Mass spectrometer

An atmospheric pressure ionization mass spectrometer comprising an ionization region maintained at about atmospheric pressure for ionizing a sample, a mediate pressure region maintained at about 0.1 to 10 Torr and a highly evacuated mass spectral region provided at its upstream side with auxiliary ionization means, a first partition wall separating said ionization region and said mediate pressure region for each other, at least a part of said partition wall being made of a conductive material to form a first electrode, a first fine aperture formed in said first electrode for effecting communication of said ionization region and said mediate pressure region with each other, a second partition wall separating said mediate pressure region and said mass spectral region from each other, at least a part of said second partition wall being formed of a conductive material to form a second electrode, a second fine aperture formed in said second electrode for effecting communication of said mediate pressure region and said mass spectral region with each other, said fine apertures in combination forming a jet separator, and means for applying an electric potential between said first and second electrodes.