Principles of Operation — Ion
Source
B
Electrospray Ionization Mode
The probe is located centrally between the two turbo heaters, which are located at a 45-degree
angle on either side of the probe. The combination of the spray and the heated dry gas from the
turbo heaters is projected at a 90-degree angle to the aperture in the curtain plate.
Only compounds that ionize in the liquid solvent can be generated as gas phase ions in the
source. The efficiency and rate of ion generation depends on the solvation energies of the
specific ions. Ions with lower solvation energies are more likely to evaporate than ions with
higher solvation energies.
The interaction of the
IonSpray Voltage
and the turbo heaters helps focus the stream and
increases the rate of droplet evaporation, resulting in an increased ion signal. The heated gas
increases the efficiency of ion evaporation, resulting in increased sensitivity and the ability to
handle higher liquid sample flow rates.
A high-velocity flow of nebulizer gas shears droplets from the liquid sample stream in the
IonSpray Voltage
inlet. Using the variable high voltage applied to the sprayer, the ion source
applies a net charge to each droplet. This charge aids in the droplet dispersion. Ions of a single
polarity are preferentially drawn in the droplets by the high voltage as they are separated from
the liquid stream. However, this separation is incomplete and each droplet contains many ions of
both polarities. Ions of one polarity are predominant in each droplet, and the difference between
the number of positively or negatively charged ions results in the net charge. Only the excess
ions of the predominant polarity are available for ion evaporation, and only a fraction of these
actually evaporate.
The probe can generate multiply-charged ions from compounds that have multiple charge sites,
such as peptides and oligonucleotides. This is useful during analysis of high-molecular-weight
species where the multiple charges produce ions of a mass-to-charge ratio (
m/z
) within the
mass range of the mass spectrometer. This allows routine molecular-weight determinations of
compounds in the kiloDalton (kDa) range.
Each charged droplet contains solvent and both positive and negative ions, but with ions of one
predominant polarity. Refer to the figure:
. As a conducting medium, excess charges
reside at the surface of the droplet. As the solvent evaporates, the electrical field at the surface
of the droplet increases due to the decreasing radius of the droplet.
Principles of Operation — Ion Source
SCIEX 3200 Systems
84/114
System User Guide
RUO-IDV-05-6475-C
