Thermospray mass spectrometry ionization processes fundamental mechanisms for speciation, separation and characterization of organic complexants in DOE wastes. 1998 annual progress report

Abstract

'This is a three-year project which started on October 1, 1997. This report summarizes the progress achieved for the first eight months of the study (through May, 1998). Thermospray positive ion mass spectra of seven different organic complexants have been defined in a water:methanol (1:1 by volume) solvent system that was buffered with 0.1 M ammonium acetate (pH = 6.7). These complexants were imidodiacetic acid (IDA), ethylenediamine-N,N{prime}-diacetic acid (EDDA), N-(2-hydroxyethyl)iminodiacetic acid (HEIDA), nitrilotriacetic acid (NTA), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). Mass spectra of all these complexants showed a peak at mass [M+1] + when vaporized from this solvent medium. Only NTA showed a significant peak at mass [M+18] + which was interpreted as the formation of an adduct with an ammonium ion from the buffered solvent medium. This observation for the vaporized NTA is consistent with the measured acidity in aqueous solution where the first acid dissociation for NTA has a lower pK value than those reported for EDTA, HEDTA, and IDA (1). This tendency to form an ammonium adduct in the gas phase would indicate that NTA has a lower proton affinity than the other complexants in the vaporized phase. This lower proton affinity in the vapor phase would be consistent with the greater observed acidity in the solution phase. Accordingly, this observation provides an indication that characteristics observed after thermospray vaporization may aid in defining behavior in solution. Such interpretations of thermospray mass spectra should be useful in the characterization of DOE mixed waste solutions. Progress has also been made in developing thermospray mass spectra at source temperatures of less than 170 C. Such lower source temperatures should reduce the amount to thermal degradation for labile species. Pressures in the analyzer chamber of the mass spectrometer tend to elevate as the thermospray source temperature is reduced; however, the instrument has been operated for up to 8 hours at a source temperature of 175 C and a solvent flow of 1 ml/min. This time interval is of sufficient duration to analyze multiple liquid chromatography separations.'