The term parts per million (abbreviated ppm) described a ratio of density used for measuring small concentrations of a substance within a mass/volume of a much larger one.
Hadron count, ion trace, and ENC ratio might be measured in PpM (ENT: "Divergence")
To revive the crew in 2367, Data used five parts per million of ADTH. (TNG: "Clues")
Beta Agni II's water supply was contaminated with forty parts per million of tricyanate later that year. (TNG: "The Most Toys")
To counter a graviton field, Jadzia Dax used one part per million of tridium in 2369. (DS9: "Q-Less")
A class 17 nebula in the Delta Quadrant was composed of 7,000 parts per million of sirillium in 2373. (VOY: "Flashback")
Seventeen parts per million of argonite was well under safety margins for the air supply on Deep Space 9. (DS9: "Doctor Bashir, I Presume")