Introduction
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____________________________________________________________ Ionization Techniques
_______________________ Finnigan LTQ Hardware Manual ______________________
1-5
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any polar compound that makes a preformed ion in solution. The term
preformed ion can include adduct ions. For example, polyethylene glycols can
be analyzed from a solution containing ammonium acetate, because of adduct
formation between the NH
4
+
ions in the solution and oxygen atoms in the
polymer. With ESI, the range of molecular weights that can be analyzed by
the LTQ is greater than 100,000 u, due to multiple charging. ESI is especially
useful for the mass analysis of polar compounds, which include: biological
polymers (for example, proteins, peptides, glycoproteins, and nucleotides);
pharmaceuticals and their metabolites; and industrial polymers (for example,
polyethylene glycols).
In ESI, ions are produced and analyzed as follows:
1. The sample solution enters the ESI needle, to which a high voltage is
applied.
2. The ESI needle sprays the sample solution into a fine mist of droplets that
are electrically charged at their surface.
3. The electrical charge density at the surface of the droplets increases as
solvent evaporates from the droplets.
4. The electrical charge density at the surface of the droplets increases to a
critical point, known as the Rayleigh stability limit. At this critical point,
the droplets divide into smaller droplets because the electrostatic
repulsion is greater than the surface tension. The process is repeated many
times to form very small droplets.
5. From the very small, highly charged droplets, sample ions are ejected into
the gas phase by electrostatic repulsion.
6. The sample ions pass through an ion transfer capillary, enter the MS
detector and are analyzed.
In the LTQ, the ESI needle is orthogonal to the axis of the ion transfer
capillary that carries ions to the MS detector. This geometry keeps the ion
transfer capillary clean. The ion sweep cone serves as a mechanical barrier
that keeps large droplets and particulates from entering the ion transfer
capillary. Figure 1-3 shows the steps in the formation of ions from highly
charged droplets, and the relationship between the ESI probe and the ion
transfer capillary.
