Development of a gas chromatographic method for the simultaneous determination of trace amounts of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid in natural waters

A GC method for the simultaneous determination of ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) in natural waters is described. The method was studied with spiked samples of distilled water, sea-water (2 matrices) and humic-containing lake mater. An acidified 5-10 ml sample was evaporated and esterified with C₂H₅OH, containing H₂SO₄ and CH₃COOH catalysts and CH₃(CH₂)₁₅CN as an internal standard. The esterified sample was extracted with C₆H₅CH₃, neutralized with KHCO₃, dried with anhydrous Na₂SO₄ and analysed by GC with nitrogen-phosphorus selective detection. The best recoveries mere obtained by the esterification in 100 °C for 3 h. The tetraethyl ester of EDTA and pentaethyl ester of DTPA were further identified by mass spectrometry. In the sea- and lake water matrices, the recoveries of EDTA and DTPA were 80-110%. High concentrations (above 50 mg l^-1) of iron (Fe³⁺) interfered considerably in the determination of DTPA. For EDTA the interference was insignificant with iron concentrations below 500 mg l^-1.