Effect of affinity for droplet surfaces on the fraction of analyte molecules charged during electrospray droplet fission

The effect of uneven fissioning of mass and charge from electrospray droplets on the amount of analyte charged during the electrospray process was explored. A surface selectivity factor (S) was developed to describe the affinity of an analyte for the droplet surface, and both theoretical and experimental response curves were compared for analytes with various S values. The theoretical response curves were generated by calculating the overlap between the charge and analyte spawned from parent droplets to determine the amount of analyte charged. This overlap was then graphed as a function of analyte concentration. Differences in the amount of analyte charged during droplet fission were predicted for analytes of varying surface affinities. The issue of analyte partitioning between the surface and interior phases of the ESI droplet was also included in the discussion. This was accomplished by applying the equilibrium partitioning model to a set of offspring droplets to determine the amount of analyte on their surfaces and then calculating the overlap between fissioning analyte and excess charge. Experimental response curves resembled theoretical ones, and S values predicted from theory were in excellent agreement with those predicted on the basis of the structural characteristics of the analytes.