PURPOSE: To raise measuring sensitivity, by a method wherein a liquid specimen after freezing is transferred under high vacuum to perform freeze drying and the conc. material is introduced into an ion source.

CONSTITUTION: A liquid specimen which is cooled by passing a cooling medium such as liquid nitrogen through a cooling pipe 6 and flowed out from an LC column is introduced into a specimen introducing pipe 5 from a specimen effluent pipe 4, the solvent thereof is frozen to be formed into minute ice like crystals and the frozen specimen is extruded onto the introducing belt 8 in a high vacuum region 3 from an opening part to be subjected to freeze drying. The specimen on the introducing belt 8 is melted by an infraed lamp 13 prior to perfectly drying to be converted to a predetermined conc. solution which is in turn introduced into an ion source 2 while receives a matrix such as glycerine added from an introducing pipe 15 and the specimen as the target is irradiated with neutron beam 20 in an ionization part 19 by supplying repeller voltage to obtain ion beam 22. The non-ionized specimen and glycerine are scraped off by a scraper 16 and recovered in a waste specimen sump 18 as drain. By this system, sensitivity can be raised.