Page 376 - Tandem Techniques

Page 360

The electrospray interface differs from the thermospray in that it is operated at atmospheric pressure
whereas the thermospray usually functions at a reduced pressure, ca 1 to 10 torr. Typically the analyte
solution is sprayed from a stainless steel capillary usually situated about 1 cm form the ion sampling
orifice. A potential of 3-5 kV is applied between the jet and the orifice plate and the gradient can be
positive or negative in nature. The ions are formed by the potential between the capillary jet and the
plate The electrospray effect changes with different ionizing potentials. As the potential difference is
increased, the drop size increases, but initially there are no ions formed. As the voltage increases further
a liquid cone is formed which generates an expanding mist of droplets. In this form the droplets are
charged and ions are formed. The flow rate is limited to a few microliters /min, as the volume that can
be drawn from the jet by electrical sheer forces is limited. In most instruments solvent evaporation is
aided by a warm stream of nitrogen flowing counter current to the spray and any uncharged droplets are
swept away from the orifice. At still higher voltages a corona discharge is formed which interferes with
the ion productions and is to be avoided.

A diagram of the Hewlett-Packard electrospray ionization LC/MS interface is shown in Figure 9.21.
The column eluent is mixed with a nebulizing gas and the spray jet directed onto a disk target which is
at a high potential relative to that of the spray nozzle. In the center of the target is a pinhole entry into
the interface and the jet is directed slightly to the side of this aperture. The fine droplets at the periphery
of the spray are drawn into a chamber (held at a reduced pressure) through a pinhole aperture. Inside the
chamber, the droplets are entrained in a stream of hot nitrogen gas, that rapidly evaporates the solvent,
producing ions in the manner described above. The core of the jet is skimmed by conical screens to
remove the drying gas, and the ions then pass directly into an ionoptical arrangement, that directs them
into the mass analyzer. Ions with multiple charges are produced as a result of a molecule being
associated with more than one proton. Consequently, an ion of molecular weight 1000, carrying three
charges, will appear at an m/z value of 333.3 on the spectrum where an ion of mass 333.3 and unit
charge would normally appear.