Solid-phase extraction for metabolomic analysis of high-salinity samples by capillary electrophoresis-mass spectrometry

Environmental samples such as soil solutions contain inorganic ions such as NH4(+), K(+), Na(+), NO3(-), and PO4(3-) in high concentrations, which must be removed prior to capillary electrophoresis-mass spectrometry analysis to obtain accurate results. However, the separation of these inorganic ions from ionic metabolites, which are the target compounds in capillary electrophoresis-mass spectrometry analysis, is difficult because the physicochemical properties of the inorganic ions are similar to those of the ionic metabolites. In this study, we used various solid-phase extraction (SPE) columns for the purification of the samples containing inorganic ions in high concentrations. We found that cation-exchange SPE columns successfully filtered out the inorganic ions while retaining most of the organic compounds, which were easily collected with high recovery rates. In addition, 17 cationic metabolites in the soil solution were quantified by CE-MS analysis following the SPE purification process. The results suggest that our method can be used to analyze other environmental samples containing inorganic ions in high concentrations.