Electrospray diagnostics performed by using sucrose and proteins in the gas-phase electrophoretic mobility molecular analyzer (GEMMA) Original Research Article

The proteins α-lactalbumin (14 KDa), conalbumin (77 KDa), and ferritin (460 KDa) were dissolved in 20 mM NH4OAc along with controlled amounts of sucrose. The solution was electrosprayed into 160 nm diameter droplets in a charge-neutralized electrospray [D.-R. Chen, D.Y. Pui, S.L. Kaufman, J. Aerosol Sci. 26 (1995) 963], at concentrations such that most droplets contained no more than one protein molecule. The electrophoretic-mobility diameter spectra of the aerosol formed on droplet evaporation were measured using a differential mobility analyzer with a condensation type particle detector [S.L. Kaufman, J.W. Skogen, F.D. Dorman, F. Zarrin, K.C. Lewis, Anal. Chem. 68 (1996) 1895, 3703]. Peaks were observed corresponding to residues from droplets containing only sucrose and from droplets containing both sucrose and single protein molecules. For the peaks corresponding to the protein molecules, an increase in particle diameter with increasing sucrose concentration was observed, consistent with a simple model in which a ‘crust’ of sucrose is formed around the protein molecule as the liquid in the surrounding droplet evaporates. Spectra at high sucrose concentrations, where the pure-sucrose particles were larger than the protein molecules without sucrose, showed a peak at the diameter corresponding to the uncoated protein molecules. This somewhat surprising result appears to show that there is more than one process by which macromolecules or their ions leave the associated droplets in charge-neutralized electrospray.