parts per billion
/ parts per BIL-yun /
If one drop of dye in a bathtub is a part per million, then a part per billion is roughly one drop in a small swimming pool. It is the unit for the almost-vanishingly-rare — contaminants present at the edge of what instruments can even detect.
Parts per billion (ppb) is one part of analyte for every billion (one thousand million) parts of total sample, by mass or by volume. It is a thousand times smaller than a part per million, so 1 ppb in dilute water is about one microgram of solute per litre.
ppb is the working scale for trace toxins, heavy metals, and pesticide residues, where modern instruments routinely measure what older methods could not even see. The closer concentrations creep toward ppb, the more careful you must be about contamination, blanks, and the detection limit of the method.
A safe-water limit of 10 ppb lead means at most 10 micrograms of lead in each litre — so small that touching the sample with the wrong glassware could add more than the limit allows.
For dilute water, 1 ppb ≈ 1 microgram per litre.
The word 'billion' differs between languages and old usage; in analytical chemistry ppb always means one in 10⁹. At this level the analytical blank often contributes a meaningful share of the signal.