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parts per billion

A unit of measure used for very small quantities, it is equal to the ratio of the weight or volume of one component of a mixture to a billion weights or volumes of the mixture. When based on weight (ppbw), it is equal to the weight or mass of the component divided by the total weight or mass in a given volume, multiplied by one billion. When based on volume (ppbv) it is equal to the volume of the component divided by the total volume of the mixture, multiplied by one billion. Source: NASA (http://earthobservatory.nasa.gov/Glossary)