Kinetically labile equilibrium shifts induced by the electrospray process

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Mass Spectrometry

Background:

• Understanding complexation reactions of alkaline earth metal ions with chelating agents like EDTA is crucial in various chemical and biological applications.
• Previous studies often faced challenges in accurately measuring solution-phase equilibrium concentrations, especially for labile species.
• Electrospray mass spectrometry (ES-MS) offers a potential avenue for probing solution equilibria by analyzing gas-phase ion abundances.

Purpose of the Study:

• To investigate the complexation reactions between alkaline earth metal ions and EDTA.
• To quantify the shift in metal ion-EDTA complex equilibrium using electrospray mass spectrometry.
• To elucidate the factors contributing to equilibrium shifts observed during ES-MS analysis.

Main Methods:

• Electrospray mass spectrometry (ES-MS) was employed to study the complexation of alkaline earth metal ions (e.g., Mg2+, Ba2+) with EDTA.
• The concentration of the metal ion-EDTA complex (MY(2)(-)) was measured in the gas phase relative to solution-phase equilibrium.
• Experiments were conducted within a solution pH range of 4 to 7, ensuring the presence of free metal ions at equilibrium.

Main Results:

• A measurable shift in the metal ion-EDTA complex equilibrium was observed, with the magnitude varying across different alkaline earth metals.
• Barium (Ba2+) exhibited the largest equilibrium shift, while magnesium (Mg2+) showed the smallest.
• The observed equilibrium shift is attributed to a combination of electrolytic pH increase within the ES capillary and kinetic effects in evaporating droplets.

Conclusions:

• Electrospray mass spectrometry can effectively measure shifts in labile complexation equilibria.
• The analysis involves complex interplay between solution chemistry (pH effects) and droplet dynamics (kinetic control).
• This study highlights the potential of ES-MS for studying kinetically controlled equilibria and provides insights into the behavior of metal-EDTA complexes.