Organic Cation Distributions in the Residues of Levitated Droplets with Net Charge: Validity of the Partition Theory for Droplets Produced by an Electrospray

Abstract

In electrospray (ES) mass spectrometry experiments, the suppression of analyte ion signal intensity by a higher concentration of nonvolatile electrolyte is well documented. This phenomenon is, according to the partition theory, the result of competitive partitioning that favors electrolyte over analyte for occupancy in a surface volume that contains the net excess charge carried by droplets produced in an ES ion source. Reported here are the results of a set of experiments that were designed to learn of the final partition of cations in droplets with net charge by measurement of their distributions within the solid residues of those droplets. An electrodynamic balance was used to levitate droplets that contained Rhodamine 6G (R6G+) plus a large molar excess of Na+Cl- until they dried to a solid residue. The residues, each typically 20 μm in cross section, were then deposited onto a plate and the distributions of R6G+ were measured using confocal fluorescence microscopy and characterized using laser desorption/ionization time-of-flight mass spectrometry. Within the residues of droplets that had net negative charge, the R6G+ was contained only within the core of the residue, indicating the net excess charge carried by the droplets was contained in the surface volume. Within the residues of droplets that had net positive charge, a small fraction of the R6G+ precipitated on the residue's surface, but the majority of the R6G+ precipitated in banded regions that were underneath a distinct 0.5−3-μm-thick surface volume that was identifiable because of its lower abundance of R6G+. These measurements of R6G+ distributions within the residues of levitated droplets with net charge have provided literally solid evidence in support of the droplets having two phases, and that was a key postulate in the development of the partition theory.