Wang H, Agnes G R
Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 Canada.
Anal Chem. 1999 Oct 1;71(19):4166-72. doi: 10.1021/ac981375u.
The complexation reactions between the alkaline earth metal ions and EDTA were studied by electrospray mass spectrometry to measure the change in concentration of the metal ion-EDTA complex (MY(2)(-)) in the gas phase relative to the solution-phase equilibrium concentration. This work focused on the solution pH range from 4 to 7 where there exists free metal ions in solution at equilibrium. The equilibrium shift, measured through quantitation of the increased abundance of the MY(2)(-) species in the gas phase, was largest for barium and smallest for magnesium. The cause of the net equilibrium shift of the MY(2)(-) species is the combined effect of an electrolytic increase in pH within the capillary plus an additional shift within the evaporating droplets. In a thin diffusion-limited layer created by the products of electrolysis mixing with the bulk solution at the ES capillary tip, the labile species reequilibrate at a new, higher pH. In the evaporating droplets, the formation of new labile species due to increased solute concentrations is kinetically controlled because the ion residence time in the droplet prior to desorption is only ∼5 μs. These results are briefly discussed with respect to the potential for utilizing electrospray mass spectrometry for kinetically labile equilibrium studies.
通过电喷雾质谱法研究了碱土金属离子与乙二胺四乙酸（EDTA）之间的络合反应，以测量气相中金属离子 - EDTA络合物（MY(2)(-)）浓度相对于溶液相平衡浓度的变化。这项工作聚焦于溶液pH值范围为4至7的情况，在此范围内平衡时溶液中存在游离金属离子。通过定量气相中MY(2)(-)物种丰度的增加来测量的平衡移动，对钡而言最大，对镁而言最小。MY(2)(-)物种净平衡移动的原因是毛细管内pH值的电解增加以及蒸发液滴内的额外移动的综合作用。在由电解产物与电喷雾毛细管尖端的本体溶液混合形成的薄扩散限制层中，不稳定物种在新的更高pH值下重新达到平衡。在蒸发液滴中，由于溶质浓度增加而形成新的不稳定物种是受动力学控制的，因为离子在脱附前在液滴中的停留时间仅约为5微秒。关于利用电喷雾质谱法进行动力学不稳定平衡研究的潜力，简要讨论了这些结果。
