  A simple resonance ionization mass spectrometer is built with a Quadrupole
Mass Spectrometer (QMS) and two laser diodes aiming at trace analysis of
potassium. The overall detection efficiency is estimated comparing the yields
of resonance ionization and electron-impact ionization in the same QMS. A clear
discrimination of $^{39}\rm K$, $^{40}\rm K$ and $^{41}\rm K$ is demonstrated
with a help of isotope shifts of the atomic levels.
