A particle injector was a device used on Makull's homeworld.
As part of their demonstration against polaric ion energy in 2371, Pe'Nar Makull and Ny Terla took a particle injector out of their safe and intended to sabotage one of the polaric ion power plants with a particle injector. (VOY: "Time and Again")
The device was up to appear twice in the episode but was actually never seen, only inside the safe and in the blue bag of Makull.