Parts Per Trillion
A means of expressing extremely minute concentrations of substances in water or air.
In the United States, the concentration is the number of units of the substance found in 1x1012 units of water or air; an equivalent unit is nanograms per kilogram. Detection of concentrations this low, which was not possible until the late twentieth century, is limited to only a few types of chemical compounds. For example, one gram of sulfur hexafluoride, used as a tracer in studies of ocean mixing, can be detected in a cubic kilometer of sea water, a concentration of 1,000 parts per trillion.
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