Abstract: In a laser microprobe interface for a mass spectrometer, such as a Fourier transform mass spectrometer, the focusing optical system (1) and the viewing optical system (4) are arranged in the cell holder (5) of the spectrometer itself. The focusing optical system (1) can be moved by an adjustment means (2) so as to make allowance for the variation of the focal distance with the wavelength of the primary ionization laser beam (3) and is arranged at the center of the viewing optical system (4) of the achromatic inverted cassegrain type. The latter optical system provides perfect definition of the image and a high magnification together with a good depth of field and good laser focusing. The arrangement of the focusing optical system (1) at the center of the viewing optical system (4) also allows the focusing optical system (1) to be interchanged with other ionization means.