Parts-per notation

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In science and engineering, parts-per notation is a set of pseudo-units to describe the 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 parts-per-quadrillion – ppq, 10−15 This notation is not part of the International System of Units (the SI system) and its meaning is ambiguous. In chemistry, ambiguity arises because parts-per notation is able to be used to refer to a mole fraction and a mass fraction, which are unequal depending on the molar mass of the substance.