Determination of metal-EDTA complexes in soil solution and plant xylem by ion chromatography-electrospray mass spectrometry

Abstract

An ion chromatography-electrospray mass spectrometry (IC-MS) method was developed to quantify the metal complexes of ethylenediaminetetraacetic acid (EDTA) in soil solution and plant xylem exudate. Suitable separation of the metal-EDTA complexes was achieved on a Dionex AS5 column using 2 mM Na2CO3 as the eluant. However, satisfactory detection by eluant suppressed IC-MS, in either the positive or negative ion detection mode, could not be attained. A new eluant that still attained suitable separation and produced ionic species that could be detected by MS in the negative ion mode was developed. The eluant consisted of 2.5 mM (NH4)2CO3, 9.7 mM NH4OH, and 4% (v/v) methanol and had a pH 9.9. Even though eluant suppressed IC-MS degraded detection limits by a factor of 4 over the nonsuppressed system, using the retention time and not the m/z (mass-to-charge ratio) of the intact chelate for identification, the latter allowed the metal complexes to be detected intact and was optimized for the analysis of environmental samples. The number of metal-EDTA species that could be detected was limited by the eluant used for ion chromatog raphy (i.e. only those complexes that were stable at high pH), with metal-EDTA complexes of Al, Cd, Cu, Co, Mn, Ni, Pb, and Zn being adequately resolved. Iron(III), Ca, MgEDTA, and EDTA itself were not detected. Detection limits for the various complexes ranged from 0.1 to 1μM.