Ion Chromatography - The Thermospray Interface 1
The Thermospray Interface
The thermospray interface can be considered as a natural extension of the direct inlet system and involves simply heating the tip of the entry tube to a suitable high temperature to promote vaporisation. One of the first papers describing the successful use of the thermospray interface for LC/MS was by Covey and Henion (1). Because only the spray tip is heated, the solvent is vaporized right at the tip, and not inside the conduit tube. Consequently, the vapour has sufficient energy to change the remainder of the liquid to a mist of small droplets and allows better control of both the nebulizing and the ionizing process The pressure is low and, thus, an electric field cannot be applied (as will be seen in electrospray) as an electric discharge would form. However, if there are no ions in the mobile phase an electrolyte can be added (e.g., a 0.1M solution of ammonium acetate) prior to volatilization (in an ion chromatograph these may already be present). The presence of ions causes the droplets to assume a charge half of which will be half negative and half positive. Depending on the electrode configuration of the mass spectrometer either can be used for spectroscopic examination. Thus, the thermospray process differs considerably from electrospray as the ions assume a charge that is determined by the direction of the ionizing field. As the charged droplets evaporate, the diameter of the droplets is reduced and, as a consequence, the charge density increases. When the electrical forces on the surface become equal to the surface tension forces the droplet explodes.
This is called Raleigh's limit and the maximum permissible charge (q) that can be carried by a drop radius (r) can be mathematically expressed by the following equation.
where (g ) is the surface tension of the liquid,
and (e) is the permittivity of the surrounding free space..
The thermospray produces larger drops (ca 10mm) than electrospray (1-2mm). The small diameter of the droplets produced by electrospray probably explains the production of multiple ions, which allows very high masses to be measured. An example of the thermospray Ionizer is shown in figure 40.
The device comprises a metal cap constructed from a high-conductivity metal (e.g. copper) situated in the end of a stainless steel tube. Through the centre of the stainless steel tube and copper cap passes, a small diameter conduit, which carries the column eluent. The conduit projects slightly beyond the end of the heater cap which is situated in a cartridge heater together with a thermocouple. The thermocouple monitors the temperature of the tip of the probe and also provides the signal that maintains the cap at a selected temperature. The pumping rate of a mass spectrometer vacuum system is limited so it is still necessary to split the eluent. The generally accepted flow rate range for the thermospray interface is 100-1000ml/min. The electrospray, however, has an acceptable flow rate range of only 1 to 10ml/min. The properties of the thermospray system were examined by Voyksner et al. who noted that the thermal spray interface tended to produce parent ions and appeared to be more sensitive than other LC/MS interfaces.