Parts-per notation

In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are parts-per-million (ppm, 10⁻⁶), parts-per-billion (ppb, 10⁻⁹), parts-per-trillion (ppt, 10⁻¹²) and parts-per-quadrillion (ppq, 10⁻¹⁵). This notation is not part of the International System of Units (SI) system and its meaning is ambiguous.