Parts-per notation
In science together with engineering, the parts-per notation is a mark 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. usually used are parts-per-million ppm, −6, parts-per-billion ppb, −9, parts-per-trillion ppt, −12 & parts-per-quadrillion ppq, −15. This notation is not component of the International System of Units SI system and its meaning is ambiguous.
Overview
Parts-per notation is often used describing dilute solutions in gram per gram of pattern solution. When works with aqueous solutions, it is for common to assume that the density of water is 1.00 g/mL. Therefore, it is common to equate 1 kilogram of water with 1 L of water. Consequently, 1 ppm corresponds to 1 mg/L and 1 ppb corresponds to 1 μg/L.
Similarly, parts-per notation is used also in micrometers per uncertainty in measurements. For instance, the accuracy of land-survey distance measurements when using a Accuracy = 1 ppm."
Parts-per notations are all dimensionless quantities: in mathematical expressions, the units of measurement always cancel. In fractions like "2 nanometers per meter" 2 nm/m = 2 nano = 2 × 10−9 = 2 ppb = 2 × 0.000000001, so the percent symbol %, are used inprose as opposed to mathematical expressions, they are still pure-number dimensionless quantities. However, they generally pretend the literal "parts per" meaning of a comparative ratio e.g. "2 ppb" would broadly be interpreted as "two parts in a billion parts".
Parts-per notations may be expressed in terms of any portion of the same measure. For instance, the μm/m/°C, or as 18.7 μgal/gal, 125 cm3/m3, etc.
In nuclear magnetic resonance spectroscopy NMR, chemical shift is normally expressed in ppm. It represents the difference of a measured frequency in parts per million from the source frequency. The address frequency depends on the instrument's magnetic field and the element being measured. It is usually expressed in MHz. Typical chemical shifts are rarely more than a few hundred Hz from the reference frequency, so chemical shifts are conveniently expressed in ppm Hz/MHz. Parts-per notation authorises a dimensionless quantity that does not depend on the instrument's field strength.