A part per trillion (ppt) is a measurement of the quantity of a substance in the air, water or soil. A concentration of one part per trillion means that there is one part of that substance for every one trillion parts of either air, water or soil in which it is contained. One part per trillion is equivalent to one nanogram per kilogram.
It is very difficult to visualize how minute a particle one part per trillion really is. To get an idea, one ppt would be represented by a single drop of food coloring in 18 million gallons of water. Another way to view it as representing a single second out of 32,000 years. One part per trillion is one thousand times smaller than one part per billion.
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