Operating the Electrospray
4–7
and Corona Discharge mode. Figure 4-2 shows three photographs
of the capillary tip viewed through the viewing window.
Figure 4-2
Three Views of Capillary Tip Through the Viewing Window: (1) No liquid flow, (2)
Liquid flow but no electric field (Dripping mode), (3) Liquid flow and an electric field
(Cone-Jet mode). The third view illustrates stable electrospray operation.
Dripping mode
(view 2 in Figure 4-2) occurs when a primary
droplet builds up on the end of the capillary until it becomes
large enough to be pulled away by the weak electric field. The
current display is usually near zero in this mode. Inspection
through the viewing window shows the primary droplet
increasing in size until it is pulled away at a very low frequency.
As the voltage is increased in this mode, the primary droplet
size decreases.
In
Pulsating mode
, the liquid at the tip of the capillary
continues to drip, but at a higher frequency than in dripping
mode. In addition, a visual inspection shows a more cylindrical
liquid profile with a cone-shaped tip. During operation, the
cylindrical liquid profile increases in length and the current
display increases until the primary droplet is pulled away (and
the current display rapidly decreases). As in the dripping mode,
as the voltage is increased, the primary droplet size decreases
while the frequency increases.
The
Cone-Jet mode
(view 3 in Figure 4-2) occurs when the
electric field strength is balanced with the liquid properties. A
visual inspection shows the conical shape of the profile (Taylor
Cone), but the jet of primary droplets cannot be seen since the
primary droplets are very small. As voltage is increased, the
profile does not change significantly while in this mode; in
addition, the total current reading changes very little. The cone-
jet mode is the most stable and useful mode for generating
aerosols. The voltage setting in this mode is typically 2.00 to
2.50 kV using a 20 mM ammonium acetate buffer solution.
As the voltage is increased further, the electric field becomes too
strong for the liquid and surrounding gas and the
Corona
Discharge mode
occurs. The strong electric field causes the tip
of the liquid to become rounded. Also, the current reading is
high because of the current traveling through the CO
2
/air
sheath flow, and erratic because of the instability of this mode.
