An atmospheric pressure ionisation source comprising: an ionisation chamber, comprising an inlet for receiving at least the distal end of a capillary into the ionisation chamber in use; a desolvation heater including a heating element, for directing a stream of heated gas onto the distal end of the capillary in use; a corona discharge device arranged in the ionisation chamber; and a control system configured to operate the source in a selected one of: an analytical mode, in which the heating element is heated to a first temperature within a first temperature range, and in which a first current within a first current range is supplied to the corona discharge device; and a capillary priming mode, in which the heating element is heated to a second temperature within a second temperature range, and in which a second current within a second current range is supplied to the corona discharge device, wherein the lower limit of the second temperature range is higher than the lower limit of the first temperature range, and the second current range is higher than the first current range.

In an ionization device for an ionization chamber (11) separated from an analysis chamber (12-14) by a partition wall having an ion introduction port (113), an ionization probe (111) sprays a liquid sample. A heated-gas supply mechanism (112), which includes a gas supply source and a heating section (1122) for heating a gas supplied from the gas supply source, expels the gas in a direction intersecting with the direction in which the liquid sample is sprayed from the ionization probe. A controller (32) controls an operation of the heated-gas supply mechanism so that the gas is continuously expelled from the heated-gas supply mechanism regardless of the presence or absence of an operation by a user while the liquid sample is sprayed from the ionization probe. The continuous expulsion of the gas from the heated-gas supply mechanism prevents this mechanism from being contaminated by the sprayed liquid.