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that rapidly breaks up into droplets. The droplets then pass into a heated chamber, where they are
vaporized and the sample and solvent then enter the ionization chamber. The tip of the jet can be
suitably cooled to prevent premature evaporation of the eluent drops. The spectrometer pumping system
cannot usually cope with much more that a few microliters of liquid solvent as vapor, so the column
eluent must be split, and this is accomplished with the use of a down-stream needle valve. The interface
is usually employed with a chemical ionization interface and the solvent vapor is used as the reagent
gas. The use of the solvent as the reagent gas can either place restrictions on the choice of mobile phase
that can be employed with the chromatograph or control the nature of the ions that are produced in the
chemical ionization source. This interface has the disadvantage that a very small orifice is necessary for
vaporization which can easily become blocked. This type of interface has been most successfully used
for solutes that have a reasonable vapor pressure at the source temperature

The Thermospray Interface
The thermospray interface evolved from the simple inlet system, by the rather straightforward
modification of heating the tip of the entry tube (or column) to a relative high temperature. One of the
first reports of the successful use of the thermospray interface for LC/MS was that by Covey and
Henion [12]. As a result of the heated tip, the solvent is vaporized right at the tip, and not somewhere
inside the delivery tube or column. Under these conditions, the vapor has sufficient energy to change
the remainder of the liquid to a mist of small droplets This results in much better control of both the
nebulizing and the ionizing process. Due to the low pressure, an electric field cannot be applied (as in
electrospray), as an a electric discharge would form. Instead, if there are no ions present in the mobile
phase an electrolyte such as a 0.1 M solution of ammonium acetate may be added prior to volatilization.
Due to the presence of the ions, the droplets assume charges approximately half of which will be
negative and half positive. Either can be used for spectroscopic examination depending on the electrode
configuration of the mass spectrometer. In this way the thermospray process differs considerably