A compact resonance ionization mass spectrometer is developed using two laserdiodes and a quadrupole mass spectrometer to perform trace element analysis ofpotassium. With the help of a narrow linewidth of the laser diode, the isotopeshifts of $^{40}{\rm K}$ and $^{41}{\rm K}$ of the 405 nm line with respect to$^{39}{\rm K}$, corresponding to the transition of $4\,^2$S$_{1/2}\rightarrow$$5\,^2$P$^\circ_{1/2}$, are measured to be $207\pm13$ MHz and $451\pm10$ MHz,respectively, by comparing them to the known hyperfine splitting widths of the$4\,^2{\rm S}_{1/2}$ ground state of each potassium isotope. The overalldetection efficiency of an order of $\sim10^{-6}$ in our setup indicates theavailability of RIMS to the analysis of the trace metal impurities on or in acertain material such as the contamination assessment of semiconductor wafers.
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