Determination of anions in soil solutions by capillary zone electrophoresis

Abstract

Two simple methods have been developed for the direct quantification of low molecular mass aliphatic carboxylates and inorganic anions in soil solutions by capillary zone electrophoresis (CZE), one for di- and tricarboxylates and inorganic anions and the other for monocarboxylates. In addition to carboxylates and inorganic anions, soil solutions contain cations and unspecified humic substances which can interfere in the CZE analysis. Owing to complex formation, high concentrations of aluminium will invalidate the quantification of di- and tricarboxylates, such as citrate, malate and oxalate. This interference may be eliminated by addition of 10% v/v of 20 mM Na₄EDTA at pH 11 to the soil solution samples prior to the analysis, with no further sample preparation. Metal cations did not interfere in the quantification of monocarboxylates and inorganic anions. The detection limits of low molecular mass aliphatic carboxylates and inorganic anions were all below 2 µM. The relative standard deviations for carboxylates and inorganic anions were within the ranges 4.2–5.1% for 100 µM and 9.3–59% for 1 µM standard solutions. The recovery of oxalate added to soil solution was 100 ± 8%. The CZE method requires small sample volumes and is robust to high concentrations of humic substances in the soil solutions and it is easier to maintain the capillary than ion chromatographic columns.

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