Recently, we developed a high-resolution vacuum ultraviolet mass-analyzed threshold ionization (HR VUV-MATI) mass spectrometer. This mass spectrometer is composed of a four-wave difference frequency mixing cell for long-lasting and intense VUV laser pulses in 123.6–160.0 nm, a space-focused linear time-of-flight photoionization chamber, and a compact molecular beam chamber with a temperature-controlled pulsed nozzle for ion spectroscopy. In particular, newly invented ion source assembly and pulsing schemes can adopt extremely weak pulsed-field ionization of the molecules, which allows the first-order space focusing of the generated MATI ions. Finally, because the spectral and mass resolutions of approximately 5 cm^-1 and 2,400, respectively, were achieved, this spectrometer could be used to measure the fine vibrational spectrum from the zero-point level of the cation and the exact adiabatic ionization energy of the neutral molecule. Further, it could be used to measure the appearance energies of the photoproducts and elucidate the vibrational structures of the cationic isotopomers. Specifically, we investigated 3-chloropyridine isotopomers (C₅H₄N-³⁵Cl and C₅H₄N-³⁷Cl), which have been received much attention due to molecular characteristics of pyridine moiety, to obtain the accurate adiabatic ionization energy and cationic vibrational structures.