Concentration in Parts per Million
and Parts per Billion
Parts per million (ppm) and Parts per Billion (ppb), usually a mass/mass relationship
used to describe a very small quantity of a substance in the human body, or in the
environment. Notice that ppm does not refer to the number of particles, but to the
mass of the solute compared with the mass of the solution.
ppm =
ppb =
Mass of solute
Mass of solution
1 x 106
Mass of solute 1 x 109
Mass of solution
Example: A fungus that grows on peanuts produces a deadly toxin. When ingested in
large amounts, this toxin destroys the liver and can cause cancer. Any shipment of
peanuts that contains more than 25 ppb of this dangerous fungus is rejected. A
company receives 20 tonnes of peanuts to make peanut butter. What is the
maximum mass (in g) of fungus that is allowed?
Given: Allowable concentration is 25 ppb
Mpeanuts
= 20 t


ppb =
First convert the mass in tonnes into grams.
20 t x 1000 kg/t x 1000g/kg = _______________ g
Mass of fungus 1 x 109
Mass of peanuts
Mass of fungus
=
ppb x Mass of peanuts
1 x 109
=
__________________
=
_____ g
Therefore the maximum mass of fungus that is allowed is ______ g.