Ion Chromatography - The Electrospray Interface
The Electrospray Interface
An early report describing an electrospray interface was that of Whitehouse et al (4) and this type of interface is probably the one most commonly used in contemporary LC/MS instruments.
Figure 43 The Electrospray Interface
A diagram of the elctrospray interface is shown in figure 43. 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 10torr. The sample solution is sprayed from the end of a stainless steel capillary usually situated about 1 cm from the ion sampling orifice. A potential of 3-5kV is applied between the jet and the orifice plate and the gradient may be positive or negative. The ions are formed by the potential between the capillary jet and the plate. The ionizing efficiency of the electrospray depends on the ionizing potential. Increasing the potential difference first increases the drop size but initially no ions are formed. Increasing the voltage further results in an expanding mist of droplets. Under these conditions, the droplets are charged and ions are formed. The flow rate is restrained to a few microliters /min, because the volume of liquid that can be drawn from the jet by electrical sheer forces is also limited. In many electrospray interfaces, solvent-evaporation is aided by a warm stream of nitrogen that flows counter current to the spray. In addition, any uncharged droplets are swept away from the orifice by this nitrogen stream. Further increase in the voltage initiates a corona discharge that interferes with the ion productions and should be avoided.
Courtesy of the Hewlett-Packard Company.
Figure 44. The Hewlett-Packard Electrospray LC/MS Interface
The Hewlett-Packard electrospray LC/MS interface is shown in figure 44. The column eluent is mixed with a nebulizing gas and the resulting spray-jet directed onto a disk target that is set at a high potential relative to the spray nozzle. In the centre of the disk is a pinhole, which allows entry into the interface and the jet-stream is directed slightly to the side of this aperture. This results, in the fine droplets at the periphery of the spray being drawn into a chamber (held at a reduced pressure) through the 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. The core of the jet is skimmed by conical screens, which removes the drying gas and the ions pass directly into an ion-optical arrangement. The ion optics direct the ions into the mass analyzer. Multiple charged ions are produced as a result of a molecule being associated with more than one proton. As a result, 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.