ESI-MS of Small Molecules

Polar analytes in the m/z 100-1500 range are often involved in pharmaceutical analytics including metabolism studies. The types of ions formed are various and depend on the ion polarity, the pH of the solution, the presence of salts, and the concentration of the sprayed solution. Multiply charged ions are rarely observed. 

Example The compound below represents the functional part of an effective dmg (BM 50.0341) inhibiting HIV-1 infection by suppression of the unfolding of the gpl20 glycoprotein (Fig. 12.30). Its positive-ion nanoESI spectrum from ethanol in the presence of ammonium chloride exhibits signals due to the formation of [Mh-H] , [Mh-NHJ , and [M-i-Na] ions. In addition, cluster ions of the type 

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With respect to analyte ionization, ion-molecule reactions may take place between gas-phase buffer ions and neutral analyte molecules, either at the droplet surface, or in the gas phase, after soft desolvation of neutral molecules. These processes will be more relevant in ESI-MS of small molecules most peptides and proteins are readily present as preformed ions in solution. Amad et al. [32] demonstrated that the analyte signal can be completely suppressed in a solvent with a proton affinity higher than that of the analyte. These types of gas-phase ion-molecule reaction are identical to the ionization processes in APCI (Ch. 6.4). 

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