Technical data

Overview 1
Electrospray ionization (ESI) and Dual ESI
Agilent 6200 Series TOF and 6500 Series Q-TOF LC/MS System Concepts Guide 19
Because the sample solution is not heated when the aerosol is
created, ESI does not thermally decompose most analytes.
Desolvation and ion evaporation
Before the ions can be mass analyzed, solvent must be removed
to yield a bare ion.
A counter-current of neutral, heated drying gas, typically
nitrogen, evaporates the solvent, decreasing the droplet
diameter and forcing the predominantly like surface-charges
closer together (see Figure 2).
Coulomb repulsion – repulsion
between charged species of the
same sign
When the force of the Coulomb repulsion equals that of the
surface tension of the droplet, the droplet explodes, producing
smaller charged droplets that are subject to further
evaporation. This process repeats itself, and droplets with a
high density of surface-charges are formed. When charge
density reaches approximately 10
8
V/cm
3
, ion evaporation
occurs (direct ejection of bare ions from the droplet surface).
These ions are attracted to and pass through a capillary
sampling orifice into the ion optics and mass analyzer.
The importance of solution chemistry
The choice of solvents and buffers is a key to successful
ionization with electrospray. Solvents like methanol that have
lower heat capacity, surface tension, and dielectric constant,
promote nebulization and desolvation. For best results in
electrospray mode:
Figure 2 Desorption of ions from solution