We demonstrate a low-cost 343 nm solid-state laser delivering up to 20 µJ per pulse, with a pulse width of 2.3 ns at a repetition rate of 100 Hz. The 343 nm is obtained through a third harmonic generation of a passively Q-switched 1030 nm Yb:YAG laser with pulse energy of 190 µJ at 100 Hz and a pulse width of 5.4 ns. The IR-UV conversion efficiency is 10.4%, comparable to that achieved with mode-locked IR lasers. The light source is electronically controlled for easy synchronization with a detection circuit. The low repetition rate specifically targets applications exploiting the millisecond scale lifetime of lanthanides employed in fluoroimmunoassay measurements for time-resolved fluorescence spectroscopy. Low repetition rate and even pulse-on-demand operation is demonstrated.