Parts-per notation

Parts-per notation expresses a proportion as the number of parts of a substance in a stated number of parts of a mixture, such as parts per million.

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Parts per million: A proportion expressed as one part of a quantity per million parts of the whole. This is the standard scale for expressing trace concentrations with parts-per notation.
Mass fraction: The mass of one component divided by the total mass of a mixture. Mass-based parts-per values express the relative mass of a constituent.
Molar concentration: The amount of dissolved substance per volume of solution, commonly measured in moles per liter. Unlike parts-per notation, molarity specifies an amount per volume rather than a fraction of mixture.
SI prefixes: Standard prefixes that multiply units by specified powers of ten, such as milli-, micro-, and nano-. Metric prefixes provide an alternative way to express very small quantities relative to a reference unit.
Parts per billion: A proportion expressed as one part of a quantity per billion parts of the whole. This extends the notation to proportions smaller than parts per million.
Volume fraction: The volume of one component divided by the total volume of a mixture. Gas mixtures and solutions may report parts-per values based on volume.
Mass concentration: The mass of a substance divided by the volume of the mixture containing it. Mass concentration has units such as milligrams per liter, unlike a dimensionless parts-per ratio.
Measurement uncertainty: A quantified doubt associated with a measured value, describing the range of values reasonably attributable to it. A parts-per value without uncertainty can suggest more precision than the measurement supports.
Parts per trillion: A proportion expressed as one part of a quantity per trillion parts of the whole. This scale shows how the notation handles exceptionally small proportions.
Mole fraction: The amount of substance of one component divided by the total amount in a mixture. Mole-based parts-per values count particles through amounts of substance.